Meetings & Events


spring 1997 logo1997 MRS Spring Meeting & Exhibit

March 31 - April 4, 1997 | San Francisco
Meeting Chairs: Linda G. Griffith-Cima, David J. Eaglesham, Alexander H. King

Symposium M—Magnetic Ultrathin Films, Multilayers, and Surfaces


Katayun Barmak, Lehigh Univ
David Chambliss, Dept of MS&E IBM Almaden Research Center
Wim de Jonge, Eindhoven Univ of Technology
Peter Dederichs, Forschungszentrum Julich GmbH
Toshikazu Katayama, Electrotechnical Laboratory
David Kubinski, Ford Research Laboratory
Alain Schuhl, CSF-LCR Lawrence
James Tobin, Livermore National Lab

Symposium Support

  • Caburn-MDC Ltd.
  • Central Research Laboratory, Hitachi, Ltd.
  • Digital Instruments, Inc.
  • Ford Research Laboratory
  • Fujitsu, Ltd.
  • GMW Associates
  • Komag, Inc.
  • Lake Shore Cryotronics, Inc.
  • MMR Technologies, Inc.
  • Physical Electronics Inc.
  • Princeton Measurements Corp.
  • Tencor Instruments
  • Toshiba Corporation
  • US Inc. 

    1997 Spring Exhibitor

Proceedings published as Volume 475
of the Materials Research Society
Symposium Proceedings Series.

In sessions below "*" indicates an invited paper.

Chair: Katayun Barmak
Monday Morning, March 31, 1997
Golden Gate C2

8:30 AM *M1.1 

This talk is a tutorial for the broad materials audience on magnetic measurements of microscopic quantities. After briefly mentioning a few of the current microscopic probes that might be used, often for studies of small regions of a sample, I will concentrate on the principles of several of the most popular macroscopic magnetic measurement techniques including the vibrating sample magnetometer, SQUID magnetometer, ac susceptometer, ac force magnetometer, and possibly Faraday rotation. These types of instruments are available commercially as stand-alone systems, often with computer-controlled operation. Selected examples of measurements on thin films, small particles, and clusters of atoms will be shown in order to illustrate what can be done, and point out some of the not-so-subtle problems encountered if care is not exercised. Measurements on microscopic samples raise some interesting issues that are not addressed by of sensitivity. Questions from the audience on these and other techniques will be welcomed.

9:00 AM M1.2 
LOW-TEMPERATURE GROWTH OF NiMnSb HEUSLER ALLOY ULTRATHIN FILMS, Jeffrey R. Childress, Juan A. Caballero, Univ of Florida, Dept of MS&E, Gainesville, FL; Frederic Petroff, Thomson-CNRS, UMR, Orsay, FRANCE; Yoshishige Suzuki, Joint Res Ctr for Atom Technology, Atom Technology Group, Tsukuba, JAPAN.

Ferromagnetic Heusler alloys such as NiMnSb may have unique application to magnetoresistive devices because of their predicted half-metallic bandstructure (i.e., 100 spin-polarized at the Fermi level). However, the high temperatures (>500C) used to date for the synthesis of the ordered alloy in bulk or thin-film form is incompatible with the fabrication of nanoscale multilayer structures from this material. We report on the growth of high-quality polycrystalline ultrathin films of Cl-structured NiMnSb by RF-magnetron sputtering of a composite NiMnSb target onto glass and silicon substrates at temperatures as low as 200C. The films were characterized using x-ray diffraction (XRD), TEM, AFM, energy-dispersive x-ray spectroscopy, SQUID magnetometry, magneto-optical Kerr spectroscopy and electrical resistivity measurements. We have established that substrate temperature, deposition rate, and argon gas pressures all play a critical role in obtaining the Cl-structured phase by direct deposition. Optimal conditions result in films whose properties, including lattice parameter, saturation magnetization, resistivity and Kerr spectrum, are identical to those of bulk NiMnSb. Additionally, coercive fields as low as a few Oersteds make these films compatible with low-field device applications. Relationships between fabrication conditions and film properties will be discussed.

9:15 AM M1.3 
HALF-METALLIC HEUSLER ALLOYS FOR SPIN VALVE STRUCTURES, J. P. Nozieres, CNRS, Laboratoire Louis Neel, Grenoble, FRANCE; C. Hordequin, CNRS, Laboratoire L Neel, Grenoble, FRANCE; P. Dauguet, CNRS, CRTBT, Grenoble, FRANCE; Philip Gandit, Jacques Chaussy, CNRS, Inst for Low Temperature Physics, Grenoble, FRANCE.

The half metallic Heusler compounds are expected from zero-Kelvin band structure calculations to exhibit a gap in the minority subband (semi conducting behavior), whereas the majority subband has a metallic character. In other words, the conduction electrons are in principle fully spin-polarized, which in principle yield important giant magnetoresistance amplitude in multilayered structures. We report here on spin valve structures based on the cubic NiMnSb Heusler compound. 
Plain films and spin valves with an amorphous SmCo biasing layer were deposited by facing target sputtering, The magnetic, transport, and NMR properties of the plain films were found to be identical to that of bulk single crystal. In particular, a change in the behavior of M(T) and (T) occurred near 80 K. This suggests a cross-over from a half metallic to a fully metallic behavior, in agreement with recent polarized neutron diffraction studies. Spin valve structures built with a Cu spacer showed little GMR amplitude, which was ascribed to a perturbation of the Heusler structure at the Cu-interface. On the other hand, good structural properties were maintained with a Mo spacer although the GMR amplitude in the conventional CIP configuration is again small due to the large resistivity of the Mo layer. The magnetotransport properties have thus to be accessed by CPP measurements. Millimetric structures were built between crossed superconducting electrodes by in-situ masking techniques. Measurements are under progress.

9:30 AM M1.4 
STRUCTURAL AND MAGNETIC PROPERTIES OF XMnSb/YMnSb SUPERLATTICES (X,Y=Pt, Ni,Cu), Jean-Francois Bobo, Paul R. Johnson, Michael C. Kautzky, Kazuhiro Bessho, Frederic B. Mancoff, Robert L. White, Bruce M. Clemens, Stanford Univ, Dept of MS&E, Stanford, CA.

The C1b heusler alloy PtMnSb exhibits the largest known kerr rotation and, reportedly, half-metallic ferromagnetism, which should imply 100 spin-polarization at the Fermi level, and could lead to very high giant magnetoresistance (GMR) values. However, neither the magnetooptic (MO) nor the transport properties of PtMnSb-based thin films have lived up to potential for applications, as neither perpendicular anisotropy nor antiferromagnetic spin-valve alignment have been observed. To stabilize thin, flat layers for spin-dependent transport devices and to induce perpendicular anisotropy, we have grown epitaxial Heusler alloy multilayers, including PtMnSb/NiMnSb, PtMnSb/CuMnSb and NiMnSb/CuMnSb. These (111)-oriented superlattices were grown by dc magnetron cosputtering on (0001) sapphire. While each system exhibited superlattice diffraction features, the most perfect structures were obtained in PtMnSb/NiMnSb superlattices, where we observed a coherent layering down to individual layer thickness of 4 (one repeat unit). We correlate the lattice mismatch strain and the magnetic anisotropy as a function of layer thickness, and report the influence of intermixing on the value of the saturation magnetization and interlayer interactions

10:15 AM M1.5 
MAGNETIC PROPERTIES, THERMAL STABILITY AND MICROSTRUCTURES OF FeMoN FILMS FOR HIGH DENSITY RECORDING HEADS, Shan X. Wang, Stanford Univ, Dept of MS&E, Stanford, CA; Lee Nguyentran, San Jose State Univ, Dept Matl Eng, San Jose, CA; Kyusik Sin, Stanford Univ, Dept of MS&E, Stanford, CA; Patrick Pizzo, San Jose State Univ, Dept of Matls Engr, San Jose, CA.

High saturation soft magnetic thin films are required for high density recording heads in magnetic data storage devices. This paper will report the soft magnetic properties, thermal stability, microstructures, and compositions of single layer FeMoN films as deposited by RF reactive sputtering on Al-TiC substrates. At a N/Ar gas flow rate ratio of about 6.2, the FeMo(2.2 at.)N film has a minimum easy axis coercivity of 1.2 Oe and an anisotropy field of 5.5 Oe. As the content of Mo in FeMoN films increases to 3.3 at., the minimum H increases to 3.3 Oe. The optimum N/Ar gas flow rate ratio where the minimum coercivity occurs increases to 9. X-ray diffraction studies indicate that there is a strong peak from (110) planes of -FeMo (bcc) for the pure FeMo films. As the N/Ar gas flow rate ratio increases, the intensity of the (110) peak decreases significantly. At a N/Ar gas flow rate ratio where the coercivity is minimum, the -(FeMo)N phase is observed in the diffraction pattern. Further increases in the N/Ar gas flow rate ratio lead to stronger -(FeMo)N peaks, weaker -FeMo peaks, and corresponding changes in peak widths. This suggests that the variations in the coercivities of the FeMoN films are related to the grain sizes, which are affected by the incorporation of N into -FeMo, and the saturation of N in -FeMo matrix, as indicated by the formation of -(FeMo)N phase. FeMoN films show much better resistance to thermal degradation than FeN films. After 2 hours of vacuum annealing, the magnetic properties of FeN films are stable only up to 300C, whereas those of FeMoN are still stable at 350C. It is found that annealing leads to increases of -FeMo and -(FeMo)N peak intensities in the FeN films, but no appreciable changes in the FeMoN films, indicating an important role of Mo inclusion.

10:30 AM M1.6 
STRUCTURAL AND MAGNETIC CHARACTERIZATION OF IRON-NITRIDE LAYERS SYNTHESIZED WITH LT GASEOUS NITRIDING, Dirk O. Boerma, Univ of Groningen, Materials Science Center, Groningen, NETHERLANDS; D. K. Inia, Univ of Utrecht, Dept of Atomic & Interface Physics, Utrecht, NETHERLANDS; A. M. Vredenberg, Utrecht Univ, Utrecht, NETHERLANDS.

A new method was developed for the synthesis of iron nitride layers at the relatively low temperature of 300C. The iron layers are covered with a 40 nm thick Ni layer before gaseous nitriding in an ambient of NH and H. The pore-free layers are composed of , , or nitrides. The composition, the structure, and the magnetic properties depend on the growth conditions, which can be tuned by varying the gas composition, the pressure, and the temperature. The structure also depends on the presence of small admixtures in the Fe layer of metals like Cr, Zr, etc., which form precipitates of very stable nitrides during the early stages of the nitriding process. Since the growth method has a potential for applications, a patent has been claimed. 
The composition and the structure of the layers is determined with ERD, XRD, and TEM. The magnetic properties are derived from Mossbauer spectroscopy and from measurements of the magnetization.

10:45 AM M1.7 
SYNTHESIS AND CHARACTERIZATION OF METALLIC AND NITRIDE DOPED (CoFe/Cu) MAGNETIC MULTILAYERS, Prabath Perera, Ray Selestino, Anival Ayala, Derick Scott, R. W. Dail, David Medrano, M. J. Sablik, Carlos Gutierrez, SW Texas State Univ, Dept of Physics, San Marcos, TX.

For Si-based ferromagnet-semiconductor magnetoelectronic hybrid devices, it is crucial to implement robust magnetic film materials capable of withstanding substantial thermal processing (typically C). In this work, a series of [Co (where x = 2.5 nm and y = 2, 3, and 4 nm) multilayers on Si were prepared by magnetron sputtering of Co/Cu alloy and Cu targets, without and without a trace of nitrogen gas. Differences in magnetic properties of these multilayers resulting from the introduction of a nitrogen trace were observed by VSM and magnetotransport measurements. These multilayers were deposited on seeded 100 nm thermal oxide coated Si substrates, in a high vacuum system (P Torr), under an Ar atmosphere of 7 mTorr. Multilayers were subjected to a uniform 160 Oe biasing field applied along the film plane to establish a weak uniaxial anisotropy during the deposition process, and were synthesized without external substrate heating. As-deposited metal multilayers of y = 2, 3, and 4 nm showed a coercivity of 8, 10, and 11 Oe. Both metal and nitride multilayers showed an enhancement of coercivity as a function of Cu layer thickness and annealing temperature (at 350C and 400C) without a measurable loss of saturation magnetization (magnetization results are bulk-like for all multilayers). The remanent and saturation magnetizations of the nitride multilayers are typically lower than the metallic multilayers. The increase in coercivity of multilayers with annealing is consistent with the observed differences in interface roughness measured by grazing incidence x-ray reflectivity. The results of SEM, EDS, and resistivity measurements will be presented for both the metal and nitride multilayers.

11:00 AM M1.8 
MAGNETIC THIN FILMS OF COBALT NANOCRYSTALS ENCAPSULATED IN GRAPHITE-LIKE CARBON, Takayoshi Hayashi, Shigeru Hirono, NTT Integrated Information & Energy Systems Lab, Tokyo, JAPAN; Masato Tomita, NTT Science & Core Technology Lab, Tokyo, JAPAN; Shigeru Umemura, Jean-Jack Delaunay, NTT Integrated Information & Energy Systems Lab, Tokyo, JAPAN.

Nanocrystals encapsulated in multilayered graphitic nanocages formed by the carbon arc-discharge method was first reported in 1993 [1, 2]. Since then a variety of materials have been encapsulated in graphitic nanocages, including magnetic materials such as Fe, Co, and Ni. This new class of materials is attractive for ultrahigh-density (10 Gbit/in2) magnetic recording applications, because graphitic carbon cages can reduce exchange coupling among encapsulated magnetic nanocrystals as well as protect encapsulated air-sensitive materials against degradation. 
Ultrahigh-density magnetic recording media must be in the form of granular thin films with smooth surfaces on a nanometer scale. The diameter of the magnetic grains must be precisely controlled around 10 nm. Since the commonly used carbon arc-discharge method is not appropriate for precise control of the required nanostructure, we have taken a different approach to wrapping cobalt nanocrystals with graphite-like carbon. This approach is to co-deposit cobalt and carbon by using ion-beam sputtering, followed by subsequent annealing [3]. It is found that the grain size and the crystal structure of the as deposited Co-C films depend on the substrate temperature and the carbon concentration. The high carbon concentration films () deposited at 200 degree C are amorphous but still have a grain-like structure with a grain size of less than 10 nm. The amorphous grains transform into crystalline h.c.p. cobalt encapsulated in graphite-like carbon during the succeeding annealing at 300 degree C, but the initial grain size is not affected. These films meet the above-mentioned structural requirements for ultrahigh-density magnetic recording media and have promising magnetic properties.

11:15 AM M1.9 

Monolayer and multilayer ultrathin films comprised of nano-sized ion oxide (Fe3O4) particles and polyimide molecules have been fabricated on single crystal silicon, and quartz substrates by a novel molecular self-assembly process. This process involves the alternate dipping of a substrate into an aqueous solution of anionic polyimide precursor (polyamic acid salt, PAATEA) followed by dipping an aqueous solution of polycation which coats on nanoscale Fe3O4 particles as stabilizer. The growth process and the structure are probed with UV/Vis spectroscopy, contact angle and ellipsometry measurements as well as FT-IR spectroscopy. The results suggest that well-ordered uniform monolayer and multilayer magnetic films have been formed on silicon and silica surfaces. A recently developed fiber optical magnetic sensor was used to measure the magnetic field intensity of the magnetic multilayer films.

11:30 AM M1.10 
FORMATION AND PROPERTIES OF FERROMAGNETIC MNAS SUBMICRON PARTICLES IN LOW TEMPERATURE GaAs BY MANGANESE IMPLANTATION PARTICLES IN LOW TEMPERATURE GAAS BY MANGANESE IMPLANTATION, Peter Juergen Wellmann, Jorge M. Garcia, Jiang-Lin Feng, Pierre M. Petroff, Univ of California-S Barbara, Dept of Materials, Santa Barbara, CA; Mark Field, David Awschalom, Univ of California-S Barbara, Dept of Physics, Santa Barbara, CA.

The formation of ferromagnetic particles of the size of several nanometers () embedded in semiconductors is of particular interest for device applications such as magnetic sensors, switches or memories based on integrated semiconductor devices. 
We have investigated the formation as well as the structural and magnetic properties of MnAs nanomagnets in low temperature GaAs (LT-GaAs). We report the formation of MnAs crystallites which show ferromagnetism at room temperature using ion implantation with Mn (, E=180keV) and subsequent rapid-thermal-annealing (RTA) (T=600C ... 900C and ). 
TEM (transmission-electron-microscope) analysis of the Mn-implanted and annealed LT-GaAs layers show a significant change in the crystal structure in comparison to conventional (not implanted but annealed) LT-GaAs. The LT-GaAs layer which has been amorphised due to Mn-implantation, recrystallises during annealing and forms an epitaxial layer with a dislocation density of about 10cm. As opposed to the unimplanted but annealed LT-GaAs, the presence of As-precipitates is dramatically reduced in these samples. On the other hand, crystalline structures, which we attribute to be MnAs precipitates, are found near dislocations. 
SQUID-measurements have been done to study the magnetic properties of the precipitates. For an annealing temperature of 750C we find the ferromagnetic hysteresis with the highest -product. The Curie temperature T of this sample coincidences very well with the T=318C for bulk MnAs.

11:45 AM M1.11 
MAGNETORESISTANCE EFFECT IN GRANULAR THIN LAYERS FORMED BY HIGH-DOSE IRON IMPLANTATION INTO SILICON, Sai-Peng Wong, Chinese Univ of Hong Kong, Dept of Electronic Engr, Shantin NT, HONG KONG; W. Y. Cheung, Chinese Univ of Hong Kong, Dept of Electical Engr & Matls Tech, Shantin NT, HONG KONG.

High dose iron implantation into silicon substrates has been performed with a metal vapor vacuum arc ion source to doses ranging from 1x10 to 1x10 at various beam currents. The magnetoresistance (MR) effect in these implanted granular layers was studied at temperatures from 15K to 300K. The magnetic field applied was transverse to the current in the measurements. A positive MR effect, i.e., an increase in the resistance at the presence of a magnetic field, has been observed at temperatures lower than about 35K. The magnitude of the MR effect, defined as DR/R = (R(H) - R, was found to depend not only on the implantation dose but also on the beam current. This is attributed to the beam heating effect which affects the size distribution and the phase of the microstructures formed during the implantation. The ratio DR/R was found to attain high values larger than 200 for some samples. The dependence of the MR effect on temperature, implantation dose, and beam current will be presented and discussed in conjunction with the results of other characterization techniques such as transmission electron microscopy.

Chairs: David Kubinski and James G. Tobin 
Monday Afternoon, March 31, 1997
Golden Gate C2

1:30 PM *M2.1 
MAGNETIC FIELD SENSORS FOR MAGNETIC POSITION SENSING IN AUTOMOTIVE APPLICATIONS, Joseph P. Heremans, General Motors R & D Center, Dept of Physical Chem, Warren, Michigan.

Magnetic position sensors are transducers that create an electrical signal which is a function of a mechanical motion. They consist of a permanent magnet, a magnetic field sensor, and a moving magnetic circuit. Being contactless, they do not wear out. Unlike optical systems, they are impervious to contamination. With the proliferation of microprocessor control in many on-board automotive applications, position sensing becomes a crucial function. Magnetic position sensors are found in cam and crankshaft sensors, where they are used for ignition timing and engine misfire detection, in brushless electrical motors, in wheel speed sensors and other applications.
This presentation first reviews the requirements put on the magnetic field sensors when used in this type of applications. There are five main types of field sensors used or potentially used in position sensing: Si Hall sensors with integrated amplifiers, GaAs Hall sensors, InSb-based magnetoresistors, anistropic magnetoresistors (AMR), and metal-multilayer magnetoresistors (GMR). Magnetoresistors are almost always used as differential pairs, either in Wheatstone bridges or with matched constant current sources. The characteristics of each type of field sensors will be reviewed: their sensitivity to magnetic field, and the drift of the outputs with external parameters, mainly temperature. These characteristics will be compared with the needs of the different position sensing applications.

2:00 PM M2.2 
A NEW SPIN FILTER: THE MAGNETIC SCHOTTKY DIODE, Antoine Filipe, Thomson CSF, UMR CNRS, Orsay, FRANCE; Henri-Jean Drouhin, Georges Lampel, Jacques Peretti, Ecole Polytechnique, Lab de Physique de la Matiere Condensee, Palaiseau, FRANCE; Alain Schuhl, Thomson-CNRS, CSF-LCR, Orsay, FRANCE; Yves Lassailly, Ecole Polytechnique, Lab de Physique de la Matiere Condensee, Palaiseau, FRANCE; V. I. Safarov, Univ de la Mediterranee, Fac des Sciences de Luminy, Marseille, FRANCE.

Spin-dependent electronic transmission through a Fe/GaAs Schottky diode is observed, with an upper value of 20. 
The source of spin-polarized electrons is a p-doped GaAs substrate activated to Negative Electron Affinity. Under illumination with a circularly polarized light, it provides a current polarization of 25. The photoemitted electrons are then injected from the vacuum into the sample: a 3.5 nm thick layer of Fe deposited on a n-doped GaAs substrate. The magnetization of the Fe-layer is in plane, and colinear to the spin polarization of the incident electrons. Whereas the majority of electrons undergo inelastic collisions in the metallic layer and do not overcome the Schottky-barrier, a small number of electrons enter the semiconductor and are detected as a transmitted current. In a simple ballistic model, the transmission coefficient is exp(-d/), where d is the metal thickness and is the inelastic mean-free path in the metal. The spin-dependence of (and therefore of the transmission coefficient) is checked by changing the spin-polarization of the incident electrons (i.e., the light polarization) and/or reversing the Fe-layer magnetization. 
We measure a transmission coefficient of the order of a few 10. The transmission spin-dependence is observed to vary with the energy of the incident electrons (between 5 to 15 eV above the Fermi level); it reaches 20 at 5 eV. We present a simple theoretical model explaining all the experimental features. Moreover, we discuss why the association of ferromagnetic and semiconductor materials appears to be very promising for the fabrication of solid-state spin-detectors.

2:15 PM M2.3 
GIANT HALL EFFECT IN PERCOLATING MAGNETIC NANOCOMPOSITE FILMS, X. Yan, B. Zhao, A. B. Pakhomov, X. N. Jing, Y. Xu, E. Z. Luo, Hong Kong Univ Sci & Tech, Dept of Physics, Kowloon, HONG KONG.

Since the initial discovery of giant magnetoresistance (GMR), there are numerous systematic studies of GMR in various kinds of magnetic multilayers and granular films, with nonmagnetic parts being either metallic or insulating. One common feature of GMR materials is the existence of magnetic inhomogeneity at a length scale below the electron mean free path which is typically at 5 nm in metals. GMR is often the largest when individual magnetic constituents are separated having antiferromagnetical or random alignment between nearby constituents at zero magnetic field due to a spin-dependent transport process. Recently, in magnetic metal-insulator nanocomposite films, giant Hall effect (GHE), or a 10fold enhancement of Hall resistivity, was discovered when the magnetic volume fraction is just above the percolation threshold. GHE represents a new kind of nanostructure-enhanced property in magnetic nanostructures. In this talk, I will review the discovery of GHE in NiFe-SiO films, studies of magnetoresistance, Hall effect, and magnetic properties for various temperatures and magnetic field at various metallic volume fractions, and their relationship with the annealing controlled microstructures characterized by transmission electron microscope and novel conducting atomic force microscope.

2:30 PM M2.4 
HEMISPHERICAL MULTILAYER STRUCTURES FOR CAP-GMR, Karen Attenborough, Katholieke Univ Leuven, Dept of MTM, Leuven, BELGIUM; Jo De Boeck, IMEC, Dept of MAP/MBE, Leuven, BELGIUM; Jan Fransaer, Jean-Pierre Celis, Katholieke Univ Leuven, Dept of MTM, Leuven, BELGIUM; Gustaaf Borghs, IMEC, Dept of MAP/MBE, Leuven, BELGIUM.

In order to increase the MR characteristics in planar multilayer structures, Current at an Angle to the Plane (CAP) geometries have been proposed where the substrate is patterned prior to directional deposition of multilayers. We propose an electrodeposition process for fabricating CAP-GMR layers. The use of electrodeposition from aqeous solutions for fabricating multilayers and nanowires is well documented. However, the practical use of such nanowires in fabricating sensors is hindered by the complexity of making electrical contact onto such wire structures. In the proposed process pores in a porous substrate are filled using electrodeposition of alternating magnetic and non-magnetic layers. The CAP structure is formed during the hemispherical outgrowth occuring once electrodeposition is continued after the nano-pores are filled up. The multilayer hemisheres connect at the bottom and form a string. Contacts are formed on the string and the current path crosses a large fraction of the interfaces perpendicularly. We have studied various possible porous templates including e-beam written structures, porous anodised aluminium, and structures formed by natural lithography. The formation of the connected hemispheres will be illustrated and we will discuss the technological parameters (pore size, diameter, substrate - wire electrical resistance) in relation to the GMR response.

3:15 PM M2.5 
(Co+Cu)/Cu MULTILAYERS WITH REDUCED MAGNETORESISTIVE HYSTERESIS, David Kubinski, Harry Holloway, Ford Research Laboratory, Dearborn, MI.

The high sensitivity [S = (I/R)R/H] of Co/Cu multilayers (MLs) at the second antiferromagnetic maximum (AFM) would seemingly make them an attractive candidate for most position sensing applications. Unfortunately, these films typically exhibit a large enough magnetoresistive hysteresis (MRH) to exclude their use in many applications. We previously reported [1] that we could overcome this problem and practically eliminate MRH at 300 K with only a small decrease in S by reducing the cobalt layer thickness to 3 . We report here similar magnetoresistive behavior found by replacing the Co layers in Co/Cu MLs at the second AFM with codeposited Co + Cu layers. Increasing the fractional amount of the co-deposited Cu results in both a reduction of the maximum magnetoresistance (MR) as well as MRH. With a ferromagnetic layer composition consisting of Co and Cu codeposited in a5:4 ratio, the magnetoresistance at 300 K is hysteresis free. High S is maintained at this composition since the reduction in MR is also accompanied by a decrease in the width of the magnetoresistance curve. The dependence on the overall thickness of the codeposited layer is discussed. Unlike the previously reported Co/Cu MLs with the very thin Co, the (Co + Cu)/Cu MLs are hysteresis free over a substantial range of ferromagnetic layer thickness. This facilitates the production of hysteresis free material at the second AFM.

3:30 PM M2.6 
SCANNING AUGER MICROSCOPY CHARACTERIZATION OF MAGNETIC HARD DISC, David Neiman, David W. Harris, Jingyu Huang, Charles Evans & Associates, Sunnyvale, CA.

Applications of Zalar rotation and Field Emission Auger analysis in various magnetic hard disk problems will be presented. These applications include thin layer structure characterization, low concentration contaminant (0.1-0.5 atomic percent) detection at deeper interfaces (greater than 1000 ), and small particle () contaminant identification.

3:45 PM M2.7 
PHASE TRANSFORMATIONS IN MAGNETO-OPTIC MnBi THIN FILMS, Prabhakar R. Bandaru, Univ of California-Berkeley, Berkeley, CA; Timothy D. Sands, Yukiko Kubota, Univ of California-Berkeley, Dept of MS&ME, Berkeley, CA; Ernesto Marinero, IBM Almaden Research Center, San Jose, CA.

Of the candidate magnetooptic media for ultrahigh density rewritable optical recording, polycrystalline MnBi films are unsurpassed in two of the most important criteria: the magnitude of the Kerr signal in the blue wavelength regime and the degree of perpendicular anisotropy. Attempts to exploit these properties have revealed poor thermal cycling behavior and high media noise, both of which are attributed in part to hysteresis in the first order phase transformation between the low-temperature phase (LTP) and the high temperature phase (HTP). In this paper, we report on in- situ TEM and Kerr effect studies of phase transformations in MnBi films on quartz and SiN/Si substrates. Electron-beam evaporation of Mn/Bi bilayers followed by vacuum annealing at 360C was used to produce 20-120 nm thick, (0001) textured MnBi films that exhibit Kerr rotation angles of 1.8 and reflectivities of 48% at : 630 nm (He-Ne laser). Measurements of the Kerr angle as a function of temperature at furnace heating and cooling rates revealed an increasing coercive field H (from 1.7 KOe at room temperature to 6.3 KOe at 300C) up to the phase transformation temperature, comparable to the unusual magnetic behavior of bulk MnBi. Cycling resulted in an irrreversible segregation of Bi coincident with a reduction in the Kerr signal with each cycle. Segregation of Bi was also observed by in-situ TEM as the temperature was raised above the MnBi eutectic at 260C. These results for 70 nm indicate fundamental barriers to application of MnBi in optical film recording unless the quenched high temperature phase, with its lower Tc (180C) can be stabilized. We will discuss our efforts to stabilize this phase in MnBi films and multilayers with thicknesses in the 20-70 nm range.

4:00 PM M2.8 
INFLUENCE OF DIELECTRIC FILM THICKNESS ON MAGNETIC PROPERTIES OF THE MAGNETO-OPTICAL MULTILAYER FILMS, Xiangshui Miao, Y. C. Chan, City Univ of Hong Kong, Dept of Electronic Engr, Kowloon, HONG KONG; Y. M. Huang, W. Z. Liu, Z. Y. Lee, Huazhong Univ of Science & Technology, Wuhan, CHINA.

The optical interaction of the magneto-optical multilayer films (including magneto-optical recording film, dielectric film and reflector film) have widely been studied; however, there are few reports for the purpose of the magnetic interaction of magneto-optical multilayer films. The magnetic properties (the coercivity, magnetic anisotropy, etc.) of magneto-optical multilayer films are closely related to the dielectric film. In this paper, we will discuss the relation between the coercivity, the magnetic anisotropy constants of multilayer films, and the dielectric film thickness. 
The coercivity of multilayer films (AlN/TbFeCo/Glass) firstly increased, then decreased and finally tended to be steady with the increase of the AlN film thickness; there was a maximum coercivity at the dielectric film thickness of 44 nm, and the steady trend is after the film thickness of 66 nm. The change of magnetic anisotropy K, of multilayer films is similar to the coercivity. But the maximum of magnetic anisotropy constant was found at the dielectric film thickness of 33 nm, and the steady trend was found after the film thickness of 71.5 nm. 
Because the TbFe series RE-TM magneto-optical recording film have larger magnetostrictive coefficient, the distribution of stress on the coercivity is very large. And the stress was changed from the compressive stress to the tensile stress step by step during the sputtering of the dielectric film onto the magneto-optical recording film. The relation of the magnetic coercivity and the dielectric film of magneto-optical multilayer films have been analyzed and successfully explained on the stress mechanism of the coercivity. According to the single ion model for perpendicular magnetic anisotropy in RE-TM magneto-optical films, the uniaxial magnetic anisotropy is related with the stress. The explanation of the magnetic anisotropy of the multilayer films is similar to the coercivity.

4:15 PM M2.9 
GIANT MAGNETOTRESISTANCE INDUCED BY ULTRATHIN Co LAYERS , Henk J.M. Swagten, Gustav J. Strijkers, Marc M.H. Willekens, Wim J.M. de Jonge, Eindhoven Univ of Technology, Dept of Physics, Eindhoven, NETHERLANDS.

Spin-resolved photoemission studies on Cu(100)/Co/Cu wedges have demonstrated that the formation of sp- and d- Quantum Well states requires a characteristic barrier thickness of less than 3Å Co. It was suggested that the evolution of spin- polarized QWs may be related to Giant MagnetoResistance, since investigations by Parkin revealed that very thin Co layers at the interfaces of a Py/Cu/Py spin-valve are able to saturate the GMR. To measure the GMR induced by ultrathin magnetic (interfacial) layers in a straightforward way, we have developed a spin-engineered device consisting of a ferromagnetic layer separated by a non-magnetic spacer from the thin ferromagnetic probe layer PR. On top of the trilayer a thick non-magnetic cap layer is subsequently grown, allowing the electrons that are polarized by the probe layer to experience their full mean free path. We have measured the GMR effect as a function of a Co probe layer thickness in magnetron sputtered systems with Co and Cu, from which an extremely small characteristic length of 1.3Å at T = 10 K could be deduced, thereafter increasing to roughly 2.3Å at room temperature. The magnetic behavior of ultrathin Co layers has been separately investigated in Co/Cu multilayers by Co NMR and SQUID magnetometry. It is observed that below t 10Å the probe layers gradually transform into a layered array of superparamagnetic Co clusters, which explains the increase of at higher temperatures in this regime. Only at sufficiently low temperatures the probe layer responds ferromagnetically and becomes extremely small. Using classical Boltzmann theory we will demonstrate that the observed (almost) discontinuous relaxation of GMR is induced by the interfacial nature of spin dependent scattering.

Chair: David D. Chambliss
Tuesday Morning, April 1, 1997
Golden Gate C2

8:30 AM *M3.1 
THE DEVELOPMENT AND CHARACTERIZATION OF CRYSTALLOGRAPHIC TEXTURE IN THIN FILMS FOR MAGNETIC RECORDING, David Laughlin, Li Tang, Carnegie Mellon Univ, Dept of MS&E, Pittsburgh, PA; Li-Lien Lee, Yu Nu Hsu, Carnegie Mellon Univ, Dept of MS & E, Pittsburgh, PA; David Lambeth, Carnegie Mellon Univ, Dept of Electrical & Computer Engr, Pittsburgh, PA.

The crystallographic mechanisms involved in the development of a preferred orientation of the grains of thin films will be discussed Polycrystalline thin films may obtain their crystallographic texture by means of a nucleation process such as epitaxial nucleation on a polycrystalline underlayer or by means of a process involving a preferred growth direction. Both of these mechanisms will be discussed with emphasis on that of epitaxial nucleation. Our examples will be drawn from our work on magnetic recording media. Once the thin film is produced, its crystallographic texture can be characterized by several techniques. We will discuss our recently developed technique which we have called the tilted electron beam technique. In this technique selected area electron diffraction patterns are obtained at different angles of tilt and the development of arcs in the patterns is analyzed so as to determine the type and amount of crystallographic texture present in the films. Examples from cubic and hexagonal crystalline substances will be presented, a well as examples of the study of the crystallographic textured bilayered films.

9:00 AM M3.2 
L1 PHASE FORMATION IN CoPt THIN FILMS, Roger A. Ristau, Katayun Barmak, Lehigh Univ, Dept of MS&E, Bethlehem, PA; Kevin R. Coffey, Sensormatic, Boca Raton, FL; J. Kent Howard, IBM, Storage Systems Div, San Jose, CA.

The high magnetic anisotropy and high coercivity of equiatomic cobalt platinum thin films make them attractive as potential materials for magnetic recording applications. Magnetic coercivity (H) over 10 kOe has been measured in films in which the as-deposited fcc phase has been partially transformed to the atomically ordered L1 phase. Very high H has been related to the high volume fraction and small size of the L1 precipitates. A better understanding of the L1 phase formation and quantification of the volume fraction is critical to optimizing the magnetic properties of this material. 
As we have previously reported, an increase in H was observed with an increase in L1 volume fraction in 10 nm thick equiatomic CoPt films. In our current investigation we have observed that, at annealing temperatures far from the order/disorder transition temperature (T), e.g. at T = 0.67 T, numerous, very fine L1 precipitates cluster at the parent phase grain boundaries. At T = 0.85 T, very few, large L1 regions, subdivided by antiphase boundaries were seen. This is consistent with a heterogeneous nucleation and growth mechanism for the formation of the L1 phase throughout the temperature range studied. As the precipitates of the ordered phase grow to impingement, antiphase boundaires (APB) are formed. Pinning of the magnetic domain walls at the APB may be one of the mechanisms responsible for the high H of this material.

9:15 AM M3.3 
MICROSTRUCTURAL DEVELOPMENT OF THIN CoCrTa FILMS ON Cr UNDERLAYERS, Caroline A. Ross, Komag Inc, Milpitas, CA; Frances M. Ross, Lawrence Berkeley National Laboratory, Natl Ctr for Electron Microscopy, Berkeley, CA.

Longitudinal hard disk recording media commonly include a Cr underlayer with a CoCrTa or CoCrPt magnetic layer. The Cr is grown at elevated temperatures and has a (200) preferred orientation. The CoCrTa grows epitaxially with a (1120) orientation which places the magnetically easy c-axis parallel to the film plane, which is desirable for longitudinal recording. The CoCrTa can grow in two possible variants on the Cr, with its c-axis parallel to either (011) or (01 1). As recording densities increase, the magnetic film is made thinner and the early stages of film growth become more critical. We have investigated the morphology and microstructural development of thin Co-lOatCr-6atTa (2.5 - l5 nm thick) grown on Cr using atomic resolution microscopy. The CoCrTa nucleates as 2.5 nm regions in both orientations on a single Cr grain. The thinnest films have a discontinuous microstructure with several CoCrTa nuclei forming on each Cr grain. For films of 10 nm thickness and above, the grains coalesce and stacking faults are seen. The contrast of small nuclei and stacking faults depends on the film thickness and matches computer simulations. Magnetic switching volumes of the films were measured from the change in coercivity with the scan rate of the external magnetic field. The ratio KV/kT of magnetic to thermal energy within the switching volume V decreases as the film thickness decreases, and films thinner than 10 nm have KV/kT < 60 and would show significant decay of high density transitions in longitudinal media applications. For most films the switching volumes are of similar size to the physical grain volume, suggesting that the entire grain participates in magnetic switching.

9:30 AM M3.4 
MICROSTRUCTURAL EVOLUTION OF ION BEAM DEPOSITED GMR SPIN VALVES, William E. Bailey, Shan X. Wang, Stanford Univ, Dept of MS&E, Stanford, CA; Sunita Gangopadhyay, Seagate Technology Inc, Minneapolis, MN.

One of the greatest challenges in characterizing giant magnetoresistive spin valves is in resolution of the interfaces between successive layers. The mass contrast between the compounds used, as between Co/Cu or NiFe/FeMn, is not typically visible from HRXTEM (high-resolution cross sectional transmission electron microscopy) imaging. These interfaces were brought into question in previous work, where HRXTEM images showed pronounced (20 AA) bowing of the top FeMn surface of an ion-beam deposited, strongly (111)-textured Si/SiO/NiFe/Co/Cu/NiFe/FeMn spin valve, accompanied by an uncommonly low pinned-layer coercivity (<80 e) and GMR ratio (2.5). To circumvent the limitation of mass contrast, we have analyzed a series of films with the deposition halted at each relevant interface. Full GMR spin valves of the form Si/SiO/Ta(25)/ 
NiFe(50)/Co(20)/Cu(t=25,33)/Co(20)/NiFe(50)/FeMn(150 ) were prepared, as well as stacks capped by Ta after deposition of the first ferromagnetic bilayer NiFe/Co, nonmagnetic spacer Cu, and second ferromagnetic bilayer Co/NiFe. High-intensity XRD shows a monotonic increase of the (111) peak intensity, with clear contributions observable from the additional 8 of Cu. In confirmation of previous work, no (200) or (220) orientations are observed. HRXTEM investigation verifies the nondisperse textural orientation and columnar structure of the samples. The evolution of interface morphology will be discussed in the full paper.

9:45 AM M3.5 
MICROSTRUCTURAL STUDIES OF SPUTTERED CoFe/Ag GMR MULTILAYERS, James D. Jarratt, Univ of Alabama, Dept of Metallurgical and Matls Engr, Tuscaloosa, AL; T. J. Klemmer, Univ of Alabama, Center for Materials for Information Tech, Tuscaloosa, AL; J. A. Barnard, Univ of Alabama, Dept of Metallurgical & Materials Engr, Tuscaloosa, AL.

The microstructure of [Si<111>/1 m SiO/50 Ta/(Co 7.5-25 /Ag 20-50 )x 20/120 Ta] films has been investigated using cross-sectional transmission electron microscopy (XTEM), selected area electron diffraction (SAED), and high-angle x-ray diffraction (HXRD). (111) textured columnar grains are observed. A direct correlation exists between an intensity histogram made from a line drawn radially through the (111) texture SAED spots and the log of the HXRD scan intensity of the same corresponding 2 region. We observe a comparison of these both as a function of 1/d-spacing as measured from the radius of the diffraction spots and the 2 values from the SAED pattern and the HXRD scan, respectively.

10:00 AM M3.6 
IMAGING OF MAGNETIC METALLIC MULTILAYERS WITH FRESNEL CONTRAST, Pierre Galtier, Thierry Valet, Thomson CSF, Lab Central de Recherches, Orsay, FRANCE.

The Fresnel imaging technique is applied to study metallic multilayers by Transmission Electron Microscopy (TEM). This contrast mode is found to be a powerful tool to image the details of the multilayers when they are composed of elements with close atomic numbers (Co, Cu, Fe, Ni...). In that case, the lack of contrast usually observed in TEM images can be overcome by recording the images with large defocus. Under such viewing conditions, Fresnel fringes are generated and this allows to visualize the detail of the multilayer stacking. This technique has been applied to characterize Co/Cu and NiFe/Cu multilayers sputtered on silicon. Parameters like the thickness of the layers, their waviness, as well as possible discontinuities can be assessed from a straightforward analysis. The advantages and limitations of the technique are discussed with regard to the structure and texture of the samples and the experimental conditions. In addition, we show that a careful analysis of the experimental images can help to quantify the interface sharpness. A quantitative analysis of experimental focus series combined with kinematical simulations have been performed. This allowed to measure a value of 10 for the interface sharpness in Ca/Cu multilayers.

10:45 AM *M3.7 

We have performed ultrahigh vacuum scanning tunneling microscopy studies of thin multilayer metal films produced in situ by Magnetron or Ion Beam Sputtering onto single crystal and amorphous substrates. For Co/Cu multilayers deposited on Cu(100) we find strong differences in the growth morphology which manifest themselves in radically different magnetic behavior as observed by MOKE. The structural differences can be qualitatively accounted for by considering the effects of kinetic energy on lateral diffusion and the magnetic differences can be ascribed to the consequences of topography on oscillatory antiferromagnetic coupling. Epitaxial spin valves of the structure Al:sub2O/Pt/Co/Cu/Co/MnFe/Ta have also been examined. We find that the presence or absence of oscillatory coupling, evident from GMR measurements, can be traced to the deposition technique used for individual layers. We further find that the ferromagnetic coupling, usually ascribed to interfacial roughness, does depend on the surface roughness as determined by STM but its magnitude does not agree well with Neel's model. Finally we characterize amorphous spin valves of the form Ta/Co/Cu/Co/MnFe/Ta and speculate on the role of arrival energy in interfacial intermixing and in controlling the amplitude of GMR.

11:15 AM M3.8 
DYNAMIC EVOLUTION OF THE MAGNETIC PROPERTIES OF ULTRATHIN Co/Cu(110), Stephen Hope, Erdan Gu, James Anthony Bland, Cambridge Univ, Dept of Physics, Cambridge, UNITED KINGDOM.

High sensitivity magneto-optic Kerr effect (MOKE) measurements of the magnetic hysteresis behaviour have been carried out in-situ during the growth of ultrathin (< 40ML) fcc Co/Cu(110) films. The films were structurally characterised by LEED measurements. Such Co/Cu(110) epitaxial films are found to have unique and novel magnetic properties which are substantially different from both the Co/Cu(100) and Co/Cu(111) systems. In particular the results reveal a striking time dependence (up to 1 hour) evolution of the magnetic properties at a constant Co thickness (up to 40ML). Three magnetic phase/spin orientation transitions have been found, i.e. one paramagnetic to ferromagnetic phase transition with Co thickness and two 90 degree in-plane spin reorientation transitions (due to a change of sign in the in-plane anisotropy field), as described below. The phase transition occurs sharply at a film thickness of 4.51.1 monolayers, which is much higher than the value of the critical thickness for ferromagnetism for the Co/Cu(100) system. Once a ferromagnetic signal is detected, angle dependent studies show that the film has a uniaxial anisotropy with easy axis parallel to the [100] direction. Surprisingly, with no further Co deposition it is found that the easy axis switches from the [100] to the [-110] direction within a repeatable duration of about one hour. This behaviour is repeatable and is seen after many growth runs for a wide range of Co thicknesses. After the first spin reorientation transition is complete the easy axis is stable along the [-110] direction. The second spin reorientation transition shows strikingly different behaviour. Whereas the first spin reorientation transition is a time dependent dynamic process, the second anisotropy transition can be initiated by sub-monolayer Co or Cu coverages or by simply heating the sample above 100 degrees centigrade. We discuss the origin of these new phenomena in terms of structural and chemical changes at the surface of the Co film.

11:30 AM M3.9 
GIANT IN-PLANE MAGNETIC ANISOTROPY IN NiFe/FeMn(110) FILMS WITH MESOSCOPIC-SCALE INTERFACE ROUGHNESS, Reiner M. Jungblut, Philips Research Laboratories, Eindhoven, NETHERLANDS; Juergen T. Kohlhepp, Eindhoven Univ of Technology, Dept of Physics, Eindhoven, NETHERLANDS; Reinder R. Coehoorn, Philips Research Laboratories, Dept of Magnetism, Eindhoven, NETHERLANDS; Martijn M.J. Dekker, Philips Research Laboratories, Eindhoven, NETHERLANDS.

In Cu(110)/NiFe/FeMn samples, originally made with the intention to create exchange biasing of the permalloy layer due to the exchange interaction with the antiferromagnetic FeMn layer, a giant interface related in-plane twofold anisotropy was discovered. In-plane anisotropy fields up to 300 kA/m were found. We provide evidence that this effect is related to the presence of mesoscopic-scale interface roughness. 
All samples were grown by Molecular-Beam-Epitaxy. Cu(110)/NiFe samples were observed to display a moderate uniaxial in-plane volume-type anisotropy with a [001] easy direction of the magnetization. The surface of the NiFe films consists of islands with 2--4 monolayers height, a width of typically 4--6 nm along the [001]-direction and a length along the [10]-direction of 25--60 nm. This surface morphology was observed for thicknesses between 2--9 nm. Covering the NiFe films (thickness < 3.5 nm) with antiferromagnetic FeMn films (thickness > 3 nm) a strong uniaxial in-plane interface-type ( mJ/m) anisotropy preferring an [10] easy direction is introduced. For thinner FeMn layers of for sample-temperatures beyond the (thickness-dependent) blocking-temperature the uniaxial anisotropy of the NiFe stays unaffected. From this observation we conclude that the pinning of the ferromagnetic spins by exchange coupling to the antiferromagnetic spins is responsible for the observed large interface-type anisotropy. We introduce a new mechanism for the uniaxial in-plane anisotropy. Antiferromagnetic FeMn(110) films are expected to have uncompensated surface moments along the [001]-directions assuming the ideal <111> bulk spin-structure. Mesas and valleys with a height of an odd number of monolayers have an opposite sign of the orientation of the surface moments. If the dimensions of the mesas and valleys are smaller than a critical exchange length, the moments of the neighboring ferromagnetic layers are frustrated and point along a [110] in-plane direction. This mechanism is similar to Slonczewski's model [PRL 67, 3712 (1991)] for biquadratic interlayer exchange coupling caused by spatial fluctuations of the coupling strength. This model has been extended to the strong coupling regime and qualitative agreement with our experimental data has been obtained.

11:45 AM M3.10 
MAGNETIC ANISOTROPY AND GROWTH TEXTURE IN CoPd FILMS, Shien-Uang Jen, J. Y. Huang, K. B. Huang, Academia Sinica, Inst of Physics, Taipei, TAIWAN.

Pd-rich Co-Pd alloy films are known to exhibit perpendicular anisotropy. In this study, CoPd films were made by the vapor deposition method. The thickness of the films ranged from 80 to 2000 . Following measurements were carried out on each film sample: the impurities and alloy compositions were checked by Auger depth profile analysis (AES), the surface topology was mapped out by an atomic force microscope (AFM), the structural analysis was done by using the x-ray diffractometer (XRD), and the magnetic properties, such as magnetic anisotropy and saturation magnetization, were measured by a SQUID magnetometer and/or a MOKE (using the longitudinal or polar effect) apparatus. Magnetic quantities, such as the out-of-plane (either hard or easy axis) squareness ratio M/M saturation magnetization M, and anisotropy energy K, are closely related to the structural properties, such as the degree of (111) texture, grain size D, mode of growth (nucleation texture or growth texture), and impurities in the films. The emphasis is placed on the thickness dependence of these quantities, and the interrelationships among them.

Chair: David Kubinski
Tuesday Afternoon, April 1, 1997
Golden Gate C2

1:30 PM *M4.1 
LOCAL MAGNETIC PROPERTIES AND STRUCTURE OF METALLIC MULTILAYERS INVESTIGATED BY NMR, Pierre Panissod, Christian Meny, Inst de Physique et Chemie des Materiaux, Strasbourg, FRANCE; Ewa Jedryka, Marek Wojcik, Inst of Physics, Warsaw, POLAND.

As a probe of the short range chemical and topological order, NMR is useful for structural investigations of magnetic multilayers (interface structure and composition, bulk phases and defects, strains) In such inhomogeneous systems, the magnetic character of the raw data makes it also possible to study local magnetic properties and to correlate them with local structural features. The hyperfine field is the first output of an NMR experiment in ferromagnets which, despite some uncertainties, gives a direct insight on the local magnetization. Zero field NMR is also sensitive to any restoring torque exerted on the electronic moment (magnetic anisotropy, exchange and dipolar couplings). Combined with the fact that the various parts of a sample (atomic planes involved in the interfaces, bulk phases and bulk defects) can be discriminated in the spectra, the magnetic information contained in the data allows to probe quantitatively or, at least, to place on a relative scale, the different magnetic responses of the different parts of a sample. We present here the relevant results which we have obtained in the course of our investigations of Co-based multilayers and spin valves (interface magnetic profiles, magnetic anisotropy inhomogeneities, local magnetic coupling).

2:00 PM M4.2 
UNUSUAL MAGNETIC BEHAVIOUR OF bct Fe THIN FILMS EVIDENCED BY Fe M/"OSSBAUER SPECTROMETRY, O. M. Lemine, Michel Piecuch, Univ Nancy I, CNRS/LMPSM, Vandoeuvre, FRANCE; Stephane Andrieu, Univ Nancy I, Lab de Metallurgie Physique, Vandoeuvre, FRANCE; Ph. Bauer, Univ Henri Poincare, LMPSM, Vandoeuvre, FRANCE.

In this study, we show that unusual magnetic behaviors occur in thin bct Fe films epitaxially grown on Ir(001) by Molecular Beam Epitaxy. Four different magnetic sites are actually observed in these layers by Fe Conversion Electron Mossbauer Spectrometry. The correct assignment of these magnetic sites was achieved by depositing Fe atomic planes at the appropriate place in the iron layers. 
Up to 4 ml, iron is in a strained FCC structure, and two magnetic sites are observed. At 4.2 K, the central layers of each iron slab exhibit a strong in plane hyperfine field of 34 Tesla above a critical Ir thickness of 5 , whereas iron in contact with Ir keeps a low hyperfine field of 5 Tesla with perpendicular anisotropy, irrespective of Ir thickness. Large emission lines point to strong magnetic relaxation effects. For thicknesses larger than 4 atomic planes, the transition from the pseudomorphic regime (strained FCC) to the relaxed regime (strained FCC) results in new iron magnetic components. A sharp disymmetry is observed along the growth direction. For the 6-8mlFe/15Ir(100) series, the evolution from the strained FCC structure at the bottom Ir/Fe interface towards the strained BCC structure at the top Fe/Ir interface is demonstrated. In both 6-8 ml series, the thermal behavior of the large hyperfine field (Bhf = 34.6 Tesla) occurring in the stained BCC structure departs from the usual T spin law: with 6 Fe atomic planes, a quasi linear Bhf(T) dependence is found, evidencing 2D ferromagnetism. The magnet relaxation phenomena found in the strained FCC structure, together with the spin wave softening observed in the strained BCC structure, suggest small magnetic correlations along the growth direction.

2:15 PM M4.3 
STRUCTURAL AND MAGNETOTRANSPORT PROPERTIES OF NiMnSb/Cu AND NiMnSb/Ag MULTILAYERS, Juan A. Caballero, Jeffrey R. Childress, Univ of Florida, Dept of MS&E, Gainesville, FL; Frederic Petroff, Thomson-CNRS, UMR, Orsay, FRANCE; Anthony Cabbibo, Augusto A. Morrone, Univ of Florida, Dept of MS&E, Gainesville, FL.

High-quality polycrystalline thin-films of the predicted half-metallic (100 spin-polarized) ferromagnetic Heusler alloy NiMnSb have been recently obtained by using a low-rate sputtering technique at substrate temperatures as low as 200C without post-deposition annealing. Here we report on the integration of these films in NiMnSb/Cu and NiMnSb/Ag multilayer structures, and on their magnetic, microstructural and magnetotransport properties. The NiMnSb layer thickness was fixed at 30 and those of Cu and Ag were varied from 20 to 60 . The multilayers were characterized using x-ray diffraction (XRD), TEM, AFM, SQUID and magnetotransport measurements (in the current-in-plane geometry). Moderate substrate temperatures (250C) are shown to be sufficient to produce stoichiometric Clb-type NiMnSb. XRD data confirm that the NiMnSb ultrathin layers retain their crystalline quality and texture (<111> and <220> directions). SQUID measurements shows that the bulk saturation magnetization (760 emu/cm) is maintained and that it is strongly sensitive to interlayer diffusion and roughness. AFM studies show that the surface roughness, which can be as low as 5 , depends on the nonmagnetic material, layer thickness and substrate temperature. The relationship of the measured magnetoresistance to the NiMnSb crystalline quality, magnetic properties and interfacial roughness will be presented.

2:30 PM M4.4 
ANISOTROPY INDUCED BY EPITAXIAL STRAINS IN THIN FILMS OF LAVES PHASE COMPOUNDS, V. Oderno, C. Dufour, Univ Henri Poincare, LMPSM, Vandoeuvre, FRANCE; Dumesnil Karine, Univ Henri Poincare, Metallurgie Physique & Sciences des Materiaux, Vandoeuvre, FRANCE; Ph. Bauer, Ph. Mangin, G. Marchal, Univ Henri Poincare, LMPSM, Vandoeuvre, FRANCE.

(110) Rare Earth-Fe (DyFe, TbFe and ErFe) thin films have been for the first time epitaxially grown on (110) Nb/(110) sapphire by molecular beam epitaxy. The epitaxy is initiated by the deposition of a thin layer of iron on niobium. RHEED patterns confirm the high quality of epitaxy. X-Ray diffraction measurements evidence that the films are strained compared to the bulk alloy (the e shear is around 0.6). Mossbauer spectroscopy spectra performed at various temperatures for samples which have been elaborated from an Fe enriched source permit the determination of the easy magnetization axis and of its thermal evolution. At 4.2 K, the easy magnetization axis in the films is the same as in bulk compounds (i.e., <100> for DyFe and <111> for ErFe and TbFe). At 300 K, the magnetic moments remain in the same direction as at 4.2 K in the TbFe films, as in bulk compounds. On the contrary, they rotate towards <110> directions in the DyFe and ErFe thin films; they are close to the in-plane [110] direction in DyFe films, and close to the perpendicular-to-the-plane [110] direction in ErFe films. The room temperature easy magnetization directions have been confirmed by Kerr rotation measurements. These modifications of the magnetic anisotropy can be qualitatively understood if we consider the influence of the strains induced by epitaxy on the various energy terms which govern the easy magnetization direction. Moreover, different behaviors are observed among the Rare Earth-Fe alloys because this effect is also related to the sign and the relative importance of the anisotropy and magnetoelastic constants of the compound.

2:45 PM M4.5 
PERPENDICULAR MAGNETIC ANISOTROPY IN NANOSTRUCTURED PSEUDOMORPHIC Fe(110) FILMS ON W(110), Hans-Joachim Elmers, Nils Weber, Klaus Wagner, Jens Hauschild, Ulrich Gradmann, Technische Univ Clausthal, Dept of Physics, Clausthal, GERMANY.

Pseudomorphic Fe(110) films, deposited on W(110) at room temperature, with a mean thickness of 1.5 monolayers, are composed of double layer islands with a diameter of the order of 10 nm on a monolayer sea. The double layer islands are ferromagnetic (superparamagnetic) below 500 K, the monolayer sea becomes ferromagnetic below 222 K. Magnetic anisotropy is uniaxial in both components. However, the easy axis in the double layer islands is perpendicular to the film plane and in the monolayer sea within the film plane (along . The perpendicular anisotropy in the double layer islands apparently is caused by the pseudomorphic strain. The lateral scale for the anisotropy switching roughly coincides with the exchange lengths in both media. The exchange interaction between the areas of different anisotropy directions induces a rich variety of new micromagnetic phenomena which remain to be studied in detail.

3:00 PM M4.6 
A METHOD OF FABRICATION OF CO-PD ALLOY FILMS WITH PERPENDICULAR MAGNETIC ANISOTROPY, Sergey A. Gusev, Yuri N. Nozdrin, Inst for Physics of Microstructures, Novgorod, RUSSIA; Dmitry D. Rozenstein, Inst for Physics of Microstructures, Dept of Surface Physics and Multilaye, Novgorod, RUSSIA; Alexander E. Tselev, Inst for Physics of Microstructures, Dept of Surface Physics , Novgorod, RUSSIA.

Co/Pd and Co/Pt multilayered films, which are considered as strong candidates for next-generation magneto-optical recording media, have some disadvantages. Some of them are necessity to produce the structures with sharp interfaces and thermal instability. As alternatives Co-Pd and Co-Pt alloy films attract attention. In this work we propose a method of fabrication of Co-Pd alloy films with perpendicular magnetic anisotropy. To obtain perpendicular anisotropy we annealed Co-Pd alloy films deposited onto substrates with coefficient of linear expansion (CLE) much smaller than that of the film. After rapid enough cooling down to room temperature the films are under tensile stress. Since Co-Pd alloy has large negative magnetostriction in wide range of Co concentrations, after annealing and cooling the films with appropriate Co concentrations have easy axis perpendicular to the film plane. For sample fabrication pulsed laser deposition method was used. In the films coercivity 790 Oe, Kerr rotation 0.22 at 630 nm and perfectly square loops are attained, which indicates the possibility of their use as magneto-optical recording media. To make sure that differences in CLE of films and substrates is the deciding factor in the change of anisotropy as a result of annealing, experiments with films on substrates with different CLE were carried out.

3:45 PM M4.7 
MAGNETISM AND STRUCTURE OF Co/Cu(001) FILMS, Wolfgang Weber, Andreas Bischof, IBM Zurich, Inst fur Festkorperphyisk, Zurich, SWITZERLAND; Rolf Allenspach, IBM Zurich, Inst fur Festkorperphysik, Ruschlikon, SWITZERLAND; Christian H. Back, Stanford Univ, Stanford Linear Accelerator Ctr, Stanford, CA.

The magnetic properties of Co films grown on stepped Cu(001) crystals have been investigated by the magneto-optical Kerr effect during film growth. In the pseudomorphic growth regime up to a critical thickness of 16 monolayers, oscillations of the uniaxial magnetic anisotropy with a period of one monolayer have been found. These oscillations can be attributed to the periodic variations of the film morphology alternating between filled and incompletely filled layers. Oscillations of the cubic magnetic anisotropy, on the other hand, are not found for symmetry reasons. Above the critical thickness strong changes of both the uniaxial magnetic anisotropy and the coercive field are observed. In particular, the easy magnetization axis switches by 90 from the [10] (parallel to the step edges) to the [110] direction (perpendicular to the step edges). These strong anisotropy changes are correlated with the onset of relaxation of the misfit-induced strain in the Co lattice upon growth.

4:00 PM M4.8 
MAGNETIZATION REVERSAL BY ROTATION IN MAGNETIC THIN FILMS DEPOSITED ON STEP BUNCHED Si(111) SUBSTRATES, Magali Sussiau, Thomson CSF, Orsay, FRANCE; Armando Encinas, Frederic Nguyen Van Dau, Alain Schuhl, Annie Vaures, Thomson-CNRS, CSF-LCR, Orsay, FRANCE; Pierre Galtier, Thomson CSF, Lab Central de Recherches, Orsay, FRANCE.

Magnetic and magnetoresistive properties of magnetic thin films deposited on periodically structured substrates have been studied. By using a thermodynamical heat treatment, we activate the stop bunching phenomenon on vicinal Si(111) substrates misoriented towards [11-2], which results in a laterally modulated surface with a period around 0.l m. 
Epitaxial magnetic thin films deposited on these nonplanar substrates exhibit an in-plane uniaxial anisotropy and a magnetization reversal by rotation observed at room temperature on millimeter scale samples [1]. 
In this contribution, we demonstrate that these effects can also be observed in textured structures grown either by molecular beam epitaxy or by sputtering. In particular, magnetotransport measurements show that by varying parameters such as the nature of the magnetic metal, the layer thickness or the strength of the induced anisotropy, we can modulate the interplay between the anisotropy and the exchange stiffness and, consequently, observe gradual magnetization reversal behaviors between a Stoner-Wolhfarth uniform mode and a nonuniform mode.

4:15 PM M4.9 
STEP-INDUCED MAGNETIC DEFECTS IN Fe/Cr SYSTEMS, Michel Freyss, Univ de Strasbourg, Groupe d'Etude des Materiaux Metalliques, Strasbourg, FRANCE; Daniel Stoeffler, IPCMS, GEMM, Strasbourg, FRANCE; Hugues Dreysse, Univ de Strasbourg, Groupe d'Etude des Materiaux Metalliques, Strasbourg, FRANCE.

The study of complex magnetic systems with structural defects is essential since many physical properties are governed by the presence of such defects. In particular the exchange coupling between ferromagnetic layers separated by a spacer is highly sensitive to the geometry of the interface. Various model proposed to describe the exchange coupling show that the biquadratic term, responsible for non collinear couplings, arises from the presence of terraces at the interface. 
In order to study the influence of such defects on the magnetic structure, we present results of calculations of the magnetic order of Fe/Cr systems with an interfacial monoatomic step. The magnetic moments are computed by means of a -band tight-binding model and the recursive method. Two situations are considered: A Fe/Cr(001) surface with collinear magnetic moments, and a Fe/Cr(001) superlattice with non collinear magnetic moments. The assumption of collinear moments induces in Cr a very extended magnetic defect with almost vanishing moments. On the other hand, when the moments are free to orientate in any direction, the magnetic defect is much more localized around the step and a rearrangement of the moments occurs in Cr leading to a 90 coupling between the Fe layers.

4:30 PM M4.10 
MAGNETIC ANISOTROPY IN Fe/Cu OVER-AND INTERLAYERSLAYERS., Balazs Ujfalussy, Research Inst for Solid State Physics, Ctr of Computational Matls Sci, Wien, AUSTRIA; Laszlo Szunyogh, Tech Univ of Budapest, Dept of Theoretical Physics, Budapest, HUNGARY; Peter Weinberger, Research Inst for Solid State Physics, Ctr of Computational Matls Sci, Wien, AUSTRIA.

The perpendicular magnetic anisotropy in Fe/Cu thin film over- and interlayers is investigated in terms of a fully relativistic spin-polarized band theory. The crucial role of the volume (lattice spacing) on the formation of the ground state is addressed and a layer-resolved analysis of the magnetic anisotropy energies for the Fe/Cu(100) and the Cu/Fe/Cu(100) system is presented both for the ferromagnetic and the anti-ferromagnetic cases. Based on this analysis, the effects of alloying at the interface is demonstrated. By emphasizing the effects of interface hybridization an interpretation of the so called ``anomalous magnetic anisotropy'' is given for the case of the Fe/Cu(100) system.

Chair: Toshikazu Katayama
Wednesday Morning, April 2, 1997
Golden Gate C2

8:30 AM *M5.1 
MAGNETIC AND TRANSPORT PROPERTIES OF MICRON SIZE MAGNETIC PARTICULATE ARRAYS FABRICATED ON A 2D SUPERCONDUCTING Nb FILMS AND SUB-MICRON MAGNETIC WIRES, Yoshichika Otani, Kazuaki Fukamichi, Tohoku Univ, Dept of Matls Science, Sendai, JAPAN; Osamu Kitakami, Yutaka Shimada, Tohoku Univ, Research Inst for Scientific Measurements, Sendai, JAPAN; Bernard Pannetier, CRTBT-CNRS, Grenoble, FRANCE; J. P. Nozieres, CNRS, Laboratoire Louis Neel, Grenoble, FRANCE.

Micron size particulate arrays of amorphous R-Co (R = Sm and Gd) alloys and Fe-Ni alloys with thickness around 200 nm were prepared on top of a 20 nm thick niobium film by means of e-beam lithography and sputter deposition. The arrays consist of 2 micron square particles with spatial periodicity ranging from 4 to 8 micron along the edge of the square. The demagnetization curves appeared strongly different from those of bulk films with no pattern. The obtained curves were compared to those predicted by computer simulation. Calculations show that the evolution of dipolar fields leads to successive reversal of the particles column by column. When these arrays were illuminated by a laser beam, a set of Bragg reflections was observed. The obtained diffraction hysteresis loops were found to yield novel information about the domain structure on a submicron scale. Interesting oscillations due to the fluxoid quantization were observed in the transition lines of underlying Nb films. The superconducting state of the Nb is modulated by stray fields by the square magnetic particles. 
Submicron size ferromagnetic Fe wires were also prepared in the same manner in order to study pinning and depinning processes of 180 degree domain walls inside the wire. Low temperature transport and electron holography measurements demonstrated a clear change in resistivity associated with depinning of a pinned single 180 degree domain wall.

9:00 AM M5.2 
MAGNETIC PROPERTIES AND MUTUAL ANISOTROPIC INTERACTION OF MICRON PERMALLOY DOTS FORMING A SQUARE LATTICE, Sergei O. Demokritov, Univ Kaiserslautern, Dept of Physics, Kaiserslautern, GERMANY; Ch. Mathieu, S. Reidling, M. Bauer, O. Buttner, C. Hartmann, B. Hillebrands, Univ Kaiserslautern, Fachbereich Physik, Kaiserslautern, GERMANY; B. Bartenlain, C. Chappert, Univ de Paris-Sud, Orsay, FRANCE; D. Decanini, F. Rousseaux, E. Cambril, CNRS, Bagneux, FRANCE; U. Hartmann, Univ Saarbrucken, Saarbrucken, GERMANY.

We have investigated the static and spin wave properties of regular square lattices of magnetic dots patterned in permalloy films. The frequencies of the spin wave excitations were measured by means of Brillouin light scattering. 
Permalloy films were prepared by UHV-evaporation onto Si/SiO wafers. The thicknesses of the films were chosen between 50 and 100 nm. The high quality of the films are confirmed by a low coercive field (H = 2 Oe) and a low induced uniaxial anisotropy (H = 10 Oe). The samples were structured using synchrotron radiation based x-ray lithography and ion beam etching. Square arrays of round dots with sizes as large as 1 mm have been obtained with lateral dot diameter/array periods of 1/1.1, 2/2.2, 2/4 m. 
The spin wave frequencies can be well described if one takes into account the demagnetization factor of each single dot. For the samples with small dot separation (1/1.1 m) a four-fold in-plane magnetic anisotropy with the easy axis directed along the pattern diagonal was observed, which is absent for the samples with larger dot separation. The magnetization loops measurements confirmed the existence of the anisotropy. We attribute this finding to a dipole-dipole interaction between nonsaturated parts of the dots. This is supported by the fact that the effective anisotropy strength decreases with increasing the applied field. The pictures obtained by means of magnetic force microscopy show the correlation between the domain structures of the neighboring dots for lattices with small dot separations. The observed domain walls have preferred orientations along the lattice <10> axes, reflecting the dipole-dipole interaction between dots.

9:15 AM M5.3 
MAGNETIZATION PROCESSES IN SUB-MICRON ARRAYS OF EPITAXIAL Fe(110) PARTICLES, J. P. Nozieres, O. Fruchart, Dominique Givord, J. C. Toussaint, B. Kevorkian, CNRS, Laboratoire Louis Neel, Grenoble, FRANCE; Francoise Rousseaux, D. Decanini, CNRS, Bagneaux, FRANCE.

Magnetization processes in particle arrays is of considerable interest both for fundamental research and applications to storage media. Most of the attention has been devoted so far to particles with perpendicular magnetization in which complex domain structures have been observed. On the other hand, monodomain states are expected to be stabilized in ultrathin particles with in-plane magnetization. We report here on particle arrays obtained from epitaxial ultrathin W(110)/Fe(110) 60 /W(110) films deposited by pulsed laser deposition. The main effects of patterning the film are an increase of the coercive field, a broadening of the transition and the emergence of an irreversible contribution to the hard axes loops. Detailed analysis reveals that the magnetization in the continuous film reverses as soon as one phase (i.e., magnetization direction) becomes energetically favored, whereas in a particle the initial phase has already become highly metastable at the reversal. This difference can be explained by considering that in a plain film the reversal is governed by isolated defects, whereas the coercive field in a patterned film reflects the mean nucleation field in defect-free particles. The hysteresis cycle of an individual particle and the distribution function of the coercive field in the array have been derived from magnetization and dc susceptibility measurements. The interparticle dipole-dipole interactions were neglected as confirmed by numerical calculations. The reconstructed magnetization loop of a single particle, which is in good agreement with numerical simulations, can be described as a three states process: reversible departure from saturation into a flower state, irreversible nucleation-propagation reversal at the coercive field and final reversible alignment. A broad ( 80 OE) coercive field distribution function is derived, which origin remains unclear. Dipolar interactions (20 Oe max) and thickness fluctuations (15 Oe for a one ML step) may account partially as well as eventual twinned domains and/or etching defects.

9:30 AM M5.4 
OXIDIZED NANOWIRES: MAGNETIC AND TRANSPORT PROPERTIES, Scott Gilbert, Bernard Doudin, Jean-Philippe Ansermet, Ecole Polytechnique Federale, Dept de Physique, Lausanne, SWITZERLAND.

Electrodeposited ferromagnetic nanowires can be oxidized, either in -situ by anodization or ex-situ by air exposure. The effect of exchange coupling was measured as a function of diameter (35-250 nm), temperature and magnetic history. Ferromagnetic Nuclear Resonance (FNR) was used to characterize the local crystalline environment. Spin transport can be studied on samples formed by electrodeposition metal over the oxide.

10:15 AM *M5.5 
MAGNETO-OPTICAL EFFECTS OF ULTRATHIN FERRO- AND ANTIFERRO-MAGNETIC FILMS, Yoshishige Suzuki, Joint Res Ctr for Atom Technology, Atom Technology Group, Tsukuba, JAPAN; Toshikazu Katayama, Electrotechnical Laboratory, Materials Research Div, Tsukuba, JAPAN; Patrick Bruno, Univ de Paris-Sud, Inst d'Electronique Fundamentale, Orsay, FRANCE; W. Geerts, Electrotechnical Laboratory, Materials Research Div, Tsukuba, JAPAN; K. Takanashi, R. Schreiber, P. Grunberg, Forschungszentrum Julich GmbH, IFF, Juelich, GERMANY.

The magnetoptical effects of ultrathin ferro- and antiferromagnetic films were measured carefully employing wedge shaped samples. Oscillation of the polar Kerr rotation and/or ellipticity as a function of the film thickness was observed for both the Fe ultrathin films and the Cr ultrathin films. The oscillation for the Fe ultrathin films revealed a formation of quantum well states in the bcc-Fe (100) layer sandwiched by Au (100). On the other hand, bcc-Cr (100) ultrathin layer grown on 3 ML of Fe(100) layer shows about 2 ML period oscillation for the photon energy in between 2.25 and 3.75 eV, indicating an existence of a long range antiferromagnetic order. The Cr film also showed energy dependent oscillation peaks for the very thin region (d <10 ML). The longitudinal Kerr effects were measured also for above samples; however, no oscillation had been observed within the detection limit (2 x 10 deg.). The mechanism of the MO oscillations will be discussed from the viewpoint of quantum size effects and the antiferromagnetic order in Cr films.

10:45 AM M5.6 
UNIAXIAL MAGNETIC ANISOTROPHY IN THIN EPITAXIAL Fe FILMS CUT INTO NANOSTRIPES BY DISLOCATION SLIPPING, Laurence Ressier, Lab de Physique de la Matiere Condensee de Toulouse, INSA, Dept of Physics, Toulouse, FRANCE; H. Jaffres, INSA, Dept de Physique, Toulouse, FRANCE; Alain Schuhl, Frederic Nguyen Van Dau, Thomson-CNRS, CSF-LCR, Orsay, FRANCE; M. Goiran, A. R. Fert, J. P. Peyrade, INSA, Dept de Physique, Toulouse, FRANCE.

A plastic deformation of a thin epitaxial Fe film deposited on a (001) MgO substrate is used to activate dislocation slipping in the MgO planes. The lattice shifts induced by the dislocation crossing throughout the whole sample, cut the thin Fe film into adjacent nanostripes aligned along the [110] direction of Fe. Atomic force microscopy observations of the surface of the deformed thin Fe film reveal the creation of adjacent stripes characterized by a mean width ranging from 1.4 m for a 4 deformation to 50 nm for a 8 deformation. Both components of the magnetization, parallel and perpendicular to the in-plane applied magnetic field, are studied by magneto-optical measurements for different thicknesses of the starting Fe film and different deformation rates. Surprisingly, a strong uniaxial magnetic anisotropy is revealed after deformation, with an easy axis along the [110] direction of Fe, that is to say, perpendicular to the created Fe nanostripes. This uniaxial magnetic anisotropy increases with the deformation rate while it is not significantly modified by the thickness of the starting thin Fe film. For magnetic fields parallel to the nanostripes, experimental results exhibit a coherent rotation of the magnetization, confirming that the direction of the nanostripes is a hard axis. The complete rotation is obtained for a magnetic field from 1450 Oe in the case of a 4 deformation up to 2000 Oe in the case of a 8 deformation, corresponding to a uniaxial anisotropy constant Ku + 8 and Ku = 11, respectively. This original magnetic behavior is explained by the partial relaxation of the 4 misfit existing between the thin Fe film and the MgO substrate before deformation, induced by dislocation slipping.

11:00 AM M5.7 
MAGNETIC PROPERTIES OF Ni-MgO NANO-COMPOSITE FILMS OBTAINED BY PARTIAL REDUCTION REACTIONS, Jeffrey A. Smith, Pimpa Limthongkul, Linnea Hartsuyker, Cornell Univ, Dept of MS&E, Ithaca, NY; Ersan Ustundag, Los Alamos National Laboratory, Matls Science & Tech Div, Los Alamos, NM; Michael O. Thompson, Stephen L. Sass, Cornell Univ, Dept of MS&E, Ithaca, NY.

Two new processing techniques were developed for the synthesis of metal-ceramic films containing nanosize single crystal Ni particles embedded in a MgO matrix. These methods allow the control of magnetic anisotropy by use of residual stresses and crystallographic orientation of the Ni particles. In the first, Ni was electron beam evaporated onto MgO single crystal substrates. The film was heated to oxidize the metal and diffuse Ni ions into the substrate, and then reduced to form topotactically oriented Ni particles in a MgO film In the second. sol-gel films were spun on different substrates with varying coefficients of thermal expansion, oxidized by heating in air and then reduced to form a metal-ceramic two phase mixture. Magneto-optical Kerr effect (MOKE) measurements of the films show that the Ni particles have sufficient vertical magnetic anisotropy to support perpendicular magnetization. Residual stresses measured by x-ray diffraction techniques, crystallographic orientation, and film thickness are related to magnetic properties measured by MOKE. Novel magnetic behavior has been measured for films containing Ni particles which are much smaller than the wavelength of light. For example, these composite films can have Kerr saturation rotations larger than the pure Ni films. The optimization of these films for magnetic information on storage applications will be discussed.

11:15 AM M5.8 
GIANT MAGNETORESISTANCE AND MAGNETIZATION OF ErAs SELF-ORGANIZED ISLANDS IN GaAs, Daniel R. Schmidt, Univ of California-S Barbara, Dept of Physics & Quantum Inst, Santa Barbara, CA; James P. Ibbetson, Univ of California-S Barbara, Dept of Materials, Santa Barbara, CA; Daniel E. Brehmer, San-Phay Chau, S. J. Allen, Univ of California-S Barbara, Dept of Physics & Quantum Inst, Santa Barbara, CA.

The small lattice mismatch between GaAs and rare-earth arsinides makes it possible to incorporate ErAs, a , into GaAs structures. We have used Molecular Beam Epitaxy to form two-dimensional sheets of ErAs islands within GaAs. The height of the islands (along the growth direction) was approximately 3 monolayers. The lateral size of the islands can be controlled by the the growth temperature and ranged from 10 angstroms to hundreds of angstroms, as shown by TEM micrographs. A series of GaAs/ErAs superlattice structures were grown in which the amount of ErAs deposited per period was varied from 0.5-2.0 monolayers. Since the ErAs layers were disconnected (confirmed by TEM) the lateral transport in these composite films is by hopping conduction. The resistivity dropped several orders of magnitude as the density of islands increased with the amount of ErAs deposited (island lateral size, determined by the growth temperature, was essentially constant at ). Large magnetoresistive effects were observed at low temperatures (<20K). Typical magnetoresistance (MR) behavior showed a small positive MR at low fields (<1T) and a very large negative MR as the field was increased dropping by several orders of magnitude from the zero field values. Tilting the field relative to the layers demonstrated that negative MR was isotropic with respect to field orientation, while the positive MR diminished as the field was brought parallel to the layers. The effect saturated in modest fields (4T). But the field to produce saturation increased with increasing temperature implying that the giant negative resistance is caused by fully polarizing the magnetization of the ErAs islands. The magnetization of the structures is measured with various techniques, including SQUID magnetometry. These measurements are correlated with the magneto-transport results to develop a model to explain the magneto-resistance in terms of magnetization controlled island to island hopping.

11:30 AM M5.9 
APPLICATIONS OF SELECTIVE AREA DEPOSITION: MAGNETIC MICRON SCALE ARRAYS OF DOTS, Dulip A. Welipitiya, Peter A. Dowben, Univ of Nebraska, Dept of Physics, Lincoln, NE; Jiandi Zhang, Oak Ridge National Laboratory, Solid State Div, Oakridge, TN.

We have investigated two important organometallic compounds, nickelocene and feriocene. Both have been demonstrated as useful in direct writing of quantum wires and magnetic dots by chemical vapor deposition [1,2]. This novel method for deposition has a number of unique capabilities not shared by other deposition techniques. The CVD process remains poorly understood. We have proved that direct writing can be carried in several different methods such as by laser irradiation and in a scanning electron microscope. The magnetic properties were studied by gradient force magnetometry as well as magnetic force microscopy. Magnetic measurements and optical kerr microscopy shows that the structures are spatially well defined and the magnetic properties depend on the shape and dimensions of the features.

11:45 AM M5.10 
MICRO-SQUID MEASUREMENTS OF NUCLEATION AND 180 DOMAIN WALL PROPAGATION THROUGH SINGLE DEFECT IN TRILAYERS, Stephane Mangin, CNRS, Lab Louis Neel, Grenoble, FRANCE; G. Marchal, Univ Henri Poincare, LMPSM, Vandoeuvre, FRANCE; W. Wernsdorfer, B. Barbara, D. Mailly, CNRS, Laboratoire Louis Neel, Grenoble, FRANCE; A. Benoit, CNRS, Ctr de Recherche sur les Tres Basses Temperature, Grenoble, FRANCE.

The domain wall nucleation and its crossing through a single plane defect has been followed at very low scale by micro-SQUID measurements. For this purpose, a 1 m size micro-SQUID was placed on the top of the magnetic layers and the flux entering the loop of the micro-SQUID was measured as a function of the applied magnetic field and temperature. The Barkhausen noise at the 1 m scale was detected and the integrated signal is similar to a hysteresis loop performed on such a small part of the sample. With this technique, we followed the nucleation of domain wall in GdFe amorphous alloys and the crossing of a planar defect constituted of a thin layer of TbFe or DyFe amorphous alloys. The measurements were performed between 6 K and 0.1 K at different magnetic field sweeping rates, which allowed us to study the dynamical behavior of the reversal of magnetization in such samples. The data collected by this technique have shown in which range the crossing of the barrier can still be interpreted at low temperature by thermal activation and in which extend new phenomena as quantum tunneling can occur.

Chair: Alain Schuhl
Wednesday Afternoon, April 2, 1997
Golden Gate C2

1:30 PM *M6.1 
SUBMICRON TO NANOMETER SIZE SINGLE PARTICLE MEASUREMENTS, B. Barbara, W. Wernsdorfer, E. Bonet Orozco, CNRS, Laboratoire Louis Neel, Grenoble, FRANCE; K. Hasselbach, A. Benoit, CNRS, Ctr de Recherche sur les Tres Basses Temperature, Grenoble, FRANCE; D. Mailly, CNRS, Laboratoire Louis Neel, Bagneux, FRANCE; N. Demoncy, H. Pascard, Ecole Polytechnique, SESI-CEA/CNRS, Palaiseua, FRANCE.

Low temperatures magnetization measurements of individual ferromagnetic particles and wires are presented (0.1 2:00 PM M6.2 
VORTEX SYMMETRY AND DOMAIN STABILITY IN MICRON- AND SUB-MICRON-SCALE, PATTERNED CO FILMS, Peter J. Bedrossian, Lawrence Livermore National Laboratory, Dept of Matls Science & Tech, Livermore, CA.

A dramatic variety appears in the magnetization behavior of patterned ellipses fabricated from thin Co films spanning the transition from the multi-domain (m width) to the single-domain (submicron width) regime. Using a combination of Magnetic Force Microscopy (MFM) and Vibrating Sample Magnetometry (VSM), we find that the remanent state of a patterned structure of specific size and shape can be classified according to the topology of vortex pattern in the films’s in-plane magnetization, and that the presence of hysteresis is associated with a transition between magnetization states with distinct topological indices. Micron-scale ellipses magnetized along their hard axes exhibit nearly-complete supression of hysteresis, as their magnetization proceeds via the anti-parallel displacement of two vortices with opposite circulations without a change in topological index. The absence of remanence in in-plane magnetization of structures below 4m width is associated with the formation of distinct, symmetric vortex patterns. Such patterns are stabilized by the presence of the boundary, where demagnetization favors tangential orientation of the magnetization. Measurements were performed on arrays of non-interacting ellipses with aspect ratios of 1:1, 1:2, and 1:4 patterned from Co films up to 70 nm thick. Hysteresis loops were recorded with a VSM, and domain wall patterns were imaged with an MFM with concurrent application of a magnetic field.

2:15 PM M6.3 

We have investigated the magnetization nucleation mechanisms taking place in submicronic cobalt dots by means of 3D Micromagnetic simulations. Particularly, we have centered our attention in the study of the transition from parallel-to-plane to perpendicular-to-plane magnetization occurring as the thickness of the dot is increased. Simulations show a very different nucleation mechanism for both cases leading to distinct magnetization configurations. For a 35 nm dot, the nucleation mechanism consists on the formation of stripe-like domains. Stripe domains evolve with the decrease of the in-plane applied field towards a configuration of ' 'bended stripe'' domains as a result of the need for a lateral flux closure. The same nucleation mechanism has been found for thicker dots and is responsible for the appearance of diagonal domains at high thicknesses. For a 25 nm thick dot a completely different nucleation mechanism has been observed. In this case no stripe domains are present and a fully planar magnetization configuration has been obtained. In this configuration, flux closure takes place with the formation of two distorted vortices at both sides of the dot. These two vortices show an interesting ''fine structure'': they are formed by two vortices, a cross tie wall, and a quasi-Ne\'el wall each. This magnetization configuration seems to be the analog of the one found in Permalloy thin films [1], but constrained by the finite size of the dot and distorted by the presence of the perpendicular-to-plane anisotropy of cobalt.

2:30 PM *M6.4 
THE EFFECT OF ATOMIC STEPS ON MAGNETIZATION REVERSAL IN ULTRATHIN FILMS, R. A. Hyman, Andrew Zangwill, Georgia Inst of Technology, School of Physics, Atlanta, GA; M. D. Stiles, NIST, Electron Physics Group, Gaithersburg, MD.

The presence of monoatomic height steps in ultrathin film magnets is unavoidable. Such steps can arise intentionally by deposition onto a vicinal substrate or unintentionally as a result of the roughness that always accompanies epitaxial growth onto a singular surface. For this reason, the intrinsic anisotropy of the film is augmented by two-fold in plane anisotropy at every step edge due to both reduced site symmetry and the presence of magnetic poles. We discuss the effect of steps on magnetization reversal in magnetic ultrathin (< 10 ML) films using an XY-model with four-fold anisotropy at every atomic site and two-fold anisotropy at every step site. Magnetostatic effects are treated in full. Two cases are considered: a vicinal surface where magnetostatics induces a two-fold anisotropy of its own, and a singular surface with an island geometry similar to that found in real epitaxial growth experiments. For the vicinal surface, we find hysteresis loops with a characteristic shape that should allow one to extract anisotropy parameters directly from SMOKE or magnetization measurements. Our discussion of this problem corrects previous analyses in the literature. For the island geometry, we find that reversal occurs by domain nucleation at step edges, expansion of the domains through morphological constrictions, and coherent rotation within domains. A strong coverage dependence for the coercive field is found that agrees very well with recent experiments on Cu/Co/Cu sandwich structures.

3:00 PM M6.5 
OBSERVATION OF DOMAIN TRANSFORMATIONS IN ANTIFERROMAGNETIC COUPLED SANDWICHES, Nathalie Persat, H.A.M. van den Berg, Siemens AG, ZT MF 1, Erlangen, GERMANY; F. Manders, Univ of Nijmegen, Res Inst for Mat, Nijmegen, NETHERLANDS; V. da Costa, IPCMS, dept of Gemme, Strasbourg, FRANCE; Aziz Dinia, IPCMS, GEMM, Strasbourg, FRANCE.

Magnetization and GMR curves of polycrystalline isotropic Co/Cu/Co sandwiches in the first antiferromagnetic coupling maximum exhibit, from the fundamental point of view, two different types of branches. It will be demonstrated by using the ferrofluid technique that the branch type which occurs when coming from saturation is characterized by a high density of domains. To make this submicron structure visible, SEM and AFM imaging were used Upon reducing the field, irreversible conversions take place at which the density of domains is diminished. Below a certain field H, the second branch, characterized by a low density of domains and reversible changes, is reached. The reversible field windows of both branches overlap for fields higher than . The second branch type has a higher resistivity and smaller mean magnetization than its pendant at the same field. It will be shown that the reduction in the magnetization originates in the disappearance of the domain walls which have their spin directions in the cores parallel to the initial saturation field. The polarity of the domain walls is confirmed by observations by the ferrofluid technique.

3:45 PM *M6.6 
MAGNETIC SWITCHING IN ULTRA SHORT MAGNETIC FIELD PULSES, Christian H. Back, Stanford Univ, Stanford Linear Accelerator Ctr, Stanford, CA; Dieter Weller, IBM Almaden Research Center, San Jose, CA; Jurgen Heidmann, IBM, Storage Systems Div, San Jose, CA; Daniele Mauri, IBM Almaden Research Center, Storage Systems Div, San Jose, CA; Davide Guarisco, Stanford Univ, Dept of MS&E, Stanford, CA; Edward L. Garwin, Stanford Univ, Stanford Linear Accelerator Center, Stanford, CA; Hans-Christoph Siegmann, ETH Zurich, Lab fur Festkrperphysik, Zurich, SWITZERLAND.

We have used Kerr microscopy to image magnetization patterns written into a series of perpendicularly magnetized Co/Pt multilayer films in picosecond field pulses. Ultra short magnetic field pulses with amplitudes of several tesla are obtained in the final focus test beam of the Stanford Linear Accelerator Center, where both, the length of the field pulse and the shape of the field distribution can be varied. The initial magnetization reversal is governed by spin precession which can be described by the Ginzburg-Landau-Lifshitz equation. At a fixed magnetic field pulse length this equation predicts a minimal switching field, required to reverse the magnetization direction. By varying the effective anisotropy field of our perpendicular samples we can change the switching conditions at different pulse lengths. We compare our experimental observations with the results obtained from the GLL equation.

4:15 PM M6.7 
MAGNETIC DOMAIN STRUCTURE AND SPIN REORIENTATION TRANSITION IN THE SYSTEM Co/Au/W(110), Thomas Duden, Ernst G. Bauer, Arizona State Univ, Dept of Physics & Astronomy, Tempe, AZ.

Epitaxial Co layers grown on Au(111) show strong perpendicular magnetic anisotropy resulting in out-of-plane magnetization in thin films. With increasing Co thickness, the increasing contribution of the shape anisotropy leads to a reorientation transition of the magnetization which has already been the object of many other studies. In contrast to Co on Au(111), Co on W(110) has from the very beginning a strong uniaxial in-plane component of the magnetization. Using as a substrate Au(111) layers grown on W(110) allows to study the magnetic domain structure and to determine the Co film thickness at which the transition occurs as a function of the Au thickness. The results will be compared with existing models of domain structure and of the reorientation transition.

4:30 PM M6.8 
STUDY OF THE PROPAGATION OF A 180 DOMAIN WALL IN GdFe/TbFe/GdFe TRILAYERS BY RELAXATION MEASUREMENTS, Stephane Mangin, CNRS, Lab Louis Neel, Grenoble, FRANCE; G. Marchal, Univ Henri Poincare, LMPSM, Vandoeuvre, FRANCE; W. Wernsdorfer, B. Barbara, CNRS, Laboratoire Louis Neel, Grenoble, FRANCE; A. Sulpice, CNRS, Ctr de Recherche sur les Tres Basses Temperature, Grenoble, FRANCE.

To study the passage of a 180 domain wall (DW) across an energy barrier, we prepared samples made of a thin layer of TbFe amorphous alloy (a-TbFe) between two layers of amorphous GdFe (a-GdFe). The two a-GdFe layers had different thicknesses (1000 and 500 ) to provide the nucleation of a 180 DW only in the thicker one. Moreover, we are able to control the in-plane anisotropy of the GdFe layers to provide a uniaxial anisotropy necessary to create 180 DW. As a-GdFe exhibits a very low anisotropy constant compared to TbFe, which is a hard magnetic material, the TbFe layer acts as a potential energy barrier for the domain wall. The thicknesses of the a-TbFe layers range between 2 and 20 . We studied the crossing of the barrier by relaxation measurements as a function of the applied field, the temperature and the thickness of a-TbFe. The relaxation measurements were performed after nucleation of a wall in the 1000 a-GdFe layer and its compression against the a-TbFe layer. By following the evolution of the magnetization versus time, we concluded that the barrier was crossed by thermal activation. We present the parameters of the phenomenon and show thermal and field dependence of the relaxation times. The data are compared with the results expected from barrier models.

4:45 PM M6.9 
MAGNETIC HYSTERESIS IN THIN FERROMAGNETIC FILMS WITH VICINAL SURFACES , Dai Zhao, James F. MacKay, Feng Liu, Univ of Wisconsin-Madison, Dept of MS&E, Madison, WI; Max G. Lagally, Univ of Wisconsin-Madison, Materials Science Program, Madison, WI.

Recently, the relationship between surface/interface magnetism and surface/interface roughness has become a subject of intense study. Both experiments and theory show that the intrinsic magnetic properties as well as the magnetic response to external field of a ferromagnetic thin film are strongly influenced by surface/interface roughness. In this work, Monte Carlo simulations on ferromagnetic thin films with vicinal surfaces are carried out to study the dependence of magnetic hysteresis on surface roughness. The system is described by a three-dimensional XY model in a cubic lattice, and the external field is changed linearly from positive to negative by small increments. Because of the broken symmetry at surfaces and steps, local magnetic moments, in general, have an intrinsic anisotropy. With a four-fold anisotropy assigned to surfaces sites and a two-fold anisotropy to step edges, the roughness- modified hysteresis loops are calculated as a function of film thickness and surface step density. By varying the angle () between external field and surface miscut orientation, the anisotropic property of the system is also studied. The resultant magnetic hysteresis loops change shape from squarelike to stripelike when the film thickness decreases; the coercive field shows a strong dependence on and step density in a complex fashion, which are in good agreement with our recent experimental results of Al/Co/Cu multilayer films. These results indicate a volume dependent competition between magnetic anisotropies at different surfaces sites.

Chair: Katayun Barmak
Wednesday Evening, April 2, 1997
8:00 P.M. 
Salon 7


We have studied the variation of the structural and magnetic properties of 5 m-thick self-supported Ag/NiFe multilayers upon annealing by means of transmission x-ray diffraction and Mossbauer spectroscopy as well as magnetorisistance (MR) and magnetization measurements. In the virgin state, x-ray experiments show that the (220) in-plane distances are 1.4 smaller for Ag and 2 larger for NiFe than the bulk values. Mossbauer spectra consist of a pure magnetic sextuplet, meaning that no nonmagnetic layer is present at interfaces. The 4 K hyperfine field is found only 2.4 smaller than the bulk NiFe value and the Curie temperature is estimated from both Mossbauer and magnetization data between 260 and 300C. Upon annealing, the (220) Ag distance starts increasing above 250C, whereas the decrease of the (220) NiFe distance starts at a much lower temperature. The structural relaxation of NiFe leads to an increase of the Curie temperature, without any obvious modification of the magnetic moment; this relaxation can be related to the increase of the MR ratio from 17 to 23 after annealing at 240C. The structural relaxation of both Ag and NiFe distances above 250C is linked to the onset of destratification, which explains both the appearance of a remanent magnetization and the sharper decrease of the MR ratio.

MAGNETIC ULTRATHIN RARE-EARTH CONTAINING LANGMUIR-BLODGETT FILMS, Gennady B. Khomutov, Alexander M. Tishin, Yury A. Koksharov, Moscow State Univ, Dept of Biophysics, Moscow, RUSSIA.

Langmuir-Blodgett films formed as layers of ordered planar array of magnetic atoms, separated by the nonmagnetic organic layers, were used as a model system for investigation of two-dimensional magnetism. They represent a new class of promising materials with considerable technological potential. The monolayer behaviour onto the cation-containing water subphase was analysed to find an optimal conditions of the formation of high integrated mono- and multilayer rare-earth containing stearic acid Langmuir-Blodgett films. The magnetic properties of deposited films were studied by EPR technique. For the first time the magnetic ordering in a wide temperature region was discovered in gadolinium-containing Langmuir-Blodgett films. The gadolinium EPR signal is invisible at room and lower temperatures and its intensity as temperature function has rather broaden maximum at about 500 K. The linewidth increases about three times with decreasing temperature from 550 K down to 400 K. The g-value also shows corresponding increasing on cooling. The field hysteresis of microwave absorption takes place. The results have manifested the existence of an internal field due to magnetic ordering within films at temperature region below 500 K. The data obtained proved that the formed stable planar stratified structure with magnetic layers of ultimately small thickness, corresponding to the dimensions of single atomic plane gadolinium ions, shows magnetic ordering up to rather high temperatures. Such properties open wide prospects for the device applications of developed magnetic material.

METAL ORGANIC CHEMICAL VAPOR DEPOSITION OF LaAMnO (A=Ca, Sr) FILMS, Daniel B. Studebaker, G. Doubinina, C. Seegal, A. Rakovska, Thomas H. Baum, Advanced Technology Materials Inc, Danbury, CT.

Thin films of La (A = Ca, Sr) have been deposited on MgO and LaAlO by liquid delivery Metal Organic Chemical Vapor Deposition (MOCVD). These films are uniform, and grow in the preferred c axis orientation. 2,2,6,6-Tetramethylheptanedionato (thd) derivatives of La, Mn, Ca, and/or Sr were dissolved in an organic solution and used as the precursors for film growth. Changing the precursor concentrations in solution enables the film stoichiometry and properties to be systematically varied. LCMO films can be deposited with an MR response at room temperature and in small magnetic fields (250 to 700 Oe). The film stoichiometry, crystallinity and post-CVD process annealing were found to influence the MR response. The MR response in small fields and at room temperature displays promise for future sensor applications.

STRAIN EFFECTS IN SUPERLATTICES OF DILUTED MAGNETIC SEMICONDUCTOR ZnTe/ZnMnTe, Chenjia Chen, Xi Chen, Xialoi Li, Xuezhong Wang, Peking Univ, Dept of Physics, Beijing, CHINA; Zhen Ling, Jie Wang, Xun Wang, Fudan Univ, Dept of Surface Physics Lab, Shanghai, CHINA.

ZnTe/ZnMnTe superlattices with different composition x (0 x 0.4) were grown by molecular beam epitaxy (MBE) on Ga Sb and GaAs (001) substrates, respectively. Superlattice (LS) samples consist typically of a thick 0.5 m ZnTe buffer layer and then ten or twenty periods alternating layers of AnTe and ZnMnTe with thick barriers of 100, 120 and well widths of nominally 30, 60, 100, 120 . The structures and growing qualities were studied and analyzed by x-ray diffraction (XRD) and transmission electron microscope (TEM). The multiphonon processes have been observed by near-resonant Raman scattering in the temperature range 13 K to 300 K. The strain arising from lattice mismatch gives rise to shift in the optical-phonon frequencies. Experimental results are coincident with the calculated strain-induced shift in superlattices. A two-phonon interface mode of superlattice has been observed and identified for the first time. Strain effects in different substrates of GaAs as discussed and analyzed.

TRANSPORT MEASUREMENTS OF MAGNETIC MULTILAYERS AT REDUCED LATERAL DIMENSIONS, Y. D. Park, Jeffrey R. Childress, Univ of Florida, Dept of MS&E, Gainesville, FL; H. D. Hudspeth, T. J. Shultz, Fred Sharifi, Univ of Florida, Dept of Physics, Gainesville, FL.

We report on the fabrication and transport properties of magnetic multilayers as a function of lateral width ranging from submicron to nanometer scale. The fabrication process involves an in-situ nano-mask which results in sharp and clean sidewall profiles without the need for further ex-situ processing which could deteriorate the edge structures. We employ a bilevel resist mask structure (copolymer/PMMA) which is e beam written with feature sizes down to 40 nm. We deposit the multilayer films using DC and RF magnetron sputtering through a high aspect ratio collimating body to insure minimal deposition on the resist sidewalls. A final lift-off process results in structures with incorporated leads and contacts. Each sample contains four structures of differing widths to insure identical deposition condition from structure to structure. We use an AC lock-in technique to measure transport properties from room temperature down to 3 K. This approach to fabrication can enable studies of lateral size effects in multilayer structures in the current-in-plane (CIP) geometry.

STABILITY OF NITROGEN IN SPUTTERED IRON NITRIDE THIN FILMS AND Fe/FeN MULTILAYERS, Jean-Francois Bobo, Stanford Univ, Dept of MS&E, Stanford, CA; Michel Vergnat, Univ Henri Poincare, Lab de Metallurgie Physique et Science, Vandoeuvre, FRANCE; Hassan Chatbi, Louis Hennet, Univ Henri Poincare, Lab de Metallurgie Physique et Science, Vandoeuvre, FRANCE; Michel Piecuch, Univ Nancy I, CNRS/LMPSM, Vandoeuvre, FRANCE.

Different phases of iron nitrides and Fe/FeN multilayers were prepared by reactive sputtering in a N-Ar mixture. By varying the nitrogen concentration in the gaseous flow, it is possible to obtain on unheated substrates: -Fe(N) ferrite, amorphous FeN phase, -FeN and -FeN. Heating the substrate led to the -FeN phase. The release of nitrogen and the crystallographic transformations during annealing were monitored by thermal desorption spectrometry and x-ray diffraction experiments. For nitrogen-rich phases (-FeN and -FeN), there is a first release of nitrogen at about 500 which corresponds to the transformation in -FeN followed by a second release of nitrogen at 650 which corresponds to the transformation from -FeN to -Fe. The other nitrogen-poor phases crystallize in -FeN during annealing and only the second effusion peak is observed. The temperature of this effusion peak depends on the film thickness, which indicates that the effusion is diffusion-limited. Experiments performed at different heating rates and with different film-thicknesses allowed us to evaluate the diffusivity of nitrogen in the -FeN phase. The stability of the Fe/FeN multilayers was also studied by low-angle x-ray scattering experiments. The nitrogen modulation observed at room temperature disappears below 500, which corresponds to diffusion of nitrogen between the layers with the formation of the -FeN phase. The release of nitrogen from the sample is then comparable to effusion from the -FeN phase.

MAGNETIC PROPERTIES OF NANOSIZE PARTICLES PRODUCED IN SILICA GLASSES BY IMPLANTING Ni AND N+Fe, Tetsuhiko Isobe, Keio Univ, Dept of Applied Chemistry, Yokohama, JAPAN; Robert A. Weeks, Vanderbilt Univ, Dept of Applied & Engr Science, Nashville, TN; Raymond A. Zuhr, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN.

Silica plates were implanted with Ni and with N+Fe. Ni was implanted at an energy of 160 keV and 3 A cmto a dose of 6 x 10 cm. The energies of the Nand Fe, being 52 and 160 keV, respectively, were chosen so that the distributions of N and Fe coincided. The dose ratio of N/Fe ranged from 0 to 2, and the dose of Fe was fixed to be 6 x 10 cm. Ni particles of 6 nm in diameter were observed in the Ni implanted samples by TEM. Magnetic resonance spectra of the samples were measured at 9 GHz from -150C to room temperature. In each sample the spectral component was a single absorption. In all cases the magnetic fields for resonance were a function of the angle between the normal to an implanted surface and the field. In the case of implanting Ni, a heat treatment at 800C for 4 h in Ar (96) + H2 (4%) increased the amplitude of the angular dependence and the intensity. This change is attributed to a change in the magnetic properties of the particles from superparamagnetic to ferromagnetic. In the case of the N+Fe samples, the amplitude of the angular dependence and the intensity of the component for resonance was a maximum when the ratio N/Fe was 0.2. Conversion electron Mossbauer spectrum of the N+Fe sample with N/Fe = 2/16 revealed the formation of iron nitrides, showing that compounds can be formed by sequential implantation. This research was supported in part by the Division of Materials Science, U.S. Department of Energy, under contract DE-AC05-84OR21400 with Lockheed Martin Energy Systems, Inc.

STRONG NON-LINEAR MCD-EFFECTS DUE TO PARTIALLY CIRCULAR POLARIZED LIGHT SOURCES, Hartmut Hochst, Dai Zhao, David L. Huber, Univ of Wisconsin-Madison, Synchrotron Radiation Cntr, Stoughton, WI.

Most MCD studies in the soft x-ray range are carried out with light of mixed polarization, where the effect of incomplete circular polarization is usually taken care of by linearly extrapolating the data to P = 1. We developed a simple model which includes the polarization factor in the MCD line shape. Calculations show that the MCD-signal, which is a relative measurement of the difference between reflection intensities obtained with opposite light helicity or opposite magnetization directions, is indeed proportional to P, whereas the absolute MCD-effect, which is the normalized quantity 2(I-I)/(I+I) turns out to be highly nonlinear with regard to Pcirc. MCD experiments with a specially designed quadruple reflection phase shifter which allows us to take data with variable P as well as P1. Fe and Co M MCD spectra verify the calculations. The data point out that the nonlinear response of the MCD effect to P can generally introduce large errors in the quantitative analysis of MCD spectra where spin and orbital moments are extracted by means of sum rules.

STRUCTURAL STUDIES BY NMR OF Co/Cu MULTILAYERS WITH VERY THIN AND DISCONTINUOUS Co LAYERS, Marek Wojcik, Ewa Jedryka, S. Nadolski, Inst of Physics, Warszawa, POLAND; David Kubinski, Harry Holloway, Ford Research Laboratory, Dearborn, MI.

By means of the Co NMR technique we study the structure of the Co layers in Co/Cu multilayers at the second antiferromagnetic maximum (AFM). We find that as a function of the Co layer thickness (t), the structure goes through 3 main stages: from a continuous Co layer for t, it changes to Co grains embedded in a Co-Cu alloy for 4 t, and finally to a totally alloyed Co-Cu layer for t . We also apply the Co NMR technique to study the Co structure in a series of (Co+Cu)/Cu multilayer samples at the second AFM, where a varying amount of Cu was codeposited along with the Co. (The magnetoresistive results of these films are presented as a separate contribution to this symposium.) We find that the amount of segregated Co decreases linearly with increasing amount of codeposited Cu.

MAGNETIC MEDIA ANALYSIS USING TIME OF FLIGHT SECONDARY ION MASS SPECTROMETRY (TOF-SIMS), Tom Schuerlein, C. A. Evans, Patricia M. Lindley, Charles Evans & Associates, Redwood City, CA.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a surface analysis technique which has proven to be a useful tool in the analysis of hard disk drive components. TOF-SIMS is an extremely surface sensitive technique which is capable of identifying both atomic and molecular materials with detection limits about 2 orders of magnitude better than XPS or AES. Identification of molecular contaminants is accomplished using high mass resolution mass spectra. The technique is capable of imaging small features and defects with lateral resolution of better than 0.2 micrometers. TOF-SIMS can be used to analyze both hard disks and heads. Applications investigating lubricants on hard disks that will be shown include: lube identification, identification of contaminants within the lube and segregation of lube additives. Hard disks also may be analyzed to identify and show localization of trace metal contamination and magnetic metals at the disk surface as well as defect or particle analysis and identification. TOF-SIMS has also been successful in the analysis of read-write heads. Specific capabilities that will be shown include identification and mapping of pole tip debris, and metal segregation/corrosion of the poles.

MAGNETIC ANISOTROPY AND ROTATIONAL HYSTERESIS LOSS IN EPITAXIAL TbFe(110) FILMS, Chuei-Tang Wang, Gaurav Khanna, Bruce M. Clemens, Robert L. White, Stanford Univ, Dept of MS&E, Stanford, CA.

The cubic phase TbFe possesses the largest known room temperature magnetostriction (2460 x 10) and a large magnetocrystalline anisotropy (-76 x 10 erg/cm). The large magnetostriction suggests that we can apply magnetic fields in either of two <111> easy axes in a (110)-oriented TbFe film to manipulate the film as a membrane for actuator applications. Magnetron sputtering was used to grow [110]-oriented TbFe films at 500 on AlO substrates with a [110]-oriented Nb buffer layer. The magnetization geometry was investigated using torque magnetometry, and the easy axis was found to lie along the [110] in-plane direction rather than the [111] in-plane axes. This is a result of the dominance of the in-plane tensile stress and the large magnetostriction constant. Furthermore the torque curves have a strong displacement of the center line from the zero-torque axis and do not coincide at the easy axis for clockwise and anticlockwise rotations. This displacement increases with twinning density. This unusual torque behavior was modeled by incorporating the presence of twins into the Stoner-Wohlfarth rotational loss model, and the true crystalline anisotropy constant was extracted using the K vs. method.

Chair: James G. Tobin
Thursday Morning, April 3, 1997
Golden Gate C2

8:30 AM *M8.1 
OPPORTUNITIES IN MAGNETIC MATERIALS RESEARCH USING SYNCHROTRON RADIATION, N. V. Smith, Lawrence Berkeley National Laboratory, Advanced Light Source, Berkeley, CA.

This will be an introductory overview of the utility of soft x-ray synchrotron radiation in the study of magnetic materials, surfaces and films. Perhaps the most widely used soft x-ray technique is magnetic circular dichroism (MCD) at the L edges of the elemental ferromagnets Fe, Co and Ni. The basics of MCD will be described and examples will be offered of its use in element specific magnetometry and separation of orbital and spin contributions to the magnetic moment. Spin-polarized photoemission is a more difficult technique and quite powerful in cases such as the observation of magnetic quantum well states in giant magnetoresistive layer systems. The so called ''complete'' experiment combines both circularly polarized x-rays and electron spin analysis. Examples of work done at various synchrotron light sources will be shown but with emphasis on opportunities at the Advanced Light Source including plans for the new elliptically polarizing undulator beamline.

9:00 AM M8.2 
MAGNETIC TRANSITIONS OF GD(0001) AS STUDIED BY SPIN POLARIZED PHOTOELECTRON DIFFRACTION: EXPERIMENT AND THEORY, Eric D. Tober, IBM Almaden Research Center, San Jose, CA; Ramon X. Ynzunza, Lawrence Berkeley National Laboratory, Materials Science Div, Berkeley, CA; F. P. Palomes, CSIMC-Madrid, Madrid, SPAIN; Z. Wang, Lawrence Berkeley National Laboratory, Berkeley, CA; Charles S. Fadley, Lawrence Berkeley National Laboratory, Matls Science Div, Berkeley, CA.

The rare-earth ferromagnetic system of Gd(000l) has been examined through the use of spin polarized photoelectron diffraction from the Gd 4s and 5s photoelectron multiplets, both of which exhibit highly spin polarized S and S photoelectron peaks. Temperature-dependent spin asymmetries derived from the S to S peak intensity ratio reveal distinct features near both the bulk and the previously-reported surface Curie temperatures, indicating that short range magnetic order does indeed persist beyond the bulk ordering temperature. We measure a separation of 60-80 K between the surface and bulk transition temperatures. Furthermore, our results suggest that the energy splitting between the S and S peaks exhibits a systematic variation with temperature that has not been seen in previous experiments involving core-level photoemission from Gd; this effect may be due to a spin- and temperature-dependent core-hole screening process or variations in the exchange interaction arising from the valence electrons.

9:15 AM M8.3 
MAGNETIC CIRCULAR DICHROISM IN RESONANT PHOTOEMISSION FROM RARE-EARTH MATERIALS, Z. Hu, Kai Starke, E. Navas, Freie Univ Berlin, Inst of Exp Physik, Berlin, GERMANY; G. vam der Laan, Daresbury Laboratory, Warrington, UNITED KINGDOM; Andreas Bauer, A. Muhlig, S. Langeheine, E. Weschke, E. Arenholz, Freie Univ Berlin, Inst for Exp Physik, Berlin, GERMANY; N. Brookes, ESRF, Grenoble, FRANCE.

We report on the study of Magnetic Circular Dichroism (MCD) in resonant photoemission (PE). At the example of ferromagnetic Gd, a consistent description is presented for resonant PE and x-ray absorption (XA). The close agreement of resonant PE-MCD spectra with the results of atomic multiplet calculations supports a simple picture tuning to the specific line of the XA spectrum leads to a selective enhancement of PE multiplet components, determined by the total angular momentum of the XA state. In particular, the enhancement of the -PE cross section at the giant resonance is accompanied by a substantial MCD signal. This finding makes MCD in resonant PE an attractive tool for element specific analysis of thin multicomponent magnetic systems and for surface-domain imaging by PE microscopy. Large MCD signals were also observed in resonant PE from Gd and Tb metal, suggesting its use for magnetic analysis with very high bulk sensitivity.

9:30 AM M8.4 
SPIN REVERSAL IN MN-CO ULTRATHIN FILMS INDUCED BY CHEMISORPTION: X-RAY DICHROISM SPECTROSCOPY AND MICROSCOPY STUDIES, Brian P. Tonner, Univ of Wisconsin-Milwaukee, Dept of Physics, Milwaukee, WI; William L. O'Brien, Univ of Wisconsin-Madison, Synchrotron Radiation Center, Stoughton, WI; Timothy Droubay, Univ of Wisconsin-Milwaukee, Dept of Physics, Milwaukee, WI.

We report the observation of an unusual spin orientation transition in the coupled magnetic layer system of Mn on fcc Co(001). Monolayer coverages of Mn on Co(001) are initially aligned ferromagnetically. Exposure to small amounts of oxygen causes a reversal of the coupling to of the Mn and Co layers, although each layer remains ferromagnetic. We see the spin re-orientation directly using x-ray magnetic circular dichroism. The Mn/Co(001) coupled layers also exhibit anomalous uniaxial anisotropy, and the in-plane easy axis rotates by 90 degrees during the spin-reorientation. The origin of this effect is studied using X-ray dichroism magnetic microscopy.

10:15 AM *M8.5 

Photoemission electron microscopy with cathode lens objectives is an established imaging technique. It combines high spatial resolution with surface sensitivity. By exploiting the effect of magnetic dichroism, photoelectron microscopy also becomes a very versatile tool in the investigation of magnetism and magnetic phenomena. The variety of magneto-dichroic phenomena offers contrast mechanisms with circularly polarized, linearly polarized, and even unpolarized light. A unique combination of elemental selectivity and magnetic sensitivity is achieved, when exciting core electronic states. We will discuss various aspects of this magnetic photoelectron microscopy, and present results obtained from single crystal surfaces and microstructured samples by means of a newly designed photoemission electron microscope with an integrated sample stage.

10:45 AM M8.6 
INTERRELATION BETWEEN STRUCTURE AND MAGNETISM OF ULTRA-THIN Mn FILMS EPITAXIALLY GROWN ON (001) Fe, Stephane Andrieu, Univ Nancy I, Lab de Metallurgie Physique, Vandoeuvre, FRANCE; Ph. Bauer, Univ Henri Poincare, LMPSM, Vandoeuvre, FRANCE; H. Fischer, Michel Piecuch, Univ Nancy I, CNRS/LMPSM, Vandoeuvre, FRANCE; M. Finazzi, K. Hricovini, Gerard Krill, P. Lefevre, H. Magnan, D. Chandesris, Univ Paris, LURE CNRS/CEA, Orsay, FRANCE.

A lot of work, both theoretical and experimental, was recently devoted to the study of the magnetic properties of ultrathin Mn films epitaxially grown on bcc (001) Fe surface. Several recent theoretical calculations predict that the most stable configuration for 1 Mn monolayer (ml) is a c2x2 antiferromagnetic (AF) state, with a predominant AF coupling with Fe. By using x-ray magnetic circular dichroism, Dresselhauss and coworkers (E-MRS Spring Meeting, June 96, Strasbourg) found that a 0.3 ml Mn film is ferromagnetic with a AF coupling between Mn and Fe. However, we showed that the substrate preparation used in their study does not lead to pure Mn layer on (001) Fe (same conference). Consequently, the magnetic properties of ultrathin Mn films grown on (001) Fe are still an open question. In this study, the growth, structure, and magnetic properties of Mn films from 0.5 to 6 ml thick were in situ studied by using electron diffraction (RHEED), x-ray absorption spectroscopy (SEXAFS), and x-ray magnetic circular dichroism (XMCD). The three techniques show that the Mn structural and magnetic properties drastically change at 2 ml. Up to 2 ml, the structure is found to be close to bcc Fe, and the films are ferro- or ferrimagnetic with a rather low average magnetic moment (1.7 for 1 ml and 1.2 for 2 ml) compared to theoretical prediction (3). The interfacial Mn/Fe coupling is clearly ferromagnetic. For more than 2 ml, the structure of the Mn film changes in a complex way and the films become AF. In a recent theoretical study, a ferrimagnetic state with 3 Mn spin up and 1 spin down was shown to be more stable than the AF c2x2 configuration. We will show that this prediction explains our results.

11:00 AM M8.7 
STRUCTURE AND MAGNETISM OF Fe/Pd MULTILAYERS: EVIDENCE FOR A HIGH SPIN FCC-LIKE Fe PHASE, Frederic Petroff, Vincent Cross, Thomson-CNRS, UMR, Orsay, FRANCE; Jan Vogel, CNRS, Louis Neel Lab, Grenoble, FRANCE; Jean-Paul Kappler, IPCMS, Strasbourg, FRANCE; Alain Fontaine, CNRS, Louis Neel Lab, Grenoble, FRANCE; Gerard Krill, Univ Paris, LURE CNRS/CEA, Orsay, FRANCE.

We have investigated the structural and magnetic properties of Fe/Pd multilayers as a function of Fe thickness using x-ray absorption spectroscopies (EXAFS and XMCD) at the K and L edges of Fe. The Pd/Fe (100) multilayers were grown by molecular beam epitaxy on MgO (001) substrates and the XAS experiments were performed at LURE (France). 
Our results show remarkable change in the magnetism of Fe. Up to 3 ML, the magnetic moment of Fe is strongly enhanced (), compared to the bulk value (2.2 ). At 4 ML of Fe, a sharp decrease of the magnetic moment is observed and, for increasing Fe thickness, one recovers progressively the bulk magnetic moment. We have already studied the role of the hybridization between the Pd 4d and the Fe 3d bands on the magnetic polarization of the Pd atoms in the same system [1]. In the case of Fe, changes in the magnetic properties cannot only be ascribed to the exchange interaction between the Pd and Fe atoms but also to a modification of the crystallographic structure of the Fe layers. We observed indeed that up to 3 ML, the Fe layers are in an expanded fcc high-spin phase, whereas for 4 ML there is a structural relaxation to a bct phase of lower spin.

11:15 AM M8.8 
MAGNETISM OF ORDERED AND DISORDERED FEPT STUDIED BY MCXD, Kazimierz J. Gofron, Clyde W. Kimball, Northern Illinois Univ, Dept of Physics, DeKalb, IL; Peter L. Lee, Argonne National Laboratory, Matls Science Div, Argonne, IL; Cliff Yahnke, Northern Illinois Univ, Dept of Physics, DeKalb, IL; Pedro Montano, Argonne National Laboratory, Matls Science Div, Argonne, IL.

We studied element and orbital specific magnetism of the FePt invar alloy at the L and L edges of Pt, as well as K edge of Fe using energy dispersive and monochromatized beamlines. Circularly (60%) polarized x-rays were incident normal to the sample surfaces. The samples were magnetized perpendicular to the surface with a field of 1 T. The MCXD signals for the ordered (disordered) alloy are 14% (20% ) at L edge and 12% (18%) at L edge. The MCXD signal at the L and L edges of Pt are of opposite sign.The L edge results are used to estimate orbital and spin magnetic moments of Pt. We observe that the MCXD signal at Fe K edge in FePt is twice as large and twice as broad in energy than in elemental Fe. The K edge MCXD for Fe in FePt exhibits complicated structure of positive and negative peaks which have very different intensities for ordered and disordered phases of FePt alloy. Funded by the State of Illinois under HECA, NSF(DMR93-10656) and DOE/MS(BES W-31-109-ENG-38

11:30 AM M8.9 
THE ROLE OF ANISOTROPIC STRAIN AND INTERFACIAL MIXING ON PERPENDICULAR MAGNETIC ANISOTROPY IN THE Co/Pd MULTILAYERS AND Co-Pd ALLOY FILMS, Sang-Koog Kim, Lawrence Berkeley National Laboratory, Adv Light Source Dept, Berkeley, CA; Yang-Mo Koo, POSTECH, Dept of Matls Sci & Tech, Pohang, SOUTH KOREA; Vladimir A. Chernov, Boreskov Inst of Catalysis, Spectral Research Methods Lab,; Jeffrey B. Kortright, Lawrence Berkeley National Laboratory, Materials Sciences Div, Berkeley, CA; Howard A. Padmore, Lawrence Berkeley National Laboratory, Dept of Adv Light Source, Berkeley, CA.

We have investigated interface structure of Co/Pd multilayers by quantitative analysis of fluorescence-yield Co K-edge polarized extended x-ray absorption fine structure (PEXAFS). The results show there is a considerable amount of Co-Pd alloy-like phase near interfaces. Moreover, anisotropic strains of Co atoms are observed, tensile and compressive strains of in-plane and out-of-plane atoms, respectively, which depend on Co layer thickness. The in-plane tensile strains mainly lead to perpendicular magnetic anisotropy (PMA) of Co/Pd multilayer films, combined with a much more negative value of the magnetostriction coefficient, , of the alloy phase. These results are confirmed by coevaporated Co alloy films exhibiting the PMA. The PMA origin of alloy samples is also explained mainly by magnetostrictive effect which leads to stress-induced perpendicular magnetization through a progressively more negative than that of bulk Co.

11:45 AM M8.10 
OBSERVATION OF MONOLAYER MAGNETISM OF Fe BY MEANS OF NUCLEAR RESONANT SCATTERING OF SYNCHROTRON RADIATION, Lambertus Niesen, Univ of Groningen, Dept of Nuclear Solid State Physics, Groningen, NETHERLANDS; Reinder R. Coehoorn, Philips Research Laboratories, Dept of Magnetism, Eindhoven, NETHERLANDS; Reiner M. Jungblut, Philips Research Laboratories, Eindhoven, NETHERLANDS; Alfred Q.R. Baron, ESRF, Grenoble, FRANCE.

The magnetic behavior of epitaxial probe layers of Fe down to a thickness of 1 monolayer (ML) has been investigated with the novel technique of nuclear resonant scattering in a grazing incidence geometry. To test the possibilities of this technique, epitaxial Fe layers were grown with MBE on S-passivated Ge(100) substrates. They were capped with 10 ML of Au. Each Fe layer contained 1-10 ML of Fe, at various positions with respect to the substrate and the top interface. The hyperfine interaction of the Fe probe atoms was determined from the time response of the nuclear resonance scattering process initiated by 14413 eV photons from the ESRF in Grenoble. A sample of 4 ML Fe on top of 5 ML of natural Fe appeared completely nonmagnetic. The spectrum revealed the formation of a FeGe alloy at the interface. For thicker Fe layers (50 ML Fe), we observed the same hyperfine field as in the bulk, with very narrow lines. In addition, a small FeGe component was visible (intensity 8). In contrast, 15 ML Fe layers grown on nonpassivated Ge(100), containing 1 ML of Fe, showed the bulk hyperfine interaction and no indication of FeGe alloy formation. However, in this case appreciable line broadening was present, pointing to structural imperfections. The broadening was most severe at the Fe/Au interface, where no sharp features could be detected. In the near future, the technique will be applied to multilayers of FeO and MgO.

Chair: Peter H. Dederichs
Thursday Afternoon, April 3, 1997
Golden Gate C2

1:30 PM *M9.1 
THEORETICAL INVESTIGATIONS ON THE MAGNETO-OPTICAL PROPERTIES OF TRANSITION METAL SURFACE AND MULTILAYER SYSTEMS, H. Ebert, Univ Munchen, Inst fur Phys. Chemie, Munchen, GERMANY; A. Perlov, Inst of Metal Physics, Kiev, UKRAINE; V. Popescu, Univ Munchen, Inst fur Phys Chemie, Munchen, GERMANY.

The basis of theoretical investigations on the magnetooptical properties of transition metal surface and multilayer systems in the visible and x ray regime of light is outlined. An appropriate description of the underling electronic structure has to account for both the spin-orbit coupling and the magnetic state of the investigated system at the same time. As will be shown, this can be achieved in a very satisfying way within the framework of the fully relativistic Dirac formalism. Results of corresponding calculations on the magnetooptical Kerr effect (MOKE) in the visible regime of light, as well as of the magnetic dichroism in x-ray (MXD) absorption for Fe- and Co-based surface and multilayer systems are presented and compared with experimental data. Concerning the MOKE, it is most interesting to study the changes induced by a second component compared to the properties of pure Fe or Co, respectively. As will be shown, the MXD on the other hand allows in particular to study the induced magnetic moments induced by Fe or Co on adjacent layers of atoms that are otherwise nonmagnetic.

2:00 PM M9.2 
ANGLE- AND TEMPERATURE- DEPENDENT MAGNETIC CIRCULAR DICHROISM IN CORE PHOTOELECTRON EMISSION FROM Gd(0001), Jonder Morais, Reinhard Denecke, Lawrence Berkeley National Laboratory, Matls Science Div, Berkeley, CA; John Liesegang, La Trobe University, Dept of Physics, Bundoora, AUSTRALIA; Ramon X. Ynzunza, Lawrence Berkeley National Laboratory, Materials Science Div, Berkeley, CA; Charles S. Fadley, Lawrence Berkeley National Laboratory, Matls Science Div, Berkeley, CA.

We report on angle- and temperature- resolved magnetic circular dichroism (MCD) in photoemission from Gd 4f, 4d and 5p core-levels of magnetized Gd films. The films consisted of 100ML of Gd evaporated on a W(110) substrate at room temperature, subsequently annealed to 700K to form an ordered Gd(0001) layer. The MCD measurements were performed by physically rotating the sample by 180 degrees around the surface normal so as to change the direction of the magnetization axis relative to the incident light, with the light helicity being kept constant (in this case left handed). The MCD asymmetries were obtained by calculating a normalized difference between spectra obtained with the two magnetization orientations. As a self-consistency check, it was furthermore verified that, for photoemission in a mirror-plane of the crystal, this procedure yields identical results to measurements in which the magnetization is kept fixed, but the helicity is changed from right to left handed. Our results agree very well with the published data of van der Laan et al. (P.R.B 53, R5998 (1996)). However, our energy resolution is significantly better, thereby providing a more detailed test of, and excellent agreement with the multiplet theory calculations presented in this paper. For the first time, we have also measured the dependence of the MCD peak-to-peak asymmetry as a function of the angle between the exciting light and the magnetization axis. We observe a strong variation of MCD with angle which suggests a combined influence of both free-atom effects and non-magnetic and magnetic solid-state diffraction effects. Data taken for different polar angles of electron emission were analyzed in order to clarify the origin of the observed effects, and to determine the best geometries for use in studying purely magnetic phenomena. In addition, we investigated the variation of the MCD asymmetry with substrate temperature.

2:15 PM M9.3 
SOFT X-RAY MOKE STUDY OF POLYCRYSTALLINE Fe/Cr MULTILAYER AND Fe-Cr ALLOY FILMS, Jeffrey B. Kortright, Lawrence Berkeley National Laboratory, Materials Sciences Div, Berkeley, CA; Marybeth Rice, Lawrence Berkeley National Laboratory, Berkeley, CA; Chris Walton, Sang-Koog Kim, Lawrence Berkeley National Laboratory, Adv Light Source Dept, Berkeley, CA.

Element-resolved hysteresis loops were measured using soft x-ray MOKE in sputtered Fe/Cr multilayers and Fe-Cr alloys near the Fe and Cr L edges. X-ray diffraction and high resolution transmission electron microscopy indicate that these films are polycrystalline with 110 texture, but are not as well ordered as Fe/Cr multilayers exhibiting the largest GMR values. In multilayer and alloy samples, Cr has a net moment oriented antiparallel to the Fe moment. In the alloy films, the coercive field is a strong function of the composition. In multilayer films, the Fe layers exhibit ferromagnetic and antiferromagnetic (on noncollinear) coupling depending on the nominal Cr layer thickness. The element resolved hysteresis loops of multilayers exhibiting noncollinear coupling reveal two components to the net magnetization of the sample, one being the reversible ordering of the Fe layers with respect to each other, and the other being a hysteretic feature in which fe and Cr moments are oppositely oriented. These results suggest a model in which the Cr moment in the multilayers results from interface mixing with Fe, and that the composition of this interface layer determines the coercive field of the multilayers.

2:30 PM M9.4 
ALTERATION OF MCD SPECTRA DUE TO THIN FILM INTERFERENCE EFFECTS, D. Rioux, B. Allen, Univ of Wisconsin-Oshkosh, Dept of Physics & Astronomy, Oshkosh, WI; Hartmut Hochst, Dai Zhao, David L. Huber, Univ of Wisconsin-Madison, Synchrotron Radiation Cntr, Stoughton, WI.

MCD spectra at the M edge of thin Fe films exhibit thickness dependent variations in line shape as well as in the absolute MCD-effect. Our data indicate that more information is contained in the MCD spectra than simply the evolution of a magnetic moment and ferromagnetic order. We developed a model to predict line shape modulations as a function of film thickness and angle of light incidence. Using the Fresnel-Maxwell formalism we calculate MCD-interference effects between light reflected from the vacuum-film-substrate interfaces which are verified by our MCD measurements. Since the observed interference effects are a function of the excitation wavelength, our results can be directly scaled to show the significance of these effects in the more commonly used L region of 3d ferromagnets. Our data point out that one might be ill advised to rely on L-MCD experiments which try to extract the formation of magnetic moments in films of several 10 thickness.

3:30 PM M9.6 
ATOMIC STEP-INDUCED MAGNETIC ANISOTROPY IN Fe/STEPPED Ag(001), Roland K. Kawakami, Ernesto J. Escorcia-Aparicio, Martin Bowen, Zi Q. Qiu, Univ of California-Berkeley, Dept of Physics, Berkeley, CA.

The magnetocrystalline anisotropy must respect thc symmetries of the crystal structure. Therefore, symmetry breaking should generate additional anisotropies. Periodic atomic steps in the (001) surface break the in-plane four-fold rotation symmetry, and consequently induce an in plane uniaxial anisotropy. To better understand the effect of symmetry breaking on the magnetic anisotropy, it is important to know how the step-induced anisotropy develops as a function of the step density and orientation. We investigated the step-induced anisotropy of Fe/stepped Ag(001) over a continuous range of step densities for two different step orientations: step edges parallel to the (100) and (110) in-plane directions. The steps are created by polishing the Ag substrate at an angle away from (001) orientation. Fe films were epitaxially grown in ultrahigh vacuum ( 2 x 10 Torr) with typical Fe thickness 25 monolayers. By using a curved substrate and local magnetization probe (SMOKE), we are able to explore th relation between the step-induced anisotropy and the step-density in a systematic way. The results indicate that the functional form of the anisotropy is mainly determined by Neel's nearest-neighbor pair bonding. This conclusion is also supported by studies of the spin-reorientation transition in this system.

3:45 PM M9.7 
PROPERTIES OF MAGNETO-OPTICAL Co-Pt AND MAGNETORESISTIVE Co-Cu ALLOY FILMS, YoungPak Lee, K. W. Kim, G. M. Lee, Y. V. Kudryavtsev, Sun Moon Univ, Dept of Physics, Asan, SOUTH KOREA; G. S. Chang, Chung Nam Whang, Yonsei Univ, Dept of Physics, Seoul, SOUTH KOREA.

Co-Pt and Co-Cu alloy films have received great interest recently owing to their easier fabrication than the multilayered (ML) films, as well as the excellent magneto-optical (MO) properties and giant magnetoresistance (MR), respectively. Co-Cu films of 1000-1500 thick were produced by means of vacuum flash-evaporation technique at 150 K, 400 K, 520 K, and 720 K. The CoCu films produced at 400 K or 520 K exhibit the maximum value of MR of 16.5 at 77 K and 1.5 T. Co-Pt films of 200 thick with Pt buffer layers of 200 thick were prepared at room temperature (RT) by ultrahigh-vacuum cosputtering. The polar Kerr rotation angles (t) of the CoPt and CoPt films increase with increasing photon energy up to 2.71 eV and decreasing measurement temperature down to 5 K, and t at RT of CoPt film are all larger than those of the Co/Pt ML film of the corresponding composition. The surface morphology, mean grain and magnetic domain sizes, and size distributions of the grains and magnetic domains of the Co alloy films have been investigated by MFM and AFM. The results were correlated with those obtained by using MO and optical spectroscopy at both RT and low temperatures (LT) in a spectral range of 0.5-4.4 eV, and magnetic investigations at both RT and LT including field dependence measurement of MR. The overall results reflect the changes in the electronic structures and magnetic states of the constituent elements, induced by the structural transformation or correlated with the structures of the films.

4:00 PM M9.8 
OPTICAL AND MAGNETO-OPTICAL SPECTRA OF CoCu MAGNETIC GRANULAR ALLOYS, Elena Gan'shina, A. Granovsky, V. Guschin, P. Podrugin, A. Kravetz, E. Shipil, Lomonosov Univ, Physical Faculty, Moscow, RUSSIA.

In this paper we present the results of the experimental and theoretical investigation of the optical and magneto-optical (MO) spectra of magnetic granular films CoCu. The granular films CoCu with x varying between 9 and 51 at. were prepared in double e-beam gun deposition system. The optical constants were measured using the Bittie-Kohn method. The MO properties were investigated at room temperature in the geometry of transverse Kerr effect (TKE) in the energy range 1.5-4.0 eV. The optical and MO spectra of granular alloys are quite different from the spectra of homogeneous films. The amplitude of the Kerr signal and the MO spectra profile depend on the concentration and the shape of the granules. We interpreted the obtained data in the framework of the effective medium approximation. The calculated optical and MO spectra are in qualitative agreement with experimental data that allows to make some conclusions about ferromagnetic particle concentration and their shape.

4:15 PM M9.9 
THE MECHANISM OF THE OPTICAL NONLINEARITY IN RARE-EARTH IRON GARNET FILMS , Alexey V. Melnikov, Tatyana V. Murzina, Timur V. Misuryaev, Elena E. Shalygina, Moscow State Univ, Dept of Physics, Moscow, RUSSIA; Oleg A. Aktsipetrov, Moscow State Univ, Dept of Physics, Quantum Radiophysics Div, Moscow, RUSSIA.

The nonlinear magnetooptics is an intensively studied field of nonlinear optics, that now deal predominantly with modern objects such as thin metallic films, magnetic multilayer structures, etc. Nevertheless, mechanisms of nonlinearities of traditional objects of magnetooptics stay to be unclear. In this paper we propose the mechanism of optical nonlinearity of rare-earth iron garnet films studied by means of second harmonic generation (SHG). The samples studied were epitaxially grown iron garnet films doped by Y, Bi and Gd on GaGd garnet substrates of various crystalline orientations. The output of a Q-switched YAG laser (at 1064 nm, pulse duration 10 ns) was used as a fundamental beam. The radiation at the double frequency was selected by interference filter, and detected by a PMT and gated electronics. The dc-magnetic field was applied in the plane of the film or along the films normal. The SHG intensity observed in transmission geometry for films grown at the substrate of (210) orientation was essentially larger than for (111) and (001) ones. The features of the azimuthal anisotropy show the presence of electric-dipole contribution to the SHG response that are known to be forbidden for m3m symmetrical iron garnet films. The violation of this forbiddance is proposed to be connected with strong orthorhombic anisotropy existing in the case of (210) substrate. Nonlinear Kerr and Faraday effects were investigated by SHG polarization diagrams and azimuthal anisotropy studies. The dependence of SHG parameters on the orientation of the easy axes was studied for thin iron garnet films.

4:30 PM M9.10 
MAGNETO-OPTICAL INVESTIGATION OF Fe/V,Mo,Ta AND Co/V,Mo,Ta MULTILAYERS, Elena E. Shalyguina, L. V. Kozlovsky, N. I. Tsidaeva, O. A. Shalyguina, Moscow State Univ, Dept of Physics, Moscow, RUSSIA.

n this report, we present the results on investigation of magnetic and magneto-optical properties n x [NM/FM] multilayers (with NM = V,Mo,Ta; FM = Fe,Co; n = 3,20) prepared in Penning discharge. The Fe- and Co- layer thickness t(Fe, Co) was variable from 25 to 100 A and the nonmagnetic layer (NML) thickness t(V,Mo,Ta) - from 10 to 30 A. It was experimentally proved the existence of exchange coupling between FM layers through NML and its oscillatory behavior (from antiferromagnetic [AF] to ferromagnetic [F] order). The F-, AF- and sloped AF structures was discovered. It was found that period of AF-F-AF oscillations T is equal to 5 - 10 A and the one is dependent on t(Fe,Co) (T increases with increasing of t(Fe,Co)). It was discovered that at the fixed t(Fe,Co) and t(V,Mo,Ta) 25 A the value of transverse Kerr effect (TKE) oscillates with changing of t(V,Mo,Ta). The period ranged from 9 to 12 A. Moreover, in this case the TKE spectra were distinct the ones of Fe - and Co- single layer films. In all probability these peculiarities are caused the spin polarization of NML.

Chair: James G. Tobin
Thursday Evening, April 3, 1997
8:00 P.M. 
Salon 7


The study of perpendicular magnetization has led to quantitative estimates of the magnetoelastic coupling coefficients in magnetic films and multilayers. However, as far as we know, all the analyses thus far reported have used only first-order theories in treating the large internal strains. These first-order treatments may be inadequate considering that the biaxial misfit strains in many of these systems are as large as 2.6. Furthermore, even with the first-order theories, only a few of the coupling coefficients have been estimated. We have developed a second-order pair-interaction model that incorporates the nonlinear effects due to the presence of large misfit strains. The coupling coefficients can generally be categorized as follows: first-order bulk terms, first-order surface terms, second-order bulk terms, second-order surface terms. In light of the pair-interaction model, these coefficients can usually be expressed as functions of the pseudo dipole term in the pair-interaction energy expansion and its first two derivatives. Thus, for the Cu/Ni/Cu system, by using the two known magnetoelastic coupling coefficients of bulk nickel and the thin film anisotropy data, all the coupling coefficients of the system are determined. One major conclusion that can be drawn from the determination of these constants is that much of the deviation of the first-order bulk terms in the Cu/Ni/Cu system is due to the importance of the second-order bulk terms. This is contrary to what is reported in the literature where the deviations are believed to be of Neel-type surface magnetoelastic effects.


In this report we present the results on the investigation of near surface micromagnetic structure (MMS) of Fe-rich soft magnetic amorphous ribbons. The measurements were carried out on the magneto-optical micromagnetometer. It was discovered that in the initial samples the magnetization are oriented in ribbon plane; the local magnetization curves of various near-surface regions with linear size equal to 50 -100 m distinguish; the remagnetization of samples realizes by means of the magnetization rotation; at the certain magnetic field, the blocking of magnetization processes exists. So it was proved that in this case unregular ripple structure realizes. It was studied the influence of time and temperature of thermomagnetic (H perpendicular to ribbon length) annealing (TMA) on near-surface ribbon MMS. It was established that at a determined regime of TMA, the stripe structure realizes; the blocking of the magnetization processes disappears; local magnetic properties become more homogeneous. The received data can be useful as a new class of magnetic sensors based on giant magneto-impedance develop.

Fe-Cr-N BASED NANOCRYSTALLINE SOFT MAGNETIC THIN FILMS, W. Zhu, Sungho Jin, R. B. van Dover, V. Korenivski, Bell Labs, Lucent Technologies, Murray Hill, NJ.

New Fe-Cr-N and Fe-Cr-Ta-N alloy films have been prepared, and their magnetic and structural properties have been investigated. Thin films with compositions in the range of FeN (in atomic ) were deposited by reactive sputtering in a nitrogen-containing atmosphere. The films, up to 00 nm in thickness, are nanocrystalline and exhibit highly anisotropic and excellent soft magnetic properties (e.g., H < 1 Oe and 420 kG) in the as-deposited conditions without any heat treatments. The easy-axis M-H loop is square, and the hard-axis loop is linear and closed, with the anisotropy field Ha, in the range of 20-60 Oe. The combination of high 4M, and relatively high H in these films is conducive to the suppression of the undesirable FMR (ferromagnetic resonance) interference up to the GHz frequency range. The high permeability and low loss characteristics maintained in these films are useful for high frequency microwave applications.

SPIN REORIENTATION IN SURFACE GADOLINIUM LAYER OF MULTILAYER Gd/Fe FILMS INDUCED BY MAGNETIC FIELD, Sergej L. Gnatchenko, Alexander B. Chizhik, Dmitrij N. Merenkov, Inst for Low Temperature Physics & Engr, Dept of Magnetism, Kharkov, UKRAINE; Viktor V. Eremenko, Inst for Low Temperature Physics & Engr, Dept of Spectroscopy, Kharkov, UKRAINE; Henrik Szymczak, Rita Szymczak, Krzysztof Fronc, Ryszard Zuberek, Inst of Physics, Dept of Magnetism, Warsaw, POLAND.

The magnetic moments of Gd and Fe layers in thin multilayer Gd/Fe films lie in the film plane and are ordered antiparallel due to antiferromagnetic exchange interaction at the interface. Therefore in these films magnetic field can induce the twisted phase in which magnetic moments of Gd and Fe layers are noncolinear. This type of spin reorientation is much studied in the last years. In particular it has been shown that the magnetic phase transition to noncolinear state begins near the surface of the film. We report about another kind of the field induced surface spin reorientation found in the multilayer Gd/Fe films. The phase transition to noncolinear state in surface gadolinium layer induced by magnetic field has been observed in Gd/Fe films in which magnetic moment of Gd layer was significantly less than that of Fe layer. The magnetization process of multilayer Gd/Fe films has been studied my means of longitudinal magneto-optical Kerr effect and magnetization measurements in the temperature range 20K

PERPENDICULAR GIANT MAGNETORESISTANCE OF Co/Cu MULTILAYERS WITH FLUCTUATING Co LAYER THICKNESSES, Wen-C Chiang, R. Loloee, W. P. Pratt, J. Bass, Michigan State Univ, Dept of Physics & Astronomy, East Lansing, MI.

Mathon [1] recently predicted that deliberately introducing pseudorandom fluctuations (PRF) in Co or Fe layer thickness would greatly enhance the current-perpendicular to the layer planes (CPP) magnetoresistance (MR) of Co/Cu(001) or Fe/Cr(001) superlattices, assuming that the electrons travel ballistically through a sample. More precisely, the resistance, R(AP), in the state where adjacent layer magnetizations are antiparallel (AP), becomes rapidly larger due to PRF-induced localization as the number of bilayers increases, whereas the resistance R(P) in the parallel (P) state depends only weakly on such PRF. Mathon also argued [1] that the already measured values of R(AP) for sputtered, antiferromagnetically coupled Co/Cu multilayers are compatible with PRF of magnitude similar to that of unavoidable fluctuations in layer thicknesses. Thus, he argues that the published CPP-MR measurements on sputtered Co/Cu do not rule out ballistic transport. We are trying to test Mathon's predictions for sputtered Co/Cu multilayers. We have chosen to start at the second antiferromagnetic coupling peak-Cu layer thickness tCu 2.2 nm-where variations in t are not crucial and where the MR saturates by 0.1 T. So far, we have compared the Rs for pairs of Co/Cu multilayers with N = 30 and 40-one sample of each pair with t held as ' 'constant'' as possible at t = 1.5 nm, and the other with deliberately introduced PE with amplitude 1 or 2 Co monolayers. While, as expected, the R(P)s are roughly similar for both ''constant'' and PRF samples, the estimated R(AP)s are noticeably smaller for the PRF samples than for the ''constant'' ones-opposite to the prediction. Further studies are in progress.

MAGNETIZATION CONTROLLED RESONANT TUNNELING THROUGH ErAs SEMI-METAL QUANTUM WELLS, Daniel E. Brehmer, Univ of California-S Barbara, Dept of Physics & Quantum Inst, Santa Barbara, CA; James P. Ibbetson, Univ of California-S Barbara, Dept of Materials, Santa Barbara, CA; S. J. Allen, Univ of California-S Barbara, Dept of Physics & Quantum Inst, Santa Barbara, CA; Chris J. Palmstrohm, Univ of Minnesota, Dept of CE&MS, Minneapolis, MN; San-Phay Chau, Univ of California-S Barbara, Dept of Physics & Quantum Inst, Santa Barbara, CA; B. Wilkens, Arizona State Univ, Center for Solid State Science, Tempe, AZ.

Magnetic, semimetallic ErAs quantum wells can be grown epitaxially and incorporated in III-V heterostructures to produce resonant tunneling structures. Further more the semi-metallic character ot the ErAs quantum wells allows us to selectively contact the quantum well providing a development path to three terminal resonant tunneling transisitors. Here we explore the interplay of the Er sublattice magnetization and resonant tunneling through quantized hole states which produces a giant spin slitting % 1 eV) of the resonant channels. The splitting is shown to saturate in modest magnetic fields, consistent with exchange coupling of the hole states with the Er 4f spin. Both the magnitude and relative strength of the resonant channels depends in a striking way on orientation of the Er magnetization. The splitting depends on quantum well thickness, becoming smaller in thicker quantum wells. This remarkable result can be understood in terms of the ErAs band structure which indicates that the hole states, that lead to the resonant channels, are derived primarily from As orbitals, with admixture of Er orbitals that approaches zero with the wave vector . The wave vector of the resonant channel is fixed by the thickness and we expect and observe that the exchange splitting vanishes as the wave-vector approaches zero, as the quantum well thickness becomes large. While these experiments are performed at low temperatures to be able to saturate the Er 4f magnetization, they open the scientifically interesting and potentially important possibilities of controlling resonant tunneling with ferro and ferrimagnetic quantum wells magnetically ordered at room temperature.

INTERACTION OF KONDO IMPURITIES WITH SURFACES, Laszlo Szunyogh, Tech Univ of Budapest, Dept of Theoretical Physics, Budapest, HUNGARY; Balazs L. Gyorffy, Univ of Bristol, H.H. Wills Physiscs Lab, Bristol, UNITED KINGDOM; Alfred Zawadowski, Tech Univ of Budapest, Dept of Theoretical Physics, Budapest, HUNGARY.

Recently it has been discovered that the Kondo contribution to the resistivity by magnetic impurities in thin filmsdepends dramatically on the thickness of the film [1]. The most plausible explanation for this surprising phenomenon assumes that such impurities near the surface will be subject to a local magnetic anisotropy due to the combined effects of spin-orbit coupling and the surface which breaks the crystalline symmetry of the bulk [3]. In this contribution we shall study a 'buried' Fe impurity in a semi-infinite FCC Au lattice as a function of its distance, d, from the surface on the basis of spin-polarized, relativistic first principles calculations [4]. We demonstrate that such anisotropy does indeed exist and it is of the right order of magnitude.

MAGNETIC AND MAGNETOTRANSPORT PROPERTIES OF P-TYPE (GA,MN)AS, A NEW DILUTED MAGNETIC SEMICONDUCTOR, Ann Van Esch, Katholieke Univ Leuven, Dept of Physics, Leuven, BELGIUM; Jo De Boeck, IMEC, Dept of MAP/MBE, Leuven, BELGIUM; Luc Van Bockstal, Ria Bogaerts, Fritz Herlach, Katholieke Univ Leuven, Dept of Physics, Leuven, BELGIUM; Gustaaf Borghs, IMEC, Dept of MAP/MBE, Leuven, BELGIUM.

(Ga,Mn)As is a very new III-V diluted magnetic semiconductor that can be grown by MBE. A growth procedure comparable to that of low temperature GaAs was used, allowing homogeneous incorporation of Mn concentrations up to x=0.08. In contrast to the traditional II-VI DMS's, Mn not only introduces a magnetic moment into the GaAs host lattice but it also provides a hole. Hole transport is affected by the strong antiferromagnetic interaction between the holes and Mn 3d spins. Below a critical temperature Tc, determined by the Mn concentration (about 50 K for x=0.05), magnetic long-range order of Mn-hole complexes induces a paramagnetic-ferromagnetic transition. Spontaneous magnetisation is observed and resistivity decreases as the magnetic ordering sets in. Also, negative magnetoresestance is observed below Tc. We will present a model linking the transport behaviour with the magnetic properties.


Spin dependent tunneling devices of the type NiFe/AlO/Co with a MR of 3 at RT were prepared by magnetron sputtering through metal masks. The junction area was 0.1 mm, and junction resistance was 2 M. By fitting the I-V curves to tunneling theory, we obtained an effective insulator thickness of 20 and a barrier height of 2.2 eV. However, the devices were very fragile and many were destroyed during testing. Also, it was very difficult to reproduce these results. We attributed this difficulty to pinholes and nonuniform oxide thickness partially resulting from bottom electrode roughness. In order to improve the reproducibility and ruggedness of the junctions, we have made a systematic study of the influence of the deposition parameters on the oxide properties. The index of refraction, n, was used as a measure of the oxide quality. As n approached that of sapphire (n = 1.76), the resistance to chemical attack by a 20 buffered HF solution improved, indicating a lower pinhole level density. The main parameter that influenced n was the deposition rate. As the rate decreased below 3 /min, n increased to 1.61. Biasing and sputtering pressure had little effect. The O content in the sputtering atmosphere also had little effect on n, but resulted in oxidation of the top surface of the electrode (Co) prior to deposition of AlO, as determined by RBS analysis. This additional CoO layer may cause spin depolarization. The maximum surface roughness of the oxide was about 90 , as measured by AFM. This roughness is similar to that observed by AFM on the bottom electrode surface. To overcome the problems of pinholes and nonuniformity of oxide thickness, much smaller junction areas are being fabricated by lithography. Other insulating materials are also being studied. Results on AlN insulating layers will be reported.

STRUCTURE AND MAGNETORESISTANCE OF Ni/Co MULTILAYERS, J. M. Freitag, Z. Altounian, J. O. Strom-Olsen, McGill Univ, Dept of Physics, Montreal, CANADA; R. W. Cochrane, Univ de Montreal, Dept de Physique, Montreal, CANADA.

Structural and magnetoresistance results on sputtered Ni/Co multilayers are presented. Structural characterization by grazing-angle X-ray reflectivity reveals high-quality layered structures with a well-defined composition modulation along the film growth direction and an interfacial mixing of 2-3 monolayers. The magnetoresistivity (MR) 0.35 cm is roughly constant over the entire compositional range. The MR ratio , which is as high as 3.0%, is therefore more strongly dependent on the zero-field resistivity . By fitting a semi-classical model to the resistivity compositional variation, we determined the interface contribution to the resistivity. The MR measurements as well as the magnetic anisotropy of the films are consistent with the origin of the observed MR effect being anisotropic magnetoresistance (AMR). The small saturation fields in these multilayers ( as small as 40 Oe) lead to high MR sensitivity. The highest sensitivity measured between -10 Oe to +10 Oe is 0.1%/Oe.

EFFECT OF Ru IMPURITIES ON MAGNETORESISTANCE IN Co/Ru/Co(Ru) SANDWICHES, Aziz Dinia, K. Rahmouni, Daniel Stoeffler, S. Wuchner, K. Ounadjela, N. Persat, IPCMS, GEMM, Strasbourg, FRANCE; H. A.M. van den Berg, Siemens AG, Erlangen, GERMANY.

We report on an unexpected inverse giant magnetoresistance (GMR) obtained in Co/Ru/Co sandwiches by adding 8 Ru within the second Co layer. Magnetoresistance (MR) measurements have been performed on Co(32 )/Ru(x = 5 to 30 )/Co(32 ) and Co(32 )/Ru(x)/Co(32) sandwiches prepared by ultrahigh vacuum evaporation onto mica substrates. RHEED patterns performed during the growth indicate that for both series the Co and Ru layers present an hcp (0001) structure. An oscillatory GMR behavior as a function of Ru thickness is observed for the series without Ru impurities. The most interesting result is the induced inverse GMR for all Ru thicknesses for the series with 8 Ru impurities in the second Co layer. This effect can be attributed to the change of the scattering coefficients () between the two interfaces. Indeed, for the first Co layer the scattering coefficient = 1. However, for the second magnetic layer, alloying Co and Ru may induce an increase of the density of states at the Fermi level for majority spin electrons and as a consequence l.

MAGNETIC PROPERTIES OF THIN Eu FILMS ON Gd(0001) STUDIED BY MCD IN PE, E. Arenholz, Freie Univ Berlin, Inst for Exp Physik, Berlin, GERMANY; Kai Starke, Freie Univ Berlin, Inst of Exp Physik, Berlin, GERMANY; G. Kaindl, Freie Univ Berlin, Berlin, GERMANY.

The temperature and layer-dependence of the magnetization in thin Eu films on Gd(0001) were investigated using magnetic circular dichroism (MCD) in photoemission (PE). One monolayer (ML) thick Eu adlayers were found to be magnetically ordered up to T = 300 K, with predominantly parallel orientation of the Eu moments relative to the magnetization of the underlying Gd film. Comparing the temperature dependences of the Eu- 4 and Gd-4 MCD signals with the results of mean-field calculations allowed us to estimate the Eu intralayer and the Eu-Gd Interlayer exchange coupling constants. The results indicate that the Eu intralayer coupling is weakly ferromagnetic or antiferromagnetic, whereas the Eu-Gd interlayer coupling is ferromagnetic and of about the same magnitude and the exchange-coupling constant in bulk Gd. Making use of the layer-dependent core-level shifts, it was then possible to separate spectroscopically the contributions of the monolayer, the interface, the surface, and the bulk to the 4 PE signal of Eu and with that, using MCD in PE, to determine the magnetic properties of the Eu films in a layer-resolved way The first two Eu layers exhibit the same magnetization for Eu layer thicknesses to 2 ML. Moreover, LEED investigations showed that the 6 x 6 hexagonal structure found for the Eu monolayer on Gd(0001) persists for Eu coverages up to 2 ML. A further increase of the Eu coverage leads to a breakdown of the 6 x 6 structure as well as of the ferromagnetic order in the Eu film, except for the interfacial layer.

COMPARISON BETWEEN LINEAR AND CIRCULAR MAGNETIC DICHROISM IN ANGLE-RESOLVED PHOTOEMISSION, Keith W. Goodman, James G. Tobin, Lawrence Livermore National Laboratory, Berkeley, CA; F. O. Schumann, Pennsylvania State Univ, Dept of Physics, State College, PS; T. R. Willis, Pennsylvania State Univ, State College, PA; W. J. Gammon, Univ of Missouri-Rolla, Rolla, MO; S. Mishra, D. P. Pappas, Virginia Commonwealth Univ, Richmond, VA; T. R. Cummins, G. D. Waddill, Univ of Missouri-Rolla, Rolla, MO; Jonathan D. Denlinger, Lawrence Berkeley National Laboratory, Advanced Light Source, Berkeley, CA; Eli Rotenberg, Lawrence Berkeley National Laboratory, Dept of Advanced Light Source, Berkeley, CA; A. Warwick, Lawrence Berkeley National Laboratory, Advanced Light Source, Berkeley, CA; Jeffrey B. Kortright, Lawrence Berkeley National Laboratory, Materials Sciences Div, Berkeley, CA; N. V. Smith, Lawrence Berkeley National Laboratory, Advanced Light Source, Berkeley, CA.

A direct comparison of Magnetic X-Ray Circular Dichroism (MXCD) and Magnetic X-Ray Linear Dichroism (MXLD) in high-resolution, angle-resolved photoelectron spectroscopy has been made. Crucial to the experiment was the development and use of a novel transmission multilayer that serves as a soft x-ray phase retarder (quarter wave plate), converting linearly polarized light to circularly polarized light. Dichroisms were seen in the Fe 3 levels of FeNi/Cu(001) and the Gd 4 levels of Gd/Y(0001). Comparisons will be made between the MXCD and MXLD results for several geometries and sample symmetries. In the case of the fourfold symmetric FeNi/Cu(001), it is possible to choose an experimental geometry where the shape of the dichroism in MXCD and MXLD are predicted and measured to be the same. We will present the result of using this geometry and others. These measurements were made at Beamline 7 of the Advanced Light Source at LBNL. This work was done under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.

MAGNETIC X-RAY LINEAR DICHROISM IN RESONANT AND NON-RESONANT Gd 4f PHOTOEMISSION, S. Mishra, Virginia Commonwealth Univ, Richmond, VA; W. J. Gammon, Univ of Missouri-Rolla, Rolla, MO; D. P. Pappas, Virginia Commonwealth Univ, Richmond, VA; Keith W. Goodman, James G. Tobin, Lawrence Livermore National Laboratory, Berkeley, CA; F. O. Schumann, Pennsylvania State Univ, Dept of Physics, State College, PS; T. R. Willis, Pennsylvania State Univ, State College, PA; Jonathan D. Denlinger, Lawrence Berkeley National Laboratory, Advanced Light Source, Berkeley, CA; Eli Rotenberg, Lawrence Berkeley National Laboratory, Dept of Advanced Light Source, Berkeley, CA; A. Warwick, N. V. Smith, Lawrence Berkeley National Laboratory, Advanced Light Source, Berkeley, CA.

The enhancement of the magnetic linear dichroism in resonant 4f photoemission (MLDRPE) is studied from a 50 monolayer film of Gd/Y(0001). The ALS at Beamline 7 provided the source of linearly polarized x-rays used in this study. The polarized light was incident at an angle of 30 degrees relative to the film plane, and the sample magnetization was perpendicular to the photon polarization. The linear dichroism of the 4f core levels is measured as the photon energy is tuned through the 4d-4f resonance. We find that the MLDRPE asymmetry is strongest at the resonance. Near the threshold the asymmetry has several features which are out of phase with the fine structure of the total yield.

Chair: David D. Chambliss
Friday Morning, April 4, 1997
Golden Gate C2

8:30 AM *M11.1 

The existence of colossal magnetoresistance (CMR) in the doped lanthanum manganese oxides LaMMnO (where M is a divalent ion) has been known for some time, but the recent intense interest follows the discovery of approximately a factor of 10 suppression of the resistance in thin films and their possible application in magnetic recording technology. The driving force for the ferromagnetic (FM) transition, and the coincident metal-insulator (MI) transition, is thought to be a double-exchange (DE) interaction where the mobility of the conduction electrons between heterovalent Mn/Mn pairs is greatly enhanced when the magnetic moments on adjacent Mn ions are aligned. The mixed valency also leads to the formation of small polarons, arising from Mn/Mn valence changes and Jahn-Teller (JT) distortions involving Mn that leads to incoherent hopping and high resistivity in the insulating phase. Many studies of both the crystal structure and the local structure of the LaMMnO system have now been reported. As expected, these studies show a strong coupling of the lattice to the CMR transition. On the other hand, structural analysis with powder neutron diffraction data and x-ray absorption studies for a sample of the TMnO pyrochlore, also exhibiting CMR, shows no deviations from ideal stoichiometry, mixed valency, or JT distortions of the MnO octahedron, all of which were thought to play an essential role in perovskite CMR materials. We will review the results of our crystallographic and local structural studies of these two important classes of CMR materials, compare the differences in structural response, and discuss the implications of our findings to our theoretical understanding of these materials.

9:00 AM M11.2 
MAGNETIC AND TRANSPORT PROPERTIES OF PURE AND Fe SUBSTITUTED LaSrMnO COMPOUND, Krzysztof Rogacki, Bogdan M. Dabrowski, Clyde W. Kimball, Andrei Tkachuk, Northern Illinois Univ, Dept of Physics, DeKalb, IL.

We have prepared a series of pure and Fe substituted La samples using solid state and wet chemical synthesis techniques. The magnetic and transport properties were studied in the temperature range 5-400 K and in magnetic fields to 7 T. Grain size, defect concentration and surface properties were controlled by the synthesis temperature in the range 600-1300 C and the oxygen pressure in the range 10 - 10 atm. Properties were related to the vacancy concentration on the A (La) and the B (Mn) sites of the perovskite structure by using the thermogravimetric measurements. For the LaMn sample, using high oxygen pressure annealing, the metallic phase was achieved below the transition temperature, T 90 K, distinctly different front the Curie temperature, T 160 K. Presence of vacancies (about 8) on the electronically active Mn-O network may decouple these transitions. Thus, in addition to the ferromagnetic phase and the rhombohedral structure, other factors-for example, the charge localization, defect concentration, and the grain size-may control the presence of the Colossal Magnetoresistive effect (CMR). For compositions 0 < x < 0.3, it is also possible to introduce a substantial amount of vacancies on metal sites using strongly oxidizing conditions. For similar annealing conditions, Fe substitution suppresses T by 15 K/at.. However, how the Fe substitution affects transport properties depends sensitively on the amount of strontium doping. It is possible to tune magnetic and transport properties and to optimize the CMR effect by using the double Sr and Fe substitution in the LaMnO material, and by controlling the synthesis conditions. This work was supported by DARPA/ONR and the State of Illinois under HECA.

9:15 AM M11.3 
GROWTH OF HETEROSTRUCTURE THIN FILMS OF COLOSSAL MAGNETORESISTANCE LaCaMnO AND SOFT FERRITE (Mn, Zn)FeO, Jingsong Wang, Lawrence Berkeley National Laboratory, Berkeley, CA; Hauyee Chang, Lawrence Berkeley National Laboratory, Molecular Design Inst, Berkeley, CA; Peter G. Schultz, Univ of California-Berkeley, Dept of Chemistry, Berkeley, CA; X.-D. Xiang, Lawrence Berkeley National Laboratory, Inst of Molecular Design, Berkeley, CA.

We report the growth and characterization of heterostructure thin films of colossal magnetoresistance, LaCaMnO and soft ferrite (Mn,Zn)FeO. The thin films were deposited layer by layer with corresponding oxide targets by RF-sputtering and followed by post annealing. The buffer layers of NiMnO were first deposited on MgO(100), LaAlO(100), SrTiO(100) and MgAlO(100) substrate and then the ferrite layers, and finally CMR layers. The ferrite layers on SrTiO(100) exhibited better crystallinity and stronger magnetization than those on MgO(100), LaAlO(100), and MgAlO(100). The amplifying effect of ferrite magnetic layer on CMR layer were observed. Such heterostructure thin film can be developed for the device application of CMR effect in low magnetic field.

9:30 AM M11.4 
NEAR EDGE XAS STUDIES OF LaAMnO SYSTEMS, Parviz H. Ansari, Seton Hall Univ, Dept of Physics, South Orange, NJ; Mark Croft, D. Sills, Rutgers Univ, Dept of Physics, Piscataway, NJ; M. Greenblatt, Z. Zuotao, Rutgers Univ, Dept of Chemistry, Piscataway, NJ; S.-W. Cheong, Bell Labs, Lucent Technologies, Murray Hill, NJ; K. V. Ramanujachary, D. Tran, Rowan College of New Jersey, Dept of Chemistry & Physics, Glassboro, NJ.

The question of charge transfer from the Mn-d orbitals in LaAMnO systems is crucial to understanding the interplay metal-insulator, magnetic, and charge ordering transitions in these materials. To address this issue Mn-K, near-edge, x-ray absorption spectroscopy (XAS) measurements have been performed on the A = Ca, Eu, and Pb substituted systems. Detailed results on the chemical shift and pre-edge feature spectral strength (both indicators of the Mn-d hole count) for the A = Ca system are reported. These results indicate a Mn-d hole count response which increases (with increasing x) for x<0.2, but which arrested for x<0.2. This behavior is discussed in terms of the structural variation with x and of the magnetic phase diagram. The temperature dependence of the chemical shift across this system will also be discussed. For the A = Eu system in addition to reporting structural and Mn-K edge results, Eu-L edge measurements clearly show the Eu character of the substitute. Finally the Mn-K results on the A = Pb system manifest: a pre-edge variation consistent with an increase in Mn-d hole count with increasing x : and a main edge with substantially more structure than those of the other substitutes.

9:45 AM M11.5 
RESISTANCE NOISE IN THE LARGE MAGNETORESISTANCE MANGANITES, Glenn Alers, A. P. Ramirez, Sungho Jin, Bell Labs, Lucent Technologies, Murray Hill, NJ; Yuri Suzuki, Cornell Univ, Dept of MS&E, Ithaca, NY.

Resistance noise has been measured in thin films of the large magnetoresistive materials La and related materials. The resistance fluctuations were found to have a 1/f power law form throughout the ferromagnetic-metal to paramagnetic-insulator transition where f is the frequency. This intrinsic 1/f noise was much larger than what is typical for metallic films. The magnitude and frequency dependence are compared with what would be expected from the fluctuation-dissipation theorem and spontaneous magnetization fluctuations of domains in the ferromagnetic phase. We show that the 1/f noise is proportional to the magnetoresistance coefficient of the material and will therefore set a fundamental limit on the low frequency signal to noise ratio of sensors made with these materials. The equivalent magnetic field noise at low frequencies was found to be much larger in these materials than in metallic multilayer GMR materials. At high frequencies the sensor sensitivity is limited by the Johnson noise of the material. These materials have a very high resistivity, but the ultimate signal to noise ratio for magnetic sensors was found to be independent of resistivity because the signal also increases with resistivity. Comparisons are made for the intrinsic 1/f noise in films with different growth conditions and materials properties including grain orientation and thermally induced strains.

10:30 AM *M11.6 
SPIN-DEPENDENT TRANSPORT IN MAGNETIC NANO-STRUCTURES, S.S.P. Parkin, A. C. Marley, S. Gider, R. A. Fontana, IBM Almaden Research Center, San Jose, CA; Y. Lu, G. Xiao, W. J. Gallagher, IBM T.J. Watson Research Ctr, Yorktown Heights, NY.

The resistance of magnetic tunnel junctions (MTJ), which comprise two ferromagnetic layers separated by a thin insulating AlO barrier, depends on the relative orientation of the magnetic moments of the ferromagnetic layers. The tunneling current across the junction is typically higher when the magnetic moments are arranged parallel to one another than when the moments are antiparallel, which can give rise to substantial magnetoresistance. A variety of structures and materials have been explored in MTJs with cross-sectional areas of 80 x 80 m. These structures are prepared using dc magnetron sputtering in a uhv sputtering system in which up to six or more contact masks can be placed in contact with any of 20 substrates under computer control. The AlO barrier is prepared by in-situ plasma oxidation of a deposited Al layer. By exchange-biasing one of the ferromagnetic layers in the MTJ, two well-defined states of the MTJ can be defined in which the magnetic moments of the two magnetic layers are either antiparallel or parallel to one another. MR values of more than 25 and 40 are found respectively at room temperature and 4.2 K. The dependence of MR on the ferromagnetic materials and the thickness and oxidation time of the Al layer will be discussed. Using a simple self-aligned process, MTJ devices have been fabricated to submicron sizes. The MR of these devices is similar to that of the larger junctions made directly by contact masks. Indeed, the MR is found to be independent of junction area for junctions ranging in size from 0.1 x 0.6 m to 100 x 100 m. The resistance of the MTJs scales approximately as the inverse area of the junction and can be readily varied by several orders of magnitude by varying the Al thickness and oxidation time.

11:00 AM M11.7 

We have investigated the structural, magnetic and transport properties of three types of magnetic metal/insulator systems which show magnetoresistance effects associated with tunneling of spin polarized electrons: (i) tunnel junctions comprising MgO or HfO oxide barriers (20 to 100 thick), (ii) Cermet films consisting of Co or CoFe particles embedded in an insulating SiO or HfO matrix, (iii) discontinuous metal/insulator multilayers consisting of layers of metallic particles (Co or CoFe) dispersed in an insulating matrix (SiO or HfO). All oxide layers have been deposited either by reactive sputtering from a metallic target or RF sputtering from an insulating oxide target. Magnetoresistance effects as large as 30 at low temperatures have been observed in HfO tunneling junctions with resistances in the range of k to M. Cermet films are quite easy to prepare and show magnetoresistance up to 8 at room temperature. However, they have poor field-sensitivities due to their high saturation fields. The discontinuous metal/oxide multilayers actually combine the large field sensitivity of tunnel junctions with the ease of preparation and ruggedness of Cermet films. Their transport properties are intermediate between those of tunnel junctions and of Cermet films. Furthermore, the comparison of their current-in-plane and current-perpendicular-to-the plane properties give insight into the intra and inter-plane couplings between particles. 
We will present a comparative study of the properties of these three types of systems.

11:15 AM M11.8 
SPIN DEPENDENT TUNNELING WITH COULOMB BLOCKADE, Frederic Petroff, Fettar Farid, Fernando Schelp, Paul Holody, Thomson-CNRS, UMR, Orsay, FRANCE; Jean-Luc Maurice, CNRS, Orsay, FRANCE; Annie Vaures, Thomson-CNRS, CSF-LCR, Orsay, FRANCE; Shang-Fan Lee, Albert Fert, Thomson-CNRS, UMR, Orsay, FRANCE.

We have fabricated tunnel junctions consisting of two ferromagnetic electrodes (Co or NiFe layers) separated by an insulating layer (AlO) in which one or several layers of nanometric cobalt clusters are imbedded. The Co clusters layers are prepared by alternate sputtering of Co and AlO (discontinuous multilayer). TEM characterization show that the clusters are roughly spherical with a narrow distribution of size. The I(V) curves and their temperature dependence are characteristic of Coulomb blockade effects; the gap appearing in the I(V) curves increases when the size of the Co clusters is reduced. For sizes around 2 nm, the gap reaches the tenth of eV and the tunnel resistance typically increases from 5 k at RT to 0.5 M at 4.2 K. In magnetoresistance (MR) measurements, we observe a resistance peak at the coercive field of the ferromagnetic electrodes. The MR-ratio is generally in the range 4-7 (even for resistances approaching 1 M) but reaches 20 in some samples. At room temperature, the MR effects can be obtained in a field range of about 10 Oe. This type of junction is interesting not only for application as magnetoresistive sensor with very high resistance, but also probably for fundamental studies of Coulomb blockade phenomena.

11:30 AM M11.9 
{MAGNETOREFRACTIVE EFFECT IN NiFe/Cu/NiFe SPIN VALVES, Jacqueline van Driel, J.A. J. Jansen, Philips Research Laboratories, Eindhoven, NETHERLANDS; Reinder R. Coehoorn, Philips Research Laboratories, Dept of Magnetism, Eindhoven, NETHERLANDS; J.C. S. Kools, Philips Research Laboratories, Eindhoven, NETHERLANDS; F. R. de Boer, Univ of Amsterdam, Van der Waals-Zeeman Inst, Amsterdam, NETHERLANDS.

The refractive index for the transmission of infrared light through a thin metal film depends on the conductivity of the material. Because of the low energy of the photons, only radiation absorption by intraband excitations is possible. In GMR materials it is possible to change the conductivity, and therefore also the refractive index, by switching the magnetization directions in a magnetic field, leading to a new phenomenon called the magnetorefractive effect. Using this effect it is possible to investigate spin valves or multilayers without electrically contacting them. Jacquet and Valet [1] have already shown this effect to exist in NiFe/Cu/Co/Cu multilayers. We have done measurements for NiFe.

Chair: Wim J.M. de Jonge
Friday Afternoon, April 4, 1997
Golden Gate C2

1:30 PM *M12.1 

Results of first-principles calculations for the interlayer coupling in Fe/Cr/Fe and Fe/Mo/Fe sandwiches are presented. The oscillation periods of the interlayer coupling that are expected from the respective Fermi surface calipers are well reproduced by our fitting procedure and are in excellent agreement with experiment. We investigated the influence of the lattice constant on the interlayer coupling and the dependence of the interlayer coupling amplitude on the Fe and Cr magnetic moments. We find that a spin-density wave in Cr gives rise to an additional contribution to the interlayer coupling. By inserting buffer layers of different materials between the Fe/Cr interface it is possible to tune the behavior of the interlayer coupling. We calculated the effect of interface adlayers of Cu and Ag on the interlayer coupling of a Fe/Cr/Fe sandwich. The Cu adlayers change the interlayer coupling amplitude dramatically, whereas the Ag adlayers have no appreciable effect, both in excellent agreement with recent experiments. The mechanism of the adlayer tuning is explained in terms of hybridization between Fe and the respective adlayer. The results were obtained with first principles local spin-density calculations by means of a scalar-relativistic, spin-polarized Green's function technique for interfaces developed by H.L. Skriver. The technique is based on the linear muffin tin orbitals (LMTO) method within the tight-binding, frozen core, and atomic sphere approximations.

2:00 PM M12.2 
MAGNETIC MICROSCOPY OF FE/MN/FE LAYERS17882, Herbert Hopster, Y. Iwasaki, J. Barthel, E. B. Maiken, B. P. Miller, J. Kondis, Y. Yu, Univ of California-Irvine, Dept of Physics, Irvine, CA.

Mn layers were deposited on polycrystalline Fe-based soft magnetic films. The magnetism of thin (5-50 ) overlayers of Fe on these Mn films was measured by secondary electron microscopy with polarization analysis. The Fe overlayers show spin polarization only beyond a certain critical Fe thickness at which the domain structure of the substrate starts reappearing in the Fe overlayers. This is attributed to a frustrated magnetic ordering in the Fe films due to the strong coupling to the Mn moments. For all Mn thicknesses studied (up to 170 ) the coupling between the substrate and the Fe overlayer is always purely -magnetic.

2:15 PM M12.3 
EFFECT OF CAP-LAYERS ON INTERLAYER EXCHANGE COUPLING, Josef Kudrnovsky, Vaclav Drchal, Inst of Physics, Dept of Theory of Condensed Matter, Praha 8, CZECH REPUBLIC; Patrick Bruno, Univ de Paris-Sud, Inst d'Electronique Fundamentale, Orsay, FRANCE; Reinder R. Coehoorn, Philips Research Laboratories, Dept of Magnetism, Eindhoven, NETHERLANDS; Jitze Jan de Vries, Eindhoven Univ of Technology, Dept of Physics, Eindhoven, NETHERLANDS; Peter Weinberger, Technical Univ of Vienna, Inst of Technical Electrochemistry, Vienna, AUSTRIA.

The effect of non-magnetic cap-layer on periods, amplitudes, and phases of oscillations of interlayer exchange coupling (IEC) is studied theoretically on ab initio level. We employ the spin-polarized surface Green function technique within the tight-binding linear muffin-tin orbital method and the Lloyd formulation of the IEC. Application is made to Co/Cu/Co(001) trilayers with Cu-cap layer interfacing the vacuum through the dipole barrier. We investigate in detail both the asymmetric case with one semiinfinite Co-slab and other Co-slab five monolayers thick as well as symmetric cases of Co-slabs each of which is thick one or five monolayers. In all cases we have found pronounced oscillatory behavior of both the amplitudes and phases of the IEC oscillations with the cap-layer thickness. For the thick cap-layer standard results of corresponding infinite systems are recovered. The results are interpreted in the frame of a simple RKKY-like model of the IEC. 0.2cm

2:30 PM M12.4 
STUDY OF PERPENDICULAR INDUCED MAGNETIC MOMENTS IN THE Ag OF Ag/Fe MULTILAYERS, Titia Phalet, Leuven Univ, Institut voor Kern-en Stralingsfysica, Leuven, BELGIUM; J. Dekoster, P. De Moor, P. Schuurmans, N. Severijns, M. Trhlik, A. Van Geert, L. Vanneste, B. Vereecke, Leuven Univ, Inst voor Kern-en Stralingsfysica, Leuven, BELGIUM; W. D. Brewer, Freie Univ Berlin, Inst fur Experimentalphysik, Berlin, GERMANY.

The induced magnetic moments in the non-magnetic Ag spacer layers in Ag/Fe multilayers have been measured using low temperature nuclear orientation (LTNO). The magnetism induced in such spacer layers has only been studied in a few cases. The multilayers were grown on a MgO(100) substrate with MBE and the process was followed with in-situ RHEED analysis. The measured samples, [Fe(10 ML)/Ag(4 ML] and [Fe(10 ML)/Ag(2 ML)], were irradiated in a thermal neutron flux of 10n/cm/s, which activated the Ag nuclei into Ag. The samples were connected to the cold finger of a He-He dilution refrigerator and cooled into the mK region. The anisotropy of the Ag -rays was measured as a function of the externally applied magnetic field with two germanium detectors placed at 0 and 90 with respect to the direction of the magnetic field. Both the multilayers show induced magnetic moments pointing out of the plane of the multilayer in zero and low external magnetic field. When the external field is raised, the induced magnetic moments turn until they are in the plane of the multilayer at an external field of 0.4 T. We calculated the angle between the direction of the induced magnetic moments and the direction of the external field using data from both detectors. Other measurements on multilayers with variable number of Ag and Fe monolayers are made and measurements with NMR on oriented nuclei ((NMR/ON) are in progress.

2:45 PM M12.5 
EXCHANGE COUPLING IN SINGLE CRYSTALLINE SPINEL STRUCTURE FERRITE BILAYERS, Yuri Suzuki, Cornell Univ, Dept of MS&E, Ithaca, NY; R. B. van Dover, R. J. Felder, Bell Labs, Lucent Technologies, Murray Hill, NJ.

We have grown high quality single crystalline spinel structure ferrite thin films and heterostructures by introducing a buffer layer between the spinel ferrite and the conventional oxide substrates. The introduction of a buffer layer facilitates low temperature growth of the ferrite layers, thus precluding substantial intermixing at the interfaces in heterostructures. Analyses based on x-ray diffraction, Rutherford backscattering, atomic microscopy and transmission electron microscopy provide a consistent picture of the structural properties of these films. Good magnetic properties are observed only in films with high structural quality. We have observed strong exchange coupling between single crystalline spinel structure ferrite thin films of magnetically hard CoFeO and magnetically soft (Mn,Zn)FeO at a variety of temperatures below the Curie temperature of the soft (Mn,Zn)FeO layer. These single crystalline bilayers allow the investigation of exchange coupling with the ambiguity of polycrystallinity as in NiFe/MnFe alloy bilayers. The exchange coupling constant of the interface that is estimated from the exchange biased (Mn,Zn)Fe2O4 loops is comparable to estimated exchange coupling constants within each individual ferrite layer. By contrast, the interface exchange coupling constant in polycrystalline NiFe/MnFe bilayers is at best two orders of magnitude smaller than estimates of the exchange constant of NiFe. Furthermore, studies of the temperature dependence of the exchange coupling reveal a nonmonotonic dependence of the field required to switch the soft (Mn,Zn)FeO layer with temperature.

3:30 PM *M12.6 
THE OBSERVATION OF AN EXPONENTIAL FeSi SPACER THICKNESS DEPENDENCE OF THE ANTIFERROMAGNETIC EXCHANGE COUPLING IN Fe/Si-BASED MULTILAYERS, Juergen T. Kohlhepp, Eindhoven Univ of Technology, Dept of Physics, Eindhoven, NETHERLANDS; Jitze Jan de Vries, Eindhoven Univ of Technology, Dept of Physics, Eindhoven, NETHERLANDS; Frits F.J.A. den Broeder, Philips Research Laboratories, Eindhoven, NETHERLANDS; Reinder R. Coehoorn, Philips Research Laboratories, Dept of Magnetism, Eindhoven, NETHERLANDS; Wim W.J.M. de Jonge, Eindhoven Univ of Technology, Dept of Physics, Eindhoven, NETHERLANDS; Reiner M. Jungblut, Philips Research Laboratories, Eindhoven, NETHERLANDS.

Oscillatory interlayer exchange coupling in metallic multilayers (MLs) has been well-established and is well-understood by now. However, the interlayer coupling in MLs with (nominally) semiconducting spacers is still unclear and much less explored. One of the few systems which has attracted considerable attention are Fe/Si MLs. However, up to now experimental investigations have produced numerous conflicting results concerning the structure of the spacer and the dependence of the coupling strength on the spacer thickness and on the temperature. We have performed a systematic study of the spacer-thickness and temperature dependence of the coupling strength in Fe/Si-based systems employing sputtered MLs with variations of Si-doped Fe for the magnetic layers and Fe-doped Si spacers, as well as Fe/Si-wedge/Fe(-wedge) sandwiches, MBE-grown on Ge(100), S-passivated Ge(100) and Fe-whiskers. From a structural analysis of the MBE-grown samples with LEED and AES and from a magnetic analysis with the magneto-optical Kerr effect, it is concluded that the Si spacer-layer transforms into a crystalline, epitaxial silicide with a composition close to FeSi for Å (metallic B2(CsCl-structure)-FeSi). Furthermore, the antiferromagnetic (AF) coupling strength depends on the spacer thickness in an exponential, i.e. non-oscillatory, manner with a characteristic decay length of 3-4 Å. However, due to pinholes (direct ferromagnetic coupling), which mask the intrinsic coupling at small spacer thicknesses, and the formation of amorphous Si (weak or no coupling) at large spacer thicknesses, the thickness dependence can be easily misinterpreted as a single peak of an oscillation. Although, the observed exponential behavior can be understood within the Bruno model of complex Fermi-surfaces, the additionally observed increase of the coupling strength with decreasing temperature is incompatible with this model. Thus, the thickness and the temperature dependences of the coupling in Fe/Si-based systems addresses a new aspect of interlayer exchange coupling in magnetic MLs and asks for new or improved theoretical models.

4:00 PM M12.7 
BILINEAR AND BIQUADRATIC COUPLING IN Fe/Cr/Fe(110) TRILAYERS, Hans-Joachim Elmers, Joerg Schwabenhausen, Tobias Duerkop, Technische Univ Clausthal, Dept of Physics, Clausthal, GERMANY.

We investigated the indirect exchange coupling in W(110)/Fe/Cr/Fe/Cr films. Magnetic in-plane interface anisotropies of different sign allow the preparation of Fe films with orthogonal uniaxial anisotropies. Then, the bilinear () and biquadratic () exchange coupling term can be determined from magnetization curves independent of magnitude and sign[1]. We determined and as a function of Cr interlayer thickness ( nm) and temperature (T=100-300 K). We varied the morphology of the interlayer as determined by spot profile analysis LEED (SPALEED) using different substrate temperatures ( K) during evaporation of the Cr spacer. For below room temperature, we found a pronounced maximum of antiferromagnetic coupling mJ/m and a considerable contribution of biquadratic coupling, mJ/m. Our results are in good agreement with exchange coupling values predicted by ab initio theories[2], taking into account the morphology of the interlayer.

4:15 PM M12.8 
EXCHANGE COUPLING AND GIANT MAGNETORESISTANCE IN ELECTRODEPOSITED Co/Cu MULTILAYERS, Aziz Dinia, IPCMS, GEMM, Strasbourg, FRANCE; H. El Fanity, CNRS, Faculte des Siences, Maroc, FRANCE; K. Rahmouni, IPCMS, GEMM, Strasbourg, FRANCE; M. Bouanani, CNRS, Faculte des Siences, Maroc, FRANCE; G. Shmerber, CNRS, IPCMS-GEMM, Strasbourg, FRANCE; A. Berrada, CNRS, Faculte des Siences, Maroc, FRANCE.

We present the results of the transport and magnetization measurements of electrodeposited Co/Cu multilayers grown in a single electrolyte based on CoSO H and CuSO. The samples are deposited on glass substrate covered by a 500 thick Cu buffer layer. X-ray diffraction performed on the samples show fcc structure of both Co and Cu layers. Resistivity measurements show a giant magnetoresistance effect of about 4 at room temperature for multilayers with Co and Cu thicknesses between 40t and 30t, respectively. For Co thicknesses t, the magnetoresistance completely vanishes, indicating that there is no more continuous Co layer. Magnetization curves are characteristic for canting rather than antiferromagnetic coupling between the Co magnetic layers. Indeed, the hysteresis loops are oblique with a curvature around saturation which is reached at about 2 kOe.

4:30 PM M12.9 
NONCOLLINEAR PHASE OF MAGNETIC MULTILAYERS, Alexander S. Sigov, Moscow Inst of Radioengineering, Dept of Electronics, Moscow, RUSSIA; Alexander I. Morosov, Moscow Inst of Radioengineering, Dept of Condensed Matter Electronics, Moscow, RUSSIA.

Numerous experimental data bear witness to a noncollinear state arising under certain conditions in magnetic multilayers. The chief cause for such a state is the interlayer boundaries roughness. The presence of atomic steps at the boundaries may result in different signs of the interlayer exchange interaction of different surface regions. Many authors account for this effect by introducing the biquadratic exchange interaction between magnetic moments of the layers. ln so doing so, one assumes the magnetization in each layer to remain homogeneous. We show that if the characteristic dimension of the step is over some critical value (the domain wall width ), the magnetic layers are subdivided into microdomains with parallel and antiparallel orientation of magnetizations of neighboring layers. The biquadratic exchange approximation is valid in the opposite limiting case . And yet the diminution of results automatically in a decrease in the biquadratic exchange interaction J (J ). At the same time, a noncollinear state appears when J is comparable to the bilinear exchange constant J. We obtain the ''exchange-roughness'' phase diagram and prove the two conditions mentioned above ( and J > J) to meet simultaneously within an extremely narrow arca of the multilayer parameters. Therefore, a noncollinear phase initiation is indicative of the microdomains formation in the magnetic layers. Such a microdomain state assumption might form the basis of experimental data interpretation.