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Spring 1999 logo1999 MRS Spring Meeting & Exhibit

April 5-9, 1999 | San Francisco
Meeting Chairs: Katayun Barmak, James S. Speck, Raymond T. Tung, Paul D. Calvert

Symposium J—Patterned Magnetic Structures and Magnetoelectronics


Gernot Guntherodt 
II Physikalisches Inst 
RWTH Aachen 
Aachen, D-52056 GERMANY 

T. Miyazaki
Dept of Applied Physics
Tohoku Univ
Graduate School of Engineering
Sendai, 980-8579 JAPAN

Stuart Parkin 
IBM Almaden Research Center 
San Jose, CA 95120-6099 

Michael Roukes
Condensed Matter Physics
California Inst of Technology
MS 114-36
Pasadena, CA 91125

Symposium Support 
*Alps Electric Co., Ltd., Japan 
*Asahi Komag Co., Ltd. 
*Hitachi, Ltd. 
*NEC Corporation 
*Showa Denko K.K. 
*SONY Corporation 
*TDK Corporation 
*ULVAC Japan, Ltd.

* Invited paper
Chair: Stuart S. P. Parkin 
Monday Morning, April 5, 1999 
Salon 3 (M)
9:00 AM *J1.1 
SPINTRONICS. Stuart Wolf , DARPA, DSO, Arlington, VA.

In this presentation I will describe the concept of spin dependent transport and it's various manifestations including Giant Magneto-Resistance (GMR), Spin Dependent Tunneling (SDT), and Spin Injection Devices. I will then discuss the memory, logic, and sensor applications of these concepts that may provide revolutionary new technology for the 21st Century. 

Chair: Michael L. Roukes 
Monday Morning, April 5, 1999 
Salon 3 (M)
10:15 AM *J2.1 

The prospect of semiconductor-based spin-injection devices is exciting. From a physics standpoint, control of a semiconductor spin-polarized current offers possibilities for experiments in spin dynamics without the use of high magnetic fields or optical-polarization schemes, and for the investigation of spin-scattering mechanisms in tunable-density (gated) devices. From an engineering standpoint, these same transistor-like devices are intriguing as novel memories or switches. However, despite the discovery of spin injection phenomena in all-metal systems over a decade ago, the unambiguous detection of spin-coupled transport in a ferromagnet / semiconductor system has proven difficult. Reasons for this include: 1) The low carrier density of a semiconductor two-dimensional electron gas (2DEG) results in a high sensitivity to fringe magnetic fields from the thin-film ferromagnets used as spin-filters; 2) The formation of ferromagnetic contacts to a semiconductor poses special problems; 3) Spin-scattering mechanisms in a high-mobility InAs 2DEG are fundamentally different from those in metals. The first issue requires one to minimize the interaction between magnetic fields at the edge of a ferromagnet and the underlying 2DEG, since this interaction leads to hysteretic phenomena that can mask spin-dependent phenomena. (This interaction is used beneficially in other devices to create simple bipolar non-volatile memory elements.) The second issue constrains one to using InAs (to which non-alloyed ohmic contacts are made), and to using a ferromagnetic material and deposition technique that minimizes interfacial spin scattering in the ferromagnet / InAs contact area. Assuming both of these challenges are addressed, the third issue affects one's expectations for the evidence of spin-coupled transport and leads one to fabricate small (order 1 m) devices. It also suggests that one can control the transport of spin-polarized electrons with a FET-like electronic gate. Progress and remaining challenges in these areas will be discussed.

10:45 AM *J2.2 
ROOM TEMPERATURE OPERATING SPIN VALVE TRANSISTORS MADE BY VACUUM METAL BONDING. Douwe Monsma , Stuart Parkin, IBM Almaden Research, Science and Technology, San Jose, CA.

Functional integration between semiconductors and ferromagnets is demonstrated with the spin-valve transistor concept. The spin valve transistor consists of a spin valve sandwiched between two potential barriers. The emitter barrier is used to inject hot electrons. The collector barrier accepts only electrons which travel ballistically through the spin valve base, making the collector current a direct measure of the perpendicular hot electron mean free path in the spin valve. The operative electron energy range fits the spin-split bands of ferromagnetic transition metals, making it well suited for spectroscopic studies. The solid state structure and beneficial transport properties even make it a sensitive magnetic field sensor. Room temperature operation is accomplished by preparing lithographically patterned Si-Co/Cu/Co-Si devices using vacuum metal bonding(*). This new technique proceeds at room temperature and opens a way to combine uncommon materials, to realize many innovative vertical transport device ideas and forms a permanent link for demanding adhesion applications. (*)Science vol. 281,(1998)407

11:15 AM J2.3 

We propose a new system whose magnetization direction can be controlled by an applied bias voltage without an external magnetic field. The possibility of the existence of this new type of controllable exchange coupling in a (ferromagnetic/spacer/insulator/ ferromagnetic) system is derived within a simple free-electron-like, one-dimensional approximation. An analytic expression for controllable exchange coupling is presented. According to the prediction, the exchange coupling energy between two magnetic layers oscillate from antiferromagnetic to ferromagnetic configuration with applied bias voltage. This implies that we can switch/rotate the magnetization direction without an external magnetic field. Possible applications of such a novel system are discussed.

11:30 AM *J2.4 
NANOCONTACT MEASUREMENTS OF ELECTRON SPIN FILTERING AND SPIN TRANSPORT. S.K. Upahdyay, P. Chalsami, R.N. Louie and R.A. Buhrman , Cornell University, School of Applied and Engineering Physics, Ithaca, NY.

Andreev reflection of electrons with energy below the superconducting energy gap, and the sensitivity of this enhancement to any interfacial scattering, provides a powerful means of measuring interfacial transmission rates of electrons. The degree of Andreev reflection is also quite sensitive to any net spin polarization in the non-superconducting electrode. Thus very small ferromagnet-superconductor (F-S) contacts can, if properly made, be used to quantitatively measure both the spin polarization of the electron current emerging from the ferromagnet, and the interfacial transmission probability for each spin orientation. By forming a normal metal nanocontact to a superconductor that has been overcoated by an ultra-thin ferromagnetic layer, this Andreev technique can also be extended to the determination of the spin-dependent transmission rates through magnetic layers of different thicknesses. We have produced such F-S and N-F-S nanocontacts lithographically and have determined the net spin-polarization of the direct current emerging from several bulk ferromagnetic films, and the spin filtering behavior of ultra-thin ferromagnetic layers ranging from 0.2 nm to > 3 nm. The N-F-S studies directly demonstrate that, at least for Co, the spin-dependence of the N-F interface transmission is the dominant spin-filtering effect. Measurements with different N electrodes illustrate the importance of the band structure differences in determining the amplitude of the spin-filtering. By examining the bias dependence of the nanocontact resistance in the normal state, we can also examine the degree to which the interface results in inelastic scattering processes than can impact the conservation of electron spin during transport in magnetic multilayer systems. In this talk I will review these nanocontact experiments and discuss our recent results. 

Chair: J.A.C. Bland 
Monday Afternoon, April 5, 1999 
Salon 3 (M)
1:30 PM *J3.1 
30 NM-SIZED NANOSTRUCTURES OBTAINED BY BEAM IRRADIATION MAGNETIC PATTERNING. T. Devolder , C. Chappert, Institut dí Electronique Fondamentale, Université Paris Sud, Orsay, FRANCE; H. Bernas, Centre de Spectroscopie Nucléaire et de Spectrométrie de Masse, Université Paris Sud, Orsay, FRANCE; Y. Chen, E. Cambril, Laboratoire de Microstructures et Microlectronique, Bagneux, FRANCE; J.P. Jamet, J. Ferré, L. Belliard, Laboratoire de Physique des Solides, Université Paris Sud, Orsay, FRANCE.

In a previous paper [1], we have shown that light ion (He+) irradiation can modify in a precisely controlled way the magnetic properties of multilayers, with negligible change of surface roughness and optical indices. In (Co/Pt) multilayers with perpendicular easy magnetization axis, the anisotropy decreases with the irradiation fluence. This first reduces the coercive force for perpendicular hysteresis loops, then induces in-plane magnetization. We demonstrated that magnetic patterning at the micron scale is obtained when the irradiation is performed through a lithographically defined PMMA resist mask. Using a new kind of mask, we have now fabricated regular arrays of such irradiation-patterned nanostructures with sizes down to 30 nm. Because the irradiation method leads to negligible optical contrast, far field magneto-optical microscopy allowed us to observe magnetization reversal processes, even though its lateral resolution is only about 500 nm. Striking images of nucleation and propagation in such arrays will be displayed. In a set of such nanostructures the spread of coercivity is found to be much smaller than it is for etched nanostructures, which is of crucial importance for applications. The method is very similar to planar technologies for semiconductors and thus is entirely compatible with massive production standards. Therefore it may be a powerful tool for ultrahigh density magnetic recording applications. [1] C. Chappert et al., Science 280, 1919 (1998).

2:00 PM J3.2 
MAGNETIC NANOSTRUCTURES FABRICATED BY ELECTRON BEAM-INDUCED ORGANOMETALLIC CVD. Hong Jiang , Brian W. Robertson, Univ. of Nebraska-Lincoln, Dept of Mechanical Engineering, Lincoln, NE.

Electron beam-induced organometallic chemical vapor deposition (e-OMCVD) offers a highly controllable deposition process for fabrication of research and custom nanoscale one and zero dimensional magnetic, electronic and hybrid structures with least dimensions near 10nm, at which other methods prove difficult or costly. Using focused electron beams in a modified field-emission scanning transmission electron microscope (STEM), we have deposited submicron Fe- and Ni-containing pads and wires, with uniform thickness and well-defined shapes. The best edge acuity of the pads obtained consistently so far is 4nm and the corresponding narrowest wires are 8nm wide. The deposition involves surface-mediated, electron-induced decomposition of metallocenes under near-UHV conditions on carbon and silicon film substrates and results in deposition of metal atoms only in electron beam selected substrate locations. This method uses no polymer coating of the substrate and thus avoids the possibility of surface modification of the nanomagnetic materials by contact with a polymer coating. The method is scaleable using projection instead of focused beam electron optics. We report on the investigation of the deposition process and on the structural, compositional, and magnetic characterization of the resulting deposited features, using STEM, TEM and other methods. The e-OMCVD results are compared with those obtained using UV and x-ray radiation for OMCVD of submicron magnetic features as part of a collaborative program on the size and shape effects that may limit the performance of nanoscale magnetic memory and logic.

2:15 PM J3.3 
MICROMAGNETIC MODEL OF PATTERNED SOFT MAGNETIC STRUCTURES. C.N. Borca , Ralph Skomski and P.A. Dowben, University of Nebraska, Department of Physics and Astronomy, NE.

We are studying micron scale features of different shapes and sizes of ferromagnetic soft magnetic materials. These features are fabricated by direct-writing i.e. by selective area deposition from organometallic compounds. We have developed this technique sufficiently to selectively deposit large arrays of pure metal features with excellent spatial resolution. Such features can be also deposited in multilayer geometries. We studied Ni patterned structures with different shape and sizes using MFM, MOKE and LEED. A simple theoretical magnetic model is used to describe the loop shape. The nonuniaxial nature of thin-film magnetism is emphasized, and the meaning of the magnetization curves will be presented in terms of a magnetostatic multipole expansion.

2:30 PM J3.4 
IN-SITU PLASMA CLEANING PROCESSES FOR PREVENTION OF CORROSION ON DRY ETCHED MAGNETIC MULTILAYERS. K.B. Jung , E.S. Lambers, S.J. Pearton, Univ. of Florida, Dept. of Materials Science and Engineering, Gainesville, FL; J. Marburger, F. Sharifi, Univ. of Florida, Dept. of Physics, Gainesville, FL.

MRAM elements with minimum critical dimension of 0.5m have been patterned using an Inductively Coupled Plasma Cl2/Ar process. The multilayer TaN/NiFe/Cu/NiFeCo/CrSi structure is readily etched at ion-neutral ratios of 0.02. Several processes have been investigated for removal of residual chlorine on the feature sidewalls, which produces rapid corrosion unless efficiently removed. Three different in-situ plasma cleans (H2, O2 or SF6) have been compared with simple solvent cleaning (the standard process for Al etching in Si device technology). SQUID magnetometry and AES and SEM examination of the features over an extended period (6 months) after etching have been employed to measure the magnetic and structural stability of etched MRAM elements. Both the H2 plasma clean and solvent rinsing produce excellent long-term stability of the magnetic performance.

2:45 PM J3.5 
MAGNETIC MICROSTRUCTURE FABRICATION BY GLANCING ANGLE DEPOSITION. B. Dick , M.J. Brett, University of Alberta, Dept. of Electrical and Computer Engineering, Edmonton, CANADA.

It is well known that the microstructure of a magnetic thin film significantly affects its magnetic properties [1]. Using an advanced deposition technique known as GLancing Angle Deposition (GLAD) [2], we can control the three dimensional microstructure of magnetic films to length scales down to 10nm. We can fabricate Ni and Co films that exhibit both conventional and novel microstructures, including posts, slanted posts, and ``beds'' of helices, ``zig-zags'' or chevron structures. Historically oblique incidence deposition and pillar arrays have been fabricated and measured for use in ME tape and perpendicular recording research [3,4]. We have fabricated both these structures to controlled depth between a few tens of nanometers to several microns in a one-step process using an e-beam evaporator with oblique incidence deposition and carefully controlled substrate motion. The GLAD process also enables a thin continuous capping layer to be deposited on top of the magnetic microstructure. The presentation will describe the GLAD deposition process, report structural and X-ray diffraction analysis of the films, with a particular highlighting of the varied types of microstructures we have created. Further work is underway studying the magnetic properties of our structured films. 

Chair: Christopher B. Murray 
Monday Afternoon, April 5, 1999 
Salon 3 (M)
3:30 PM *J4.1 
PROGRESS IN LASER INTERFERENCE LITHOGRAPHY FOR NANOSCALE MAGNETIC STRUCTURES. S. Poppe, P. Kimmel, M. Bauer, J. Fassbender , B. Hillebrands, Fachbereich, Physik und Forschungs- und Entwicklungsschwerpunkt Materialwissenschaften, Universitat Kaisersalutern, Kaisersalutern, GERMANY.

A laser interference lithography (LIL) setup consisting of an UV argon ion laser and a rigid optics stage is used for the fabrication of magnetic wires and elliptically shaped dots with periodicities between 176 nm and 1000 nm and lateral sizes down to 40 nm. We have developed a new sophisticated pattern transfer process, which takes the advantages of a novel double-layer resist design, to increase the sample quality regarding the side-wall definition and the aspect ratios. This process is essential in particular for patterns which require multiple exposures. In first experiments side wall angles larger than 85 degree and aspect ratios larger than one were realized.

4:00 PM J4.2 
LARGE AREA SUBMICRON-SCALE PERIODIC MAGNETIC ARRAYS. P. Vavassori , V. Metlushko, R.M. Osgood III, M. Grimsditch, U. Welp and G. Crabtree, Materials Science Division, Argonne National Laboratory, Argonne, IL; Wenjun Fan, S.R.J. Brueck, University of New Mexico, Albuquerque, NM; B. Ilic, P.J. Hesketh, EECS, University of Illinois at Chicago, IL.

Large area submicron-scale periodic magnetic arrays (square and rectangular) of holes and dots in Fe(400)/Cr(10) were fabricated using interferometric lithography. These systems are of great interest since they offer the potential for technological applications. A better understanding of the fundamental properties of patterned magnetic structures is required for the development a new generation of magnetic devices. The influence of the lattice spacings and element shape in both dots and holes arrays were determined by Brillouin light scattering, SQUID megnetometry and Transverse Magneto-Optic Kerr Effect (TMOKE). The TMOKE measurements are sensitive to the magnetization in the plane of the film both parallel and perpendicular to an external applied magnetic field. In addition, the TMOKE measurements have been carried out on the diffracted light spots. We found that TMOKE for an array of holes provides information on the magnetic structure in the vicinity of the holes. The switching mechanism of the magnetization will be discussed. Work of Argonne National Laboratory was supported by the U.S. Department of Energy, Division of Material Sciences, Office of Basic Energy Sciences, under contract W-31-109-ENG-38. P. V. acknowledges support by a research grant from INFM-Istituto Nazionale per la Fisica della Materia.

4:15 PM J4.3 
NUCLEATION AND THERMAL STABILITY IN A CONI/PT PATTERNED MEDIUM. M.A.M. Haast , I.R. Heskamp, L. Abelmann, J.C. Lodder and Th.J.A. Popma, MESA Research Inst., Enschede, THE NETHERLANDS.

In the near future patterned media may have to be introduced into magnetic recording to overcome the bit density limits of conventional thin film media. Recently we have shown the preparation of large area arrays of magnetic dots by laser interference lithography [1]. Using polycrystalline CoNi/Pt multilayers with high perpendicular magnetic anisotropy a patterned medium of 60 nm single domain dots with a dot/bit density of 16 Gbit/in2 was realised. Moreover, with increasing dot size a transition from a single to a multidomain state was observed. The magnetic properties of these dots have extensively been studied [2]. In this paper we present studies on the nucleation and thermal stability in the patterned medium. The activation volume gives information on both properties and has been determined from measurements of magnetic viscosity by polar Kerr magnetometry. The activation volume turned out to be much lower than the actual dot volume. Probably the switching of magnetisation starts at a small region of reduced magnetic anisotropy associated with induced etching damage. The calculation of the relaxation time at zero applied field results in a bit stability larger than 4000 years. In addition we have studied the low field nucleation of domains in micron sized CoNi/Pt multilayer dots. The presence of small inhomogeneities in the as deposited multilayer is assumed to be the origin of this phenomenum. To determine their density we have patterned micron sized dots with a broad size range and observed the size dependence of the nucleation field distribution by polar Kerr microscopy. The observations resulted in an estimated nucleation site density of 240/mm2. Their role in the single domain patterned medium needs further study.

4:30 PM J4.4 
MAGNETIC PROPERTIES OF 100 - 200 nm PERIOD NANOMAGNET ARRAYS. M. Hwang, T.A. Savas1, M. Farhoud1, H.I. Smith1 and C.A. Ross , Massachusetts Institute of Technology, Dept. Materials Science and Engineering, 1Dept. Electrical Engineering and Computer Science, Cambridge, MA.

Arrays of small magnetic elements can be used for micromagnetic studies of magnetic interactions and switching, and have applications in advanced data storage technologies. We have used interferometric lithography and achromatic interferometric lithography to make square arrays of nanomagnets with periods of 100 - 200 nm, in which the individual particles have diameters of 30 nm and up, over areas of several square cm. Arrays have been made by electrodeposition through holes in a template, by evaporation and liftoff, or by etching of sputtered or evaporated magnetic films. Electrodeposited nanomagnets were made from nickel, cobalt, and cobalt-phosphorus alloys, with aspect ratios (height/diameter) of up to 3. Ni and Co nanomagnets are polycrystalline while CoP alloys are amorphous or nanocrystalline depending on composition. The behavior of Co nanomagnets is dominated by the high magnetocrystalline anisotropy of the individual Co grains, and magnetic force microscopy shows deviations of the easy axis from the long axis of the nanomagnets due to a spread in grain orientations. In contrast, the behavior of Ni and CoP nanomagnets is dominated by shape anisotropy. Vibrating sample magnetometry of 180 nm diameter, 300 nm high Ni nanomagnets with 200 nm period shows in-plane magnetization, an indication of strong magnetostatic coupling. 70 nm diameter, 150 nm high Ni nanomagnets with 100 nm period show an out-of-plane easy axis and switching field of 700 Oe which is characteristic of incoherent magnetization reversal. 30 nm Ni particles with 100 nm period are superparamagnetic at room temperature but have a switching field of 550 Oe at low temperature, close to that predicted from coherent rotation. We will describe the collective behavior of nanomagnet arrays made from CoP and Ni with a variety of compositions, aspect ratios and particle spacings, relate their behavior to micromagnetic calculations, and assess their use in patterned magnetic storage media. 

Chair: Michael R. Scheinfein 
Tuesday Morning, April 6, 1999 
Salon 3 (M)
8:30 AM *J5.1 
TEM OBSERVATIONS OF MAGNETISATION REVERSAL IN PATTERNED MAGNETIC FILMS AND MULTILAYERS. J.N. Chapman , K.J. Kirk, P.R. Aitchison, Department of Physics and Astronomy, University of Glasgow, Glasgow, UNITED KINGDOM.

Transmission electron microscopy (TEM) has been used extensively to investigate the magnetisation reversal mechanisms in small magnetic elements created by electron beam lithography. The experimental procedure adopted is to subject the specimen to a vertical magnetic field of the experimenterís choosing and to then tilt the specimen about a horizontal axis so that it is subjected to a varying field in its plane. As the electron optical conditions remain unchanged throughout the experiment observations can be made in real time. We have fabricated elements, many with one or both in-plane dimensions in the sub-micron range, from single layer and a variety of multilayer films, the latter belonging to material systems which display GMR. Emphasis in recent work has been on the influence of element end-shape and field orientation on the magnetisation reversal process. Blunt-ended elements tend to retain a residual domain structure close to the element ends and reversal normally involves its growth and evolution; elements with pointed ends are essentially single domain and reverse by rapid switching at somewhat higher fields.