Meetings & Events

fall 1997 logo1997 MRS Fall Meeting & Exhibit

December 1 - 5, 1997 | Boston
Meeting Chairs:
 Harry A. Atwater, Peter F. Green, Dean W. Face, A. Lindsay Greer 

Symposium V—Metallic Magnetic Oxides


Michael Hundley, Los Alamos National Laboratory
Janice Nickel, Hewlett Packard Co
R. Ramesh, Univ of Maryland
Yoshi Tokura, Univ of Tokyo

Symposium Support

  • Hewlett Packard Laboratories
  • Joint Research Center for Atom Technology (JRCAT)
  • Lake Shore Cryotronics, Inc.
  • Los Alamos National Laboratory
  • National Institute for Advanced Interdisciplinary Research

    * Invited paper

    Chair: Yuri Suzuki
    Monday Morning, December 1, 1997
    Providence/Orleans (M)

    8:30 AM V1.1
    EPITAXIAL GROWTH OF Fe3O4 FILMS ON SAPPHIRE, MgA1-2O4 AND SrTiO3 BY PLUSED LASER DEPOSITION AND STUDY OF THEIR MAGNETIC AND TRANSPORT PROPERTIES. S.B. Ogale, R.P. Sharma, R.D. Vispute, K. Ghosh, S.P. Pai, R.L. Greene, R. Ramesh and T. Venkatesan, NSF-MRSEC and Centre for Superconductivity Research, Department of Physics, University of Marland, College Park, MD.

    Epitaxial thin films of Fe3O4 have been grown on single crystal MgAl2O4 (100 and 111), SrTiO3 (100) and Sapphire (0001) substrates by pulsed laser deposition. The film quality was examined by X-ray diffraction, Rutherford Backscattering Channeling, Atomic and Magnetic force microscopies and Vibrating Sample and SQUID magnetometries. It was found that the desired spinel phase is formed only at a pressure close to or lower than about 10-5 Torr (lowest pressure examined was 4 x 10-6 Torr) and a small pressure increase to a value of about 4 x 10-5 Torr yields the alpha-Fe2O3 phase. When the pressure is in the optimum range, the spinel phase forms at all temperatures examined viz. from 500 C to 700 C and the resistivity is in the range of tens of mOhm-cm. The films deposited at temperatures close to or above 700 C show spinel structure but do not show any Verwey transition feature. The coercive field at room temperature is close to 200 Oe. As the deposition temperature is lowered below 600 C, a clear Verwey signature appears. The coercive field rises close to 380 Oe. The quality of the Verwey transion is further improved after an appropriate annealing sequence.

    8:45 AM V1.2
    THERMAL AND OPTICAL PROPERTY MEASUREMENTS ON EPITAXIAL PULSED LASER DEPOSITED STRONTIUM RUTHENATE THIN FILMS. Jon-Paul Maria and Susan Trolier-McKinstry, The Pennsylvania State University, Department of Materials Science and Engineering, University Park, PA.

    Epitaxially grown SrRuO3 thin films exhibit metallic conductivity, ultra-smooth surface morphology, thermochemical stability, and chemical inertness. As a result, SrRuO3 is of interest for integration into oxide perovskite heterostructures and devices incorporating ferroelectric, dielectric, and superconducting materials. Though many investigations have been performed on SrRuO3, little information concerning the thermal expansion or optical properties is available. Single crystal SrRuO3 films have been grown by pulsed laser deposition on (001) oriented SrTiO3 and LaAlO3 substrates. The structure and properties of these films were found to depend strongly upon the amount of energetic bombardment during deposition. In particular, bombardment during growth resulted in expanded lattice parameters and a decrease in the ferromagnetic transition temperature. Spectroscopic ellipsometry was used to determine the dielectric function of both heavily and lightly bombarded films. It was found that the features in the dielectric function between 1 and 4 eV were washed out with increasing bombardment. In addition, the thermal expansion coefficient for SrRuO3 was measured using high temperature x-ray diffraction. The room temperature value of thermal expansion was calculated to be 13.1 ppm/C. A reproducible discontinuity in the thermal expansion curve was observed at 300C in films grown under low bombardment conditions: this suggests the presence of a structural phase transition. Heavily bombarded films showed no evidence of phase transitions. Additional data including SrRuO3 powder measurements and high temperature ellipsometry will be presented.

    9:00 AM *V1.3
    THE COMBINATORIAL SYNTHESIS AND EVALUATION OF MAGNETIC METAL OXIDES: A MATERIALS CHIP APPROACH. X.-D. Xiang, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA.

    Due to the complex nature of correlation between materials properties and compositions, discovery and optimization of novel functional materials are often serendipitous and require tedious trial and error experiments. In order to systematically and effectively search and optimize advanced functional materials, we have used recently developed combinatorial synthesis and screening technologies. In this approach, thousands (currently 1,024) of different compositions are synthesized on a small (e.g. one inch square) substrate using sequential thin film depositions through a series combinatorial masks followed by controlled annealing processes. I will review the background and infrastructure development for the combinatorial synthesis/screening of novel solid state materials at LBNL. A cryogenic probe capable of 4-point-probe contact resistance measurements of 128 member materials chips under 12 Tesla field has been build to perform rapid screening. Nondestructive electrical and magnetic measurement systems, including a novel scanning tip microwave near field microscope for quantitative electrical impedance microscopy and magneto-optical sensor based magnetic microscope, have also been developed. The application of this approach to search for new magnetoresistive metal oxides has resulted in the discovery of large magnetoresistance in perovskite cobalates. Efforts to optimize heterostructures of magnetic/normal layers using combinatorial materials chip approach will also be discussed.

    10:00 AM V1.4
    SYNTHESIS AND PROPERTIES OF CALCIUM MANGANATE RUDDLESDEN-POPPER PHASES. J.E. Sunstrom, I.D. Fawcett, Mark Croft* and Martha Greenblatt, Depts of Chemistry and Physics*, Rutgers, The State University of New Jersey, Piscataway, NJ; K.V. Ramanujachary, Rowan University, Glassboro, NJ.

    Calcium manganate Ruddlesden Popper phases with the formula (CaO)(CaMnO3)n (n = 1, 2, 3, ) have been prepared in order to examine the effect of dimensionality on the magnetic and electrical properties. Polycrystalline samples of high quality have been synthesized using the Pechini citrate gel process which allowed a better control of the stoichiometry and lower processing temperatures compared to the standard ceramic methods. In addition, samples could be prepared at ambient oxygen pressures due to the small particle sizes. The magnetic properties are consistent with the previously published data on samples prepared at very high oxygen pressures (65 Kbar). The magnetic ordering changes from antiferromagnetic to ferromagnetic with increasing dimensionality (n). X-ray absorption spectroscopy studies confirm that manganese is exclusively tetra-valent across the series.

    10:15 AM V1.5
    THIN FILM GROWTH OF La-Ca-Mn-O BY MBE CO-EVAPORATION TECHNIQUE ON LaAlO3, SrTiO3 and CeO2-BUFFERED Si SUBSTRATES. A. Miniotas, A. Brazdeikis, B. Hirschauer and U.O. Karlsson, Materials Physics, Royal Institute of Technology, Stockholm, SWEDEN; Texas Center for Superconductivity, University of Houston, Houston, TX.

    Thin films of La-Ca-Mn-O have been grown by molecular beam epitaxy co-evaporation technique on (100)-oriented LaAlO3, SrTiO3 and CeO2-buffered Si(111) substrates. As-grown films were found to exhibit strong () texture. Zero-field resistance measurements of La0.68Ca0.32MnO3 films on LaAlO3 showed insulator-metal transitions to occur in the range of 220-230 K. In magnetic fields of 5.0 T thin film resistance decrease, (R(H)-R(0))/R(H), of more than 1500 at 214 K was measured. The obtained values are similar to those reported for as-grown La-Ca-Mn-O films synthesized either by so-called ``block-by-block`` or ``layer-by-layer`` MBE techniques, but the effect appears at significantly higher temperatures. In addition, the La-Ca-Mn-O films grown on SrTiO3 showed a 200 resistance suppression at 1.0 T. Preliminary atomic force microscopy study showed film surfaces with typical values of 10 nm, independent of substrate. Experimental data demonstrate that the MBE co-evaporation approach, characterized by relatively high growth rates, may provide new insights in advancing La-Ca-Mn-O film transport properties. In the paper, we will extend the discussion by providing a detailed study of La-Ca-Mn-O film processing conditions and how they affect film microstructure and transport properties. In addition, new results of La-Ca-Mn-O growth on CeO2-buffered Si(111) substrates and in-plane orientation studies by XRD pole-figure measurements will be presented.

    10:30 AM V1.6
    ELECTRICAL AND MAGNETIC PROPERTIES OF SCREEN PRINTED La0.53Sr0.43MnO3 THICK FILMS. A.K.M. Hossain, S. de Silva, and L.F. Cohen, Blackett Laboratory, Imperial College, London, UNITED KINGDOM; A. Berenov, T. Kodenkandath and J.D. MacManus Dirscoll, Materials Department, Imperial College, London, UNITED KINGDOM.

    The powders of La0.53Sr0.43MnO3 were obtained by combustion technique. The stoichiometric compositions were confirmed by inductively coupled plasma (ICP) technique. A thick paste was obtained from the mixture of powder and organic Blyth vehicle (60% powders 40% Blyth vehicle). The patterned thick films were printed on polycrystalline A12O3, LaAIO3 single crystal (100) and yttria stabilized Zr2O3 single crystal (100) substrates. Thickness of the films were varied from 20m-100m. The films were sintered at various temperatures, after low temperature burnout of the organic vehicle. The microstructures of the prepared films were studied by scanning electron microscope (SEM) and atomic force microscope (AEM). Resistivity of the films were carried out both in field (0.65 T) and zero field. Magnetization measurements were carried out in a vibrating sample magnetometer (VSM). Variation of film properties with annealing conditions, will be discussed.

    10:45 AM V1.7
    DRY ETCH PROCESSES FOR NiMnSb, LaCaMnO3 AND RELATED MATERIALS. J. Hong, J.J. Wang, E.S. Lambers, J.A. Caballero, J.R. Childress and S.J. Pearton, Department of Materials Science and Engineering, University of Florida, Gainesville, FL; K.-H. Dahmen, Department of Chemistry/MARTECH, Florida State University, Tallahassee, FL; S. Von Molnar, Department of Physics/MARTECH, Florida State University, Tallahassee, FL; F.J. Cadieu, Physics Department, Queens College of CUNY, Flushing, NY; F. Sharifi, Department of Physics, University of Florida, Gainesville, FL.

    A variety of plasma etching chemistries were examined for patterning NiMnSb Heusler alloy thin films and associated Al2O3 barrier layers. Chemistries based on SF6, Cl2 and BCl3 were all found to provide faster etch rates than pure Ar sputtering. In all cases the etch rates were strongly dependent on both the ion flux and ion energy. Selectivities of 20 for NiMnSb over Al2O3 were obtained in SF6-based discharges, while selectivities 5 were typical in Cl2, BCl3 and CH4/H2 plasma chemistries. Wet etch solutions of HF/H2O and HNO3/H2SO4/H2O were found to provide reaction limited etching of NiMnSb that was either non-selective or selective, respectively, to Al2O3. In addition we have developed dry etch processes based on Cl2/Ar at high ion densities for patterning of LaCaMnO3 (and SmCo permanent magnet biasing films) for magnetic sensor devices. Highly anisotropic features are produced in both materials, with smooth surface morphologies (4nm RMS roughness measured by atomic force microscopy for an etch depth of several thousand angstrom). In all cases, SiO2 or other dielectric materials must be used for masking, since photoresist does not retain its geometrical integrity upon exposure to the high density plasma.

    Monday Morning, December 1, 1997
    11:00 A.M.
    Providence/Orleans (M)

    EPITAXIAL GROWTH MECHANISM AND PHYSICAL PROPERTIES OF ULTRA THIN FILMS OF La0.6Sr0.4MnO3. Yoshinori Konishi, Masahiro Kasai, Joint Research Center for Atom Technology(JRCAT); Masashi Kawasaki, Joint Research Center for Atom Technology(JRCAT) and Department of Innovative and Engineering Materials, Tokyo Inst of Technology, Yokohama, JAPAN; Yoshinori Tokura Joint Research Center for Atom Technology (JRCAT) and Department of Applied Physics, The University of Tokyo, Tokyo, JAPAN.

    Perovskite-type oxides have such versatile properties as superconductivity, ferroelectricity, and colossal magnetoresistivity. Epitaxial multilayers composed of these oxides should explore new functional devices. It is of great importance for this purpose to understand and control the epitaxy dynamics on an atomic scale. In this study, we show the optimization of epitaxial growth condition for La0.6Sr0.4MnO3 (x=0.4) and magnetic properties of ultra thin film. Thin films were fabricated using high vacuum Pulsed Laser Deposition (PLD) apparatus with an ArF excimer laser. Single crystals of SrTiO3 (100) with various surface treatment were used for the substrates. The surface morphology of the films was sensitively affected by oxygen pressure. In the case of high oxygen pressure, randomly aligned grains were nucleated on the epitaxial film. When the pressure was 100mTorr, the epitaxial film had very smooth surface. Under this condition, we investigated the thickness dependence of resistivity and magnetization of the thin film. Above 20nm, the residual resistivity was as low as 1-210-4cm, the Curie temperature (Tc) reached 350K and magnetization saturated at 3.5B. These values are comparable to those of single crystals. Even 6nm thickness film showed ferromagnetic metallic behavior. The AFM images of ultra thin films on wet etched SrTiO3 showed atomically flat terrace and 0.4nm steps. We conclude that the film grew either layer by layer or step flow modes which were tuned by preparation of substrates and growth temperature. This work, partly supported by NEDO, was performed in the JRCAT under the joint research agreement between the NAIR and the ATP.

    THIN FILM GROWTH AND MAGNETOTRANSPORT STUDY OF . Takashi Manako, Takeshi Obata, Yuichi Shimakawa, Yoshimi Kubo, Fundamental Research Laboratories, NEC Corporation, Tsukuba, JAPAN.

    Thin film samples of (x = 0.15 - 0.3) were synthesized by a pulsed laser deposition technique. Transport and magnetic properties were examined for the films grown under various growth conditions, that is, target composition, substrate material, growth temperature, oxygen partial pressure and laser power. Among the preparation conditions, the oxygen partial pressure () has the most significant influence on the electrical and magnetic properties of the films. Films grown under low had a lower ferromagnetic transition temperature () and wider resistive transition width than those of the films grown under high . None of the heat treatments conducted after growth improved these films' quality. Film morphology is much affected by the substrate material. The films deposited on MgO were c-axis oriented but had a granular structure. In contrast, grain-free thin films were epitaxially grown on substrates. Surface roughness of these films was less than 1 nm even for the films with a thickness of 200 nm. Under the optimized growth conditions, as-deposited films for showed sharp transition at in both resistivity and magnetization. Large magnetoresistance at far below the , which is often observed in polycrystal samples and believed to be a grain boundary effect, was as small as that reported for single crystal samples. These results indicate the high quality of the films .

    CRYSTALLINITY AND MAGNETORESISTANCE IN CALCIUM DOPED LANTHANUM MANGANITES. E. S. Gillman, K. H. Dahmen, S. Watts, X. Yu, S. Wirth, J. J. Heremans, Departments of Chemistry and Physics, Center for Materials Research and Technology - MARTECH, Florida State University, Tallahassee, FL.

    Calcium doped lanthanum manganites LaxCa1-xMnO3 with have been prepared on LaAlO3(001) (LAO) Yittrium stablized ZrO2(001) (YSZ), and sapphire, Al2O3(0001) (SAP) by liquid delivery metal-organic chemical vapor deposition (LD-MOCVD). The films on YSZ and SAP substrates have a textured, polycrystalline morphology with a preferred orientation of (110). The films on LAO show a single-crystalline morphology and a (100) orientation. Transport measurements show the polycrystalline films have a resistance peak approximately 60K lower than the films on LAO and, in general, have a much higher overall resistance. The magnetoreietance (MR) ratio ( ) is sharply peaked near the maximum in resistance for the films on LAO, while the polycrystalline films show a noticeable absence of this sharply peaked behavior and a flat, rather large () MR ratio over a large temperature range. These results will be discussed in terms of grain boundary scattering, crystallite size, and magnetization.

    MICROSTRUCTURAL ASPECTS OF NANOCRYSTALLINE LiZn FERRITES DENSIFIED WITH CHEMICALLY DERIVED ADDITIVES. Yong S. Cho, Vernon L. Burdick and Vasantha R.W. Amarakoon, New York State College of Ceramics at Alfred University, Alfred, NY; Elijah Underhill and Leo Brissette, Electromagnetic Science Technologies Inc., Norcross, GA.

    Densification behavior of nanocrystalline LiZn ferrites with chemically derived additives, Si, Ca and Mn were investigated. Nanocrystalline Li0.3Zn0.4Fe2.3O4 was prepared by a chemical synthesis using a combustible polyacrylic acid (PAA). A pure spinel phase having a 10 nm size was synthesized at 400C within a short soaking time by the method. Several chemical additives were incorporated into the nanocrystalline ferrites via sol-gel reactions utilizing tetraethyl orthosilicate, calcium acetate and manganese acetate. This gives a homogeneous distribution of the additives over the nanocrystalline ferrites. A uniform microstructure was obtained without any evidence of exaggerated grain growth after sintering at 1100C. Microstructural evolution of the samples will be examined with variations in quantity of the additives, temperature, time and heating rate. The results will be compared with those of the same composition, but processed by the conventional batch-mixing of corresponding oxide additives. Magnetic properties will be correlated with the observed microstructural characteristics.

    SUBSTITUTION IN Pr-BASED MANGANITES: POSSIBLE VALENCE DEGENERACY. Litty Sebastian, M.S. Ramachandra Rao, V.S. Subramanian and U.V. Varadaraju, Materials Science Research Centre, Indian Institute of Technology, Madras, INDIA.

    REMnO3 phases are antiferromagnetic insulators, Suitable doping at the RE site by alkaline earth elements can lead to creation of mixed valency at the Mn site (Mn+3/Mn+4). Depending on the ratio of Mn+3/Mn+4, the magnetic and electrical transport properties can be significantly varied in addition, substitution at the Mn site by other isovalent as well as aliovalent transition and non transition elements effect the magnetic exchange interactions and lead to changes in physical properties. Two systems of compounds viz., Pr0.7Ca0.3Mn_3 1$ and Pr$0.5$Ca$0.5$MnO$3 ^2have been chosen for study in the first composition, the Mn +3/ Mn +4 ratio is 7.3 (type 1) and that in the second compound is 1:1 (Type 11). Both the phases show complex magnetic behaviour. They undergo a series of antiferromagnetic transition including canted antiferromagnetism. The effect of substitution of Mn^4+by Ti^4+and Sn^4+has been reported^3. However, these two ions are d^0and d^10systems respectively and cannot take part in the Zener's Double Exchange. Thus, it would be most interesting to study the effect of Ru^4+at the Mn^4+sites, Ru^4+has a 4d^4electronic configuration and the perovskite SrRuO_3is a ferromagnetic metal. Thus, Ru^4+is amenable for ferromagnetic ordering and metallicity in the perovskite lattice. Ru^4+(4d^4system) substitution at the manganese site in the above two systems has been carried out. A series of compositions in the systems Pr_0.7Ca_0.3Mn_1-xRu_xO_3and Pr_0.5Ca_0.5Mn_1-xRu_xO_3with x - 0.0-0.1 in steps of 0.02 have been synthesized by high temperature solid state reaction method starting from the respective oxides/carbonates. To avoid vocalization of RuO_2the heating was carried out in several steps at 600, 800, 900 and 1200^C. All the compositions are single phasic. Ru doping in the Pr_0.7Ca_0.3MnO_3series is found to favour antiferromagnetism while in Pr_0.5Ca_0.5MnO_3Ca0.3MnO3.
    We report systematic studies of post-deposition thermal processing on the electrical and magneto-transport and magnetic properties of epitaxial thin films of La0.7Ca0.3MnO3. We find that post-deposition annealing in oxygen dramatically increases the temperature coefficient of resistance (TCR defined as 1/R (dR/dT)) near the metal-insulator transition. The changes in TCR correlate with improved magnetic homogeneity as indicated by ferromagnetic resonance line-widths as well as with reduced 1/f noise levels. The sharpness of the change in magnetization at the ferromagnetic transition (as indicated by 1/M(dM/dT)) correlates with the TCR. Annealing also induces structural changes as indicated by a decrease in the out-of-plane lattice constants as well as morphological changes. Present studies indicate that these annealing effects are a combined result of thermally induced structural changes and changes in the oxygen stoichiometry. We will present the dependence of these effects on annealing temperature, annealing time and the annealing atmosphere.

    Chair: Yoshi Tokura
    Monday Afternoon, December 1, 1997
    Providence/Orleans (M)

    1:30 PM *V3.1
    PRESSURE AND ISOTOPE EFFECTS IN THE MAGANESE-OXIDE PEROVSKITES. J.B. Goodenough and J.-S. Zhou, Center for Materials Science & Engineering, University of Texas at Austin, Austin, TX.

    Measurements of the temperature dependence of the resistivity r(T) and thermoelectric power a(T) under several hydrostatic pressures on single crystals of La1-xSrxMnO3 with x = 0.12 and 0.15 as well as on 18O/16O isotope-exchanged (La1-xNdx)0.7Ca0.3MnO3 polycrystalline samples have confirmed the existence of vibronic electronic states below Tc in the narrow range of bandwidths where static, cooperative Jahn-Teller deformations are suppressed and itinerant-electron behavior begins. A giant isotope effect on Tc at the onset of dynamic Jahn-Teller deformations is due to a mass-enhanced change in both the density and mobility of the mobile charge carriers at Tc. Cooperative dynamic Jahn-Teller deformations introduce vibrational modes into the interatomic spin-spin interactions that enhance Tc in zero order of the Hamiltonian. The La1-xSrxMnO3 crystals undergo a charge-ordering below Tco < Tc, and dTco/dP changes sign between x = 0.12 and x = 0.15. A hysteretic anomaly in r(T) and a(T) at Ts = 278 K > Tc in the x = 0.12 crystal is interpreted to mark a disproportionation into Mn(IV)-rich regions within a Mn(IV)-poor matrix, and a transition from vibronic to itinerant electronic behavior in the range Tco < T < Tc was found in the pressure range 5 < P < 6 kbar in the x = 0.15 sample. The nature of the vibronic state is modelled.

    2:00 PM *V3.2
    INTERPLANE TUNNELING MAGNETORESISTANCE IN LAYERED MANGANITES. T. Kimura, Y. Tomioka, T. Okuda, H. Kuwahara, A. Asamitsu, Joint Research Center for Atom Technology (JRCAT), Tsukuba, JAPAN; Y. Tokura, Joint Research Center for Atom Technology (JRCAT), Tsukuba, Japan and Univ of Tokyo, Dept of Applied Physics, Tokyo, JAPAN.

    We present the pressure effect on the charge-transport and the magnetic properties in a bi-layered manganite crystal, La2-2xSr1+2xMn2O7, which shows the highly anisotropic charge-transport properties and the interplane tunneling magnetoresistance (TMR) effect. The layered crystal is composed of ferromagnetic metal (FM) MnO2 bilayers with intervening insulating (I) (La,Sr)2O2 blocks, and can be viewed as an infinite array of FM/I/FM junctions. In such a quasi-two-dimensional (quasi-2D) FM, the interplane as well as inplane charge dynamics is expected to critically depend on the interlayer magnetic coupling between the FM MnO2 bilayers. We have utilized an external pressure to finely control such an interlayer coupling in this layered manganite, although the consequence of the pressure is highly nontrivial and unpredictable for the layered perovskite type structure with anisotropic compressibility and complex ionic/covalent bonding character. The applied pressure realizes the quasi-2D FM state down to low temperature, and drastically enhances the interplane TMR ratios up to 4,000 at 4.2 K. The magnetic susceptibility data suggest that the magnitude of the magnetic coupling between the adjacent bilayers can systematically be weakened by application of pressure, which makes the charge dynamics more 2D but highly incoherent. The 2D-like conduction of the nearly fully spin-polarized carrier within the magnetically interplane-decoupled MnO2 bilayers might be highly diffuse or incoherent perhaps due to the coupling with phononic and/or orbital excitations.

    2:30 PM *V3.3
    TUNNELING MAGNETORESISTANCE IN HALF-METALLIC FERROMAGNETS. S-W. Cheong, Bell Labs, Lucent Technologies, Murray Hill and Dept of Physics and Astronomy, Rutgers University, Piscataway, NJ; H. Y. Hwang, Bell Labs, Lucent Technologies, Murray Hill, NJ.

    Large low-field magnetoresistance (MR) is realized through the field-induced change of spin-polarized intergrain tunneling below the Curie temperature in the ferromagnetic materials of perovskite (La,Sr)MnO3 and pyrochlore Tl2Mn2O7. The origin of this significant tunneling MR, which is distinctly different from that of intragrain MR (so-called colossal MR) near Curie temperature in the manganites, is the half-metallic nature of the compounds. In half-metallic ferromagnets, the band for minority spin carriers shows a gap at the Fermi level so that the spins of carriers are 100%-polarized. These findings result from the systematic comparison of magneto-transport properties of single crystals and polycrystalline materials, and provide a new avenue of realizing large MR in the manganites. We have also examined this tunnling MR behaviors in other ferromagnetic oxides.

    3:30 PM *V3.4
    ADVANCES IN THE GROWTH, STUDY AND APPLICATIONS OF MANGANITE THIN FILM HETEROSTRUCTURES. T. Venkatesan, University of Maryland, NSF MRSEC on Oxide Thin Films and Surfaces and Center for Superconductivity Research, College Park, MD.

    Since the recent resurgence of interest in the manganites owing to the observation of very large magneto-resistance in these materials, activities relating to the fabrication of a variety of thin film heterostructures of these materials have accelerated over the last few years. I will give a brief overview of the highlights of the research in this area which could cover some of the interesting physics as well as some novel applications that are emerging. In single crystalline films ion channeling measurements show that the lattice distortions are strongly correlated with the transport properties. Substrate induced stress plays an important role in affecting the magnetization anisotropy of the film. Microwave response at GHz frequencies also show similar MR effects as at dc. Electric field effects in CMR channels with ferroelectric gates give large changes in the resistance along with retention of the switched state. Finally, CMR/HTS cuprate heterostructures have been demonstrated to be of value in two interesting applications: 1) flux focused sensors with higher sensitivity at lower fields and 2) spin polarized quasi particle injected FETs where a 30 fold increase in the gain was seen with respect to unpolarized electron injected FE.

    4:00 PM *V3.5
    QUANTIFYING THE STRAIN DEPENDENCE OF 'COLOSSAL' MAGNETORESISTANCE MANGAINTES. A. J. Millis, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD; T. Darling and A. Migliori, Los Alamos National Laboratory, Los Alamos, NM.

    Strain is believed to play an important role in determining the measured behavior of `CMR' materials. To gain a more precise understanding of strain effects we present measurements and phenomenological and theoretical analyses elastic anomalies at the magnetic Tc in single crystals. We find that the dependence of Tc on volume-preserving biaxial strain is extremely large (, and consistent in magnitude with theories involving Jahn-Teller lattice distortions. Implications for behavior of films will be discussed.

    Chair: H. Y. Hwang
    Tuesday Morning, December 2, 1997
    Providence/Orleans (M)

    8:30 AM V4.1
    LATTICE DEFORMATION AND MAGNETIC PROPERTIES IN EPITAXIAL Sr1-XBaXRuO3 FILMS. Noburu Fukushima, Kazuhide Abe, Shuichi Komatsu and Tatsuo Schimizu, Toshiba Corporation, Kawasaki, JAPAN.

    Crystal structure and magnetic properties in epitaxially grown Sr1-xBaxRuO3 on SrTiO3 substrates were determined. Epitaxial Sr1-xBaxRuO3 exhibits simple perovskite structure in whole region of Ba/Sr ratio, instead of complex hexagonal layered perovskite of Ba-rich bulk Sr1-xBaxRuO3 which has plane sharing oxygen octahedora. Tetragonal deformation was enhanced from pseudo cubic in SrRuO3 to highly distorted tetragonal lattice in BaRuO3. Electronic properties such as conductivity and magnetization were examined and compared with the results of band calculation in which tetragonal distortion was taken into account. Metal-insulator transition was not observed in this system neither in experiments nor in simulation, and metallic conductivity was maintained in the whole region. Ferromagnetic ordering at 150K seen in bulk SrRuO3 was observed to be suppressed in Sr1-xBaxRuO3 films with increasing tetragonal deformation and Curie temperatures decreased down to 50K in BaRuO3.

    8:45 AM V4.2
    DIRECT EVIDENCE OF DYNAMIC STRUCTURAL VARIATIONS IN LAYERED La1.2Sr1.8Mn2O7 CRYSTALS ASSOCIATED WITH MAGNETIC BEHAVIOR AS A FUNCTION OF TEMPERATURE. R.P. Sharma, P. Fournier, R.L. Greene, and T. Venkatesan, Center for Superconductivity Research, Physics Department, University of Maryland, College Park, MD; J.F. Mitchell and D. Miller, Materials Science Division, Argonne National Laboratory, Argonne, IL.

    The presence of static and dynamic distortions associated with the complex magnetic behavior is seen by ion channeling investigations in layered perovskite double sheet high quality La1.2Sr1.8Mn2O7 single crystals This material is predominantly ferromagnetic below 120 K and shows large magnetoresistance in this region. A change of 1.5 picometer in the atomic displacement is found between 70 and 120 K which is about four times larger than the thermal displacement, providing clear indication of dynamic distortions. Between 120 and 320 K, a small ferromagnetic component(0.3% by volume) is observed in magnetization measurements, in addition to paramagnetic phase. Actually there is a small non linear rise in the magnetic moment as the temperature is decreased from 320 to 120K. Above 320 K paramagnetism dominates. Anomalous displacements of Mn, La and Sr atoms are seen in ion channeling studies between 120 and 320 K, which correlate well with the features observed in magnetization measurements. The magnitude of displacement in this wide temperature region is twice as large as compared to the thermal motion. There is a large magneto crystalline anisotropy with the hard axis along the c-direction. The material seems to be intrinsically an inhomogeneous magnetic system with the presence of small regions of highly correlated spin systems parallel to the ab plane and appears to be in a state of two dimensional magnetic order. The correlation of channeling anomalies with the magnetic anomalies agrees in favour of intrinsic effects as opposed to impurity based effects.

    9:00 AM *V4.3
    MAGNETIC ANISOTROPY AND LATTICE DISTORTIONS IN THE DOPED PEROVSKITE MANGANITES Yuri Suzuki, Dept. of Materials Science and Engineering, Cornell University, Ithaca, NY; Harold Y. Hwang, Sang-Wook Cheong, R. Bruce van Dover, Bell Laboratories, Lucent Technologies, Murray Hill, NJ.

    Recent experimental and theoretical studies of the doped perovskite manganite materials indicate the important role of lattice distortions in the ìcolossalî magnetoresistance effect observed at the Curie temperature. In epitaxial manganite films, the substrate imposes a strain on the film that affects not only the Curie temperature and magnetoresistance properties but also the magnetic anisotropy. In this talk we will present our results on the magnetic anisotropy of epitaxial manganite thin films in which strain anisotropy is found to dominate over cubic and rhombohedral magnetocrystalline anisotropy. In order to measure magnetic anisotropy, remnant magnetization was measured as a function of in-plane angle. Torque magnetometry measurements of the films revealed a dominant anisotropy consistent with the remnant magnetization results. Our results based on torque magnetometry and magnetization measurements will be presented in the context of experiments on anisotropic magnetic properties by other groups. Moreover we will compare our magnetic anisotropy results on epitaxial thin films to those on single crystals of nominally the same composition.

    9:30 AM V4.4
    EVIDENCE FOR LOCAL LATTICE DISTORTIONS IN THE LAYERED MANGANITE La1.4Sr1.6Mn2O7 BY PULSED NEUTRON DIFFRACTION. Despina Louca and G.H. Kwei, Los Alamos National Laboratory, Los Alamos, NM; and J.F. Mitchell, Argonne National Laboratory, Argonne, IL.

    The layered manganites, (La/Sr)n+1,Mnn,O3n+1, with n=2 exhibit similar transitions in their magnetic and transport properties as the related perovskite compounds (nave). The structural building blocks are made of double octahedral layers alternated by a nonmagnetic La/Sr-O layer. With the application of a magnetic field, a greater change in the magnetoresistance is observed than in the perovskite compounds with equivalent doping. In compounds with a nominal charge doping of 30%, a transition to a ferromagnetic (FM) state concomitant with an insulator-to-metal (I-M) transition is observed at > 116 K. In contrast to the perovskite compounds, an additional transition in the magnetic ordering, although smaller in magnitude, is evident close to room temperature (RT). From the real-space structural analysis of pulsed neutron powder diffraction data carried out using the pair density function (PDF) technique, we observed significant structural changes occurring with temperature not otherwise seen by standard crystallographic techniques. The data were collected at the Special Environment Powder Diffractometer (SEPD) of the Intense Pulsed Neutron Source at temperatures ranging from 20 to 300 K. The local structure with respect to the manganese and oxygen environments provide evidence that the Jalm-Teller (JT) distortion of the Mn3+ systems is present in the paramagnetic (PM) insulating phase and is significantly reduced in the FM metallic state. In addition, local lattice distortions are observed in the vicinity of the magnetic transition close to RT which could be dynamic in origin. Physical implications will be discussed.

    9:45 AM V4.5
    K-EDGE X-RAY ABSORPTION SPECTROSCOPY (XAS) STUDIES OF La1-xSrxMnO3. Susan M. Mini a,b, John Mitchellb, D.G. Hinksb, Ahmet Alatasb,c, D. Rosenmannb, C.W. Kimball^a,b)%%, P.A. Montanob, aNorthern Illinois University, Dept of Physics, DeKalb, IL and bArgonne National Laboratory, Materials Science Division, Argonne, IL, cIllinois Institute of Technology, Chicago, IL.

    Systematic Mn K-edge x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD) measurements on samples of Lal-xSrxMnO3, which are precursors to colossal magnetoresistive (CMR) materials, are reported. Detailed results on the edge or chemical shift as a function of Sr concentration (hole doping) and sample preparation (air vs oxygen annealed), are discussed. For comparison, a systematic XANES study of the Mn K-edge energy shift, denoting valence change in Mn, has been made in standard manganese oxide systems. Contrary to expectations, the variation in near-edge energies for Mn in La0.7Sr0.3MnO3 were small when compared to the difference between that for manganese oxide standards of nominal valence of +3 and +4 (Mn2O3 and MnO2). X-ray diffraction measurements show lattice parameter variations which are correlated with the Mn+4 concentration and are consistent with the valence change determined from the Mn K-edge XANES edge shift.

    10:30 AM *V4.6
    X-RAY INDUCED INSULATOR-METAL TRANSITIONS IN CMR MANGANITES. B. Keimer, D. Casa, V. Kiryukhin, Dept. of Physics, Princeton University, Princeton, NJ; J.P. Hill, A. Vigliante, Dept. of Physics, Brookhaven National Laboratory, NY; Y. Tomioka, Y. Tokura, JRCAT and Dept. of Applied Physics, University of Tokyo, JAPAN.

    Manganites of composition Pr1-x Cax MnO3 (x=0.3-0.5) were investigated by synchrotron x-ray diffraction in high magnetic fields*. At low temperatures these materials undergo magnetic field induced transitions from charge-ordered antiferromagnetic insulators to ferromagnetic metals. Surprisingly, diffraction experiments revealed that exposure to the x-ray beam itself also diminishes the charge order. At the same time, the electrical conductivity is enhanced by several orders of magnitude. An applied magnetic field accelerates the x-ray induced transition. The transition occurs only at low temperatures and is reversible on thermal cycling. When Ca is partially substituted by Sr, persistent x-ray photoconductivity is observed up to temperatures in excess of 100K. In analogy to persistent photoconductivity in compound semiconductors doped with DX centers, we argue that the x-ray induced transition is evidence for strong electron-lattice interactions of which the Jahn-Teller effect is a plausible origin. However, the collective nature of the transition in the manganites, and the associated kinetics, require more elaborate models.

    11:00 AM V4.7
    METAL INSULATOR TRANSITION OF V2O3 STUDIED WITH RESONANT SOFT X-RAY EMISSION SPECTROSCOPY. L.-C. Duda, C. B. Stagarescu, J. Downes, K. E. Smith, Dept of Physics, Boston University, Boston, MA; G. Drager, Experimentelle Physik V, Martin-Luther-Universitat, Halle/Saale, GERMANY.

    A promising application of resonantly excited SXE is the study of phase transitions induced, for instance, by changing the sample temperature. Much useful information concerning both the occupied and unoccupied density of states (DOS) has come from photoemission and x-ray absorption studies. However, we demonstrate the feasibility and large potential of using resonant soft x-ray emission spectroscopy (RSXES) for studying a variety of such phase transitions. We show that RSXES at the 2p3/2-threshold of V2O3 is sensitive to temperature-induced changes in the electronic structure. Both ordinary fluorescence and inelastic scattering, explained below, differ appreciably when taken through the metal-insulator transition at 168 K. We emphasize that, in principal, spectral resolution of this technique is not core-hole lifetime limited, yet one retains the advantages of orbital and elemental selectivity inherent to ordinary x-ray fluorescence spectroscopy. Therefore, this technique lends itself particularly to the investigation of various types of unusual phase transitions which have recently been discovered, e.g. in perovskite structured 3d-oxide compounds. Furthermore, we present resonantly excited soft x-ray emission spectra recorded at the V 2p3/2 threshold of several other vanadium oxides. We discuss the effects of the oxidation state of vanadium on the spectral shape. Resonantly excited SXE spectra consist in part of ordinary fluorescence involving a real core hole and consist to another part of inelastic scattering. Ordinary fluorescence reflects the partial, i.e. 3d-orbitally resolved, DOS, whereas inelastic scattering arises from excitations between the V 3d-valence band to the V 3d-conduction band. Therefore we find, not unexpectedly, that the intensity of the inelastic scattering contribution follows the occupancy of the V 3d-band. Ordinary fluorescence can be directly compared to the theoretical partial DOS, and, in particular, weak hybridization can be studied. Moreover, we can assess the influence of electron correlations from the deviation between band structure calculations and experiment.

    11:15 AM V4.8
    PHASE DIAGRAM AND ANISOTROPIC TRANSPORT PROPERTIES OF Nd1-xSrxMnO3 CRYSTALS. H. Kuwahara, Y. Tomioka, T. Okuda, T. Kimura, A. Asamitsu, Joint Research Center for Atom Technology (JRCAT), Tsukuba, JAPAN; Y. Tokura, Joint Research Center for Atom Technology (JRCAT), Tsukuba, JAPAN and Univ of Tokyo, Dept of Applied Physics, Tokyo, JAPAN.

    We have investigated electronic transport and magnetic properties of perovskite-type Nd1-xSrxMnO3 crystals with controlled doping concentration () grown by the floating zone method. The electronic phase diagram of Nd1-xSrxMnO3 was obtained by measurements of magnetization (magnetic structure), resistivity, and crystal structure. In particular, the electronic and magnetic properties are critically changed depending on x near the commensurate value x=0.50, in which the charge- and orbital-ordering transition accompanies the concomitant CE-type AF ordering occurs below =155K. In the charge-ordered state, the nominal Mn3+ and Mn4+ ions with 1:1 ratio show a real space ordering in the (001) plane (Pbnm notation). Such a charge-ordered state completely disappears and F metallic state is stabilized when x is decreased below 0.48. On the other hand, when x is increased beyond x=0.50 the A-type layered AF structure shows up. In the case of the x=0.55 crystal showing the A-type AF with TN=220K, the metallic behavior was observed within the F layers, while along the AF coupled direction the crystal remains insulating over the whole temperature region. The anisotropy ratio of resistivity for the AF and F directions is 102 at 4.2K although the multi-domain structures cannot be eliminated in crystal. The observed large anisotropy in spite of the nearly cubic lattice structure is due to the confinement of the spin-polarized carriers within the F sheets. No spin-canting along the AF direction indicates that carriers are confined within the F plane and DE mechanism along the AF direction is quenched. In accord with these magnetic and electronic transitions, the crystallographic structural changes occur: the ab plane (F plane) expands and the c axis (AF direction) shrinks below TN. These changes mean that the x2-y2 type orbitals lie in the F plane, as simply expected.

    11:30 AM V4.9
    STOICHIOMETRY AND MAGNETIC PROPERTIES OF IRON OXIDE FILMS. D.V. Dimitrov and G.C. Hadjipanayis, Department of Physics and Astronomy, University of Delaware, Newark DE; V. Papaefthymiou, Department of Physics and Astronomy, University of Ionannina, GREECE; A. Simopoulos, NCSR Demokritos, Athens, GREECE.

    We have studied the formation, structural and magnetic properties of Fe-O films. The films were prepared by reactive DC magnetron sputtering in a mixture of Ar and O2. The stoichiometry of the films was determined by X-ray diffraction (XRD). selected area diffraction (SAD) and Mössbauer spectroscopy. Magnetic measurements were performed by SQUID magnetometry between 10 and 380 K with a maximum field of 55 kOe. It was found that by increasing the oxygen partial pressure the stoichiometry of the films was gradually varied and passed through the following sequence; mixture of Fe and FeO, amorphous Fe-O, offstoichiometric single-phase FexO, mixture of FeO and Fe3O4, single-phase Fe3O4, mixture of Fe3O4 and -Fe2O3, mixture of Fe3O4 and -Fe2O3 and single-phase -Fe2O3. X-ray studies showed that the films that were mixtures of different phases were comprised of much smaller grains as compared to the single-phase films. Unusual ferromagnetic-like behavior was observed in offstoichiometric FexO and single-phase -Fe2O3 films, which are well-established antiferromagnets in bulk. Mössbauer studies revealed that in FexO films these properties were a consequence of the presence of Fe clusters in tetrahedral oxygen coordination. However, the ferromagnetic-like behavior in -Fe2O3 was found to be due to the presence of uncompensated surface spins. Both Mössbauer and magnetic data indicated that the magnetic moments in the surface component of -Fe2O3 grains were predominantly randomly oriented and only a small fraction contributed to the net magnetization. The Vervey transition in single-phase Fe3O4 films was observed in the temperature dependence of coercivity and low field magnetization. A systematic correlation between stoichiometry, microstructure and magnetic properties will be presented.

    11:45 AM V4.10
    NATURAL MAGNETIC MULTILAYERS IN A SERIES OF BaO : TiO2 : Fe2O3 COMPOUNDS. Leonid A. Bendersky, John E. Bonevich, Robert D. Shull, Terrell A. Vanderah and Robert S. Roth, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg MD.

    In certain applications improved magnetic materials with higher dielectric constants are required. The system BaO : TiO2 : Fe2O3 offers exciting possibilities for new materials where the high dielectric properties of the barium titanates can be combined with the strong magnetic properties of hexaferrites. In this work we will show that a group of newly found ternary BaO : TiO2 : Fe2O3 compounds have structural features that allow us to describe the structures as natural magnetic multilayers. The structures have alternating layered blocks of different thickness ratio and period. The blocks are structurally and compositionally different. One of the blocks can be considered as a dielectric spacer for the blocks which have a high concentration of Fe and are expected to be magnetic. Details of high-resolution and analytical transmission electron microscopy experiments as well as measurements of magnetic properties of the compounds will be presented in the paper.

    Chair: Janice Nickel
    Tuesday Afternoon, December 2, 1997
    Providence/Orleans (M)

    1:30 PM V5.1
    PARAMAGNETIC SUSCEPTIBILITY OF THE CMR COMPOUND La1-xCaxMnO3(0 x 1). Daniel H. Goodwin, J.J. Neumeier, Florida Atlantic Univ., Boca Raton, FL; A. Lacerda, Los Alamos National Laboratory, Los Alamos, NM; M. S. Torikachvili, San Diego State Univ., San Diego, CA.

    Measurements of the magnetic susceptibility in the temperature range 5K 0) in antiferromagnetic specimens will be addressed in the framework of the double-exchange model.

    1:45 PM V5.2
    CHARACTERIZATION OF LCMO/AU GRANULAR FILMS. Mark Rubinstein, Phillip Broussard, Laura H. Allen, Krystl B. Hathaway, Naval Research Laboratory, Washington, DC; Jonathan Z. Sun, IBM T.J. Watson Res. Ctr, Yorktown Heights NY.

    We have carried out a study of composite, granular, films of LaCaMnO and Au deposited onto MgO and La-aluminate substrates. The LaCaMnO layers were grown by either laser ablation or off- axis sputtering, with the Au layers deposited either by evaporation or simultaneous laser ablation. The films were studied by x-ray diffraction, scanning electron microscopy, magnetoresistivity, SQUID magnetization measurements, and MAMMA (magnetically modulated microwave absorption). We find that the magnetoresistance decreases as the amount of Au is increased, and that a separate peak in the magnetoresistance begins to develop. We have also compared these composites to composite films of the superconductor YBaCuO and Au to look for similarities in the granularity-induced properties.

    2:00 PM V5.3
    ENHANCED FIELD SENSITIVITY IN TRANSVERSE MAGNETORESISTANCE OF LANTHANUM MANGANITE THIN FILMS. J.E. Nunez-Regueiro and A.M. Kadin, Dept. of Electrical Engineering, University of Rochester, Rochester, NY.

    We have sputtered oriented LCMO thin films on LaAIO3substrates, and patterned the as-deposited films into a simple structure for measuring longitudinal and transverse resistance in a perpendicular B-field. We observe the usual large longitudinal magnetoresistance near the Curie temperature Tc = 260 K, which requires B of at least several kG in see an appreciable effect. But we also observe another peak at 246 K in the transverse resistance in small B, which can vary by 10% in B 100 G. This peak is present for B = 0 and is independent of the sign of B. and hence is not a Hall effect (a smaller true Hall voltage becomes evident at much larger fields). The sign and magnitude of this transverse magnetoresistance vary somewhat among several similar samples, but is present in all patterned structures. We suggest that this phenomenon may be due to randomly distributed small variations in Tc ( 20 K) within the film, due perhaps to granular microstructure and oxygen content. We have carried out simulations of such a locally inhomogeneous film using a two-dimensional resistor lattice and a simple parameterization for R(T,B) within a grain. These simulations provide qualitative agreement with the observed transverse magnetoresistance, and are also consistent with key features of the longitudinal magnetoresistance and the magnetization. We suggest that a film with a properly optimized spatial variation in Tc should maximize this effect, and may provide the basis for a magnetoresistive device that is sensitive to relatively small fields.

    2:15 PM V5.4
    EFFECT OF DOMAIN STRUCTURE ON THE MAGNETORESISTANCE OF EPITAXIAL THIN FILMS OF FERROMAGNETIC METALLIC OXIDE SrRuO3. R.A. Rao, D.B. Kacedon, and C.B. Eom, Department of Mechanical Engineering & Materials Science, Duke University, Durham, NC.

    The effect of crystallographic domain structures on the magnetoresistive behavior of epitaxial thin films of the ferromagnetic metallic oxide SrRuO3 has been studied. A strong anisotropic magnetoresistance (MR) has been observed in the single domain SrRuO3 thin film on a miscut (001) SrTiO3 substrate. In contrast, the SrRuO3 thin film on (001) LaAlO3 substrate shows identical MR behavior in two orthogonal directions on the film due to the presence of 90 domains in the plane. For both the films, large negative magnetoresistance effects (10) were observed when the current and the applied magnetic field are parallel. The MR behavior is explained in terms of suppression of spin fluctuations near Tc and magnetization rotation leading to a change in the angle between the current and magnetization at low temperatures. The hysteresis in the MR behavior and magnetization data from a SQUID magnetometer will also be discussed.

    3:00 PM V5.5
    THE LOCAL ATOMIC STRUCTURE OF La0.7Ca0.3Mn1-xBxO3 WITH B = Ti, Ga, BY PULSED NEUTRON DIFFRACTION. G.H. Kwei and Despina Louca, Los Alamos National Laboratory, Los Alamos, NM; A.P. Ramirez, Bell Laboratories, Lucent Technologies, Murray Hill, NJ; and M.A. Subramanian, Dupont Central Research and Development, Wilmington, DE.

    The perovskite La0.7NmO3 has been extensively studied due to its inherent magnetic and transport properties. It has been shown that the local atomic structure plays a critical role in the mechanism that brings about the colossal magnetoresistance (CMR) effect in this material. The substitution of manganese with a non-magnetic ion, either Ti or Ga, can result in a significant change of the properties. The magnetoresistance (MR) effect in compounds doped with Ga ion which has a nominal +3 oxidation state and an ionic radius comparable to the Mn3+ ion, is reduced with increased Ga doping. In addition, the Curie temperature (Tc) is lowered with increased Mn substitution. In samples with 10% of Ga, no insulator to metal (I-M) transition is observed. Compounds doped with Ti which can be of either 3+ or 4+ nominal charge state substituting for Mn3+ or Mn4+, exhibit a smaller MR effect in addition to lower Tc values than the Ga doped samples. Pulsed neutron powder diffraction data were collected at the High Intensity Powder Diffractometer (HIPD) of LANSCE at temperatures ranging from 20 to 300 K. Results from the crystallographic analysis and the pair distribution function (PDF) analysis will be presented.

    3:15 PM V5.6
    EELS ANALYSIS OF CATION VALENCES AND OXYGEN VACANCIES IN MAGNETORESISTIVE OXIDES. Z.L. Wang, J.S. Yin, Y.D. Jiang and J. Zhang*, School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, GA; *Motorola Inc.

    Transition and rare earth metal elements with mixed valences are usually present in oxide functional materials for stimulating structural and chemical evolutions [1]. The ionic structure of La1-xAxMnO3 y is proposed to be La3+1-xA2+xMn^3+_1-x+2yMn^4+_x-2yO^2-_3-y, where the partial substitution of trivalent La^3+by divalent element A^2+is balanced by the conversion of Mn valence states between Mn^3+and Mn^4+and the creation of oxygen vacancies as well. Electron energy-loss spectroscopy (EELS) is applied to determine the ratio of Mn^3+to Mn4+ preset in the specimen at high spatial resolution, the anion deficiency, y, is thus calculated accordingly because the average valence of Mn is (3 + x - 2y). For La1-xCaxMnO3-y (x = 0.33) (LCMO), our results indicate that the anion deficiency y is less than 0.065, which is equivalent to less than 2.2 at % of the oxygen content. At ymax = 0.065, the atom ratio of Mn4+ to Mn3+ in LCMO is 0.25, thus, the charge introduced by Mn valence conversion is (x-2y) = 0.2, the charge due to oxygen vacancy is 2y = 0.13, which means that 60% of the residual charge introduced by Ca doping is balanced by the conversion of Mn3+ to Mn4+ and 40% by oxygen vacancies.A similar analysis has been carried out for La0.5Sr0.5CoO3-y. The average valence state of Co is determined to be 2+, thus the chemical composition of the film is La0.5Sr0.5CoO2.25. Based on this chemical information, the crystal structure of this anion deficient ordered perovskite-type orthorhombic phase La0.5Sr0.5CoO2.25 (or La8Sr8Co16O36) has been determined, for the first time, with a conjunction use of high-resolution transmission electron microscopy, electron diffraction, image and diffraction simulations.

    3:30 PM V5.7
    NEGATIVE MAGNETORESISTANCE IN (Bi,Pb)2Sr3Co2O9 LAYERED COBALT OXIDES. Ichiro Tsukada, Tsuyoshi Yamamoto, Mikio Takagi, Takashi Tsubone, and Kunimitsu Uchinokura, The Univ of Tokyo, Dept of Applied Physics, Bunkyo-ku, Tokyo, JAPAN.

    Transport and magnetic properties of layered oxides (Bi,Pb)2Sr3Co2O9 are investigated with changing hole concentration. Bi2Sr3Co2O9 is nearly isomorphic to Bi2Sr2CaCu2O8 high-Tc superconductors, but has smaller lattice constants. It has nonmagnetic ground state with low-spin state of Co3+ ions, which is stabilized by a strong crystal field and is maintained up to room temperature. Sr vacancy and Pb substitution for Bi introduce holes into the parent compounds. The metallic conduction is realized from room temperature to 30 K (below 10 mcm), while the steep increase in resistivity is found in lower temperature region. The low-temperature insulating behavior is rapidly suppressed by magnetic field. The negative magnetoresistance is fairly large and hard to be explained by the collapse of weak localization. It seems to be strongly related to the ferromagnetic transition, which occurs typicaly below 4 K in Pb-substituted sample. The lack of electrons, however, indicates the absence of double-exchange interaction in these compounds, which plays an important role in CMR materials. We will discuss the origin of the magnetoresistance on the assumption that itinerant holes exist in band.

    Tuesday Afternoon, December 2, 1997
    4:00 P.M.
    Providence/Orleans (M)


    Y3Fe5O12(YIG) films grown by liquid phase epitaxial(LPE) method on the Gd3Ga5O12(GGG) substrate have been used as non-volatile magnetic memory and magneto-optic device, e.g. optical isolator. Because of their very low ferromagnetic resonance(FMR) linewidth, YIG films have been studied widely for microwave application these days. FMR linewidth is sensitive to impurities, defects and lattice misfit. In this experiment, YIG and La:YIG were grown on the GGG substrate using PbO-B2O3 flux system. The characteristics of Pb and La substitution, surface morphology and lattice misfit related to microwave loss were studied. Triangular pits arrayed in accord with crystallographic orientation were observed on the film surface. As the film thickness increased, pits were enlarged. Screw-like pittings generated from triangular pits were observed on the surface and they resulted from the uniaxial substrate rotation. They could be eliminated by changing the rotation direction periodically. In thick films the and facets were observed on the edge of surface. In the case of LPE growth of YIG film, lead ions from flux are substituted inevitably, and they play an important role in controlling film misfit. For complete lattice matching, high supersaturation is necessary in pure YIG growth, but this induces high defect concentration. In this experiment La ions were added in the melt to increase sufficiently the lattice parameter of the film grown under low supersaturation. Substituted Pb and La concentration was increased as the growth temperature was lowered and growth rate increased. The effective distribution coefficient of La was estimated about 0.2 around 850C. The optimum growth conditions which bring about very small misfit were determined by measuring the misfit by double crystal diffractometer. Strain distribution around the interface according to the misfit was investigated by triple crystal diffractometer. Lattice image of the interface was obtained by high resolution electron microscopy.

    MAGNETOTRANSPORT IN THIN FILMS OF (Lan-nxCa1+nx)MnnO3n+1 (n=2,3, and ). Hidefumi Asano, Jun Hayakawa, and Masaaki Matsui, Dpt of Crystalline Materials Science, Nagoya University, Nagoya, JAPAN.

    Perovskite compounds (Lax- nxCa1+nx)MnnO3n+1 (n=2,3, and ) composed of n layers of the MnO2 sheet are double-exchange ferromagnets exhibiting colossal magnetoresistance (CMR) effect. With a use of the epitaxial a-axis thin films with fixed carrier concentration (x=0.3), the transport properties of the series compounds have been examined to be associate with the difference in the number of the MnO2 layers. Results have indicated that the many intriguing features depend in an extreme sensitive way on the layer number. Decreasing the number resulted in systematic changes in the various features, such as increase of resistivity, decrease of the resistivity peak temperature TcP corresponding to the metal-insulator transition, enhancement of maximum MR near the TcP, and appearance of the low temperature intrinsic tunneling MR. In order to explain the variation of the magnitude of these features with decreasing the number of MnO2 layers, the reduction of the c-axis transfer interaction as well as the two dimensional spin fluctuations should be taken into account.


    In this work details about the growth and characterization of a novel class of magnetic multilayers based on radio-frequency (RF) sputtered thin amorphous garnet films are presented. One, three and five thin film multilayers composed by amorphous pure yttrium iron garnet (a:YIG) and amorphous gadolinium gallium garnet (a:GGG) have been grown by RF sputtering on GGG single crystal substrates. The micromorphological characterization was carried out by means of atomic force microscopy (AFM); the texture observed for the different samples that undergone a larger number of annealing cycles. The interfaces between different layers were characterized by means of secondary ion mass spectrometry (SIMS) profiles. In both the three and five thin film multilayers the interface widths seem to be comparable. Low field susceptibility measurements, performed by a SQUID magnetometer between 4 K and 300 K, showed a paramagnetic behaviour for the single layer YIG film. For the three and five layers samples, thermal hysteresis effect (difference between measurements performed at zero field and field cooling) were observed, giving evidence of magnetic clusters and spin glass-like behaviour.

    IMPROVEMENT OF THERMAL STABILITY OF METAL/OXIDE INTERFACE FOR ELECTRONICS DEVICES. Yo Ichikawa, Masayoshi Hiramoto, Nozomu Matsukawa, Kenji Iijima and Masatoshi Kitagawa, Central Research Laboratories, Matsushita Electric Industrial Co. Ltd, Kyoto, JAPAN.

    Preparation of metal/ceramic interface with a good thermal stability is crucial to develop micro-electronics devices for mobile equipment. To develop a useful and convenient fabrication method controlling a metal and ceramic interface, a thermal stability of multi-layered structures consist of metal and metal oxide with periodicity of several nano-meters has been investigated. Fe/Fe-O layered structures were prepared by pulse reactive rf-magnetron sputtering technique. Sputtering targets were pure Fe or Si doped Fe metal and pure oxygen gas was incorporated intermittently in sputtering chamber to grow thin oxide layers. The layered structure was characterized by high resolution TEM, low angle X-ray diffraction and high frequency permeability, before and after the annealing at high temperatures. The investigation revealed that the layered structure and the magnetic properties were degraded at about 300C for the pure Fe/Fe-O films. On the other hand, doping of Si into Fe and Fe-O layers improved the thermal stability up to 500C, indicating Si promoting the formation of highly stabilized interface. We will report the microscopic structure and related magnetic properties of multilayer films. Research supported by NEDO, under the Synergy Ceramics Project of the ISTF program promoted by AIST, MITI, Japan.

    ROOM TEMPERATURE MAGNETORESISTIVE RESPONSE IN CMR PEROVSKITE MANGANITE THIN FILMS. Michael A. Todd, C. Seegal, and T.H. Baum, Advanced Technology Materials, Inc., NovaMOS Division, Danbury, CT.

    Perovskite-structured manganite thin-films have recently attracted much attention for use in magnetic sensor applications. However, the incorporation of these materials into working devices is hindered by two fundamental limitations; (1) the metal to semiconductor transition occurs at low-temperature and (2) large applied magnetic fields are required to obtain useful magnetoresistive response (MR). In this work, we describe liquid delivery MOCVD of manganite thin-films that exhibit useful MR and transition temperatures at, or above, room temperature. A - 10 % MR response was observed at 47C in a 780 Oe applied field in La1-xSrxMnO3 films deposited onto LaAlO3 substrates. These films were characterized by Hall measurements (resistivity as a function of temperature in various fixed magnetic fields), EDS and XRD. Based upon our data, the observed film properties are a direct function of the deposited film stoichiometry.

    Chair: Andrew J. Millis
    Wednesday Morning, December 3, 1997
    Providence/Orleans (M)

    8:30 AM V7.1
    THE MAGNETIC SUSCEPIBILITY IN ULTRATHIN FILMS OF MAGNETIC MATERIALS. Kamakhya P. Ghatak, Dept. of Electronic Science, Calcutta Univ., Calcutta, INDIA; P.K. Bose and G. Majumder, Dept. of Mech. Engg., Jadavpur University, Calcutta, INDIA.

    During the last two decades, prodegious work has been done on various electronic properties of multilayered thin films. In this paper we have studied the dia and paramagnetic susceptibilities of the electrons in ultrathin films of magnetic materials in the presence of a parallel magnetic field on the basis of a newly derived dispersion law for such systems. The numerical computations are performed taking various magnetic compounds. Both the susceptibilities increase with decreasing doping and film thickness respectively. It is important to note that not only the paramagnetic-to-diamagnetic susceptibility ratio for the present case deviates from (1/3) in conventional semiconductors, but also that is a critical region, where quenching of the diamagnetic occurs. The theoretical analysis is in agreement with the experimental datas as given elsewhere.

    8:45 AM V7.2
    MICROSCOPIC MECHANISM OF COLOSSAL MAGNETORESISTANCE: ELECTRON CORRELATION BY VIRTUAL PHONON EXCHANGE. Michel D. Kaplan, Boston University, Physics Dept, Boston MA, and Simmons College, Chemistry Dept, Boston MA; and George O. Zimmerman, Physics Dept, Boston University, Boston MA.

    We suggest that the microscopic mechanism of the colossal magnetoresistance is fundamentally dependent on the correlations of the orbitally degenerate d-electrons caused by virtual phonon exchange. As all compounds with colossal magnetoresistance are Jahn-Teller crystals, the virtual phonon exchange for them is the combination of the electron-phonon and electron-strain interactions and of the phonon dispersion. An external magnetic field can induce a structural phase transition accompanied by the reconstruction of the electronic structure. As a result of that, the magnetic interactions are changing their character, the charge ordering can be suppressed, and the conductivity is drastically increased. Two types of polarons should be involved in the conductivity consideration. However, while the polaronic effects are important, the role of virtual phonon exchange and the corresponding electronic states ordering in the magnetic field is crucial. The suggested concept will be considered in greater detail for the specific compound crystal of LaSr0.12Mn0.88O3.

    9:00 AM *V7.3
    SMALL-POLARON FORMATION AND MOTION IN MAGNETIC SEMICONDUCTORS. David Emin, University of New Mexico, Dept of Physics and Astronomy, Albuquerque, NM.

    Small-polaron hopping is observed in various transition-metal and rare-earth oxides. Several interactions contribute synergistically to carriers forming small polarons. Carriers' long-range electron-lattice interactions with displaceable ions are of Coulombic origin and are usually strong, having an interaction energy comparable to electrons' Coulomb repulsion energy, U. Short-range electron-lattice interactions, that provide the dominant contribution to the activation energy for small-polaron hopping, also contribute to self-trapping. Bound polarons are confined to defects and impurities that provide seats for carrier localization. A bound magnetic polaron (BMP) is a polaron bound at an impurity in a magnetic semiconductor. Experimental and theoretical studies indicate that large- (multi-site) bound states in a ferromagnetic semiconductor (e.g., EuO) abruptly collapse into small bound polarons as the temperature is raised toward the Curie point. With suitable dopant densities this collapse occurs as EuO tranforms from being a ferromagnetic metal at low temperatures to its being a paramagnetic semiconductor at higher temperatures. Then the suppression of metallic impurity-band transport accompanies impurity states' collapse. The rise of the transition temperature with an applied magnetic field produces a large (up to 15 orders of magnitude) negative magnetoresistance in the vicinity of the transition. A similar effect was discovered in p-type doped LaMnO3 by Volger in 1954. The transition in LaMnO3 would be analogous to that in EuO if 1) its transport takes place between dopant regions and 2) its ferromagnetism is not induced by the carriers. The smallness of the Seebeck coefficients in lightly doped LaMnO3 and the (anomalously signed, n-type) Hall effect in LaMnO3 are consistent with impurity conduction. The insensitivity of the Curie temperature of doped p-type LaMnO3 to reducing the carrier concentration by introducing oxygen vacancies suggests that ferromagnetism is induced by dopant's strain rather than by their carriers.

    9:30 AM V7.4
    CALCULATED TRANSPORT AND MAGNETIC PROPERTIES OF SOME PEROVSKITE METALLIC OXIDES . Gilles Santi and Thomas Jarlborg, DPMC, Universite de Geneve, Quai Ernest-Ansermet, Geneve, SWITZERLAND.

    We study some compounds of the perovskite (or pseudo-cubic perovskite) series , where M is a transition metal and A is Ca, Sr, etc., by LSDA self-consistent electronic structure calculation with the LMTO method. Transport and magnetic properties, as well as Fermi surfaces are calculated and presented. The magnetic stability of different magnetic structures is studied via the Stoner factor for paramagnetic calculations and total energy for spin-polarized ones. These materials exhibit sharp density of states features in the vicinity of the Fermi level that strongly affect their transport and magnetic properties and make them very sensitive to structural deformation and stoichiometry. This explains partly the variety of magnetic behaviours shown by this family of compounds.

    9:45 AM V7.5
    EXPERIMENTAL DETERMINATION OF THE KEY ENERGY SCALES IN THE COLOSSAL MAGNETORESISTIVE OXIDES. D.S. Dessau and T. Saitoh, University of Colorado, Boulder, CO; C.-H. Park and Z.-X. Shen, Stanford University, Stanford, CA; Y. Moritomo and Y. Tokura, JRCAT, Tsukuba, JAPAN.

    We have performed X-ray absorption (XAS) and angle-resolved photoemission (ARPES) on single crystals of both the layered and cubic colossal magnetoresistive manganites to determine the electronic structure and the relevant energy scales in the problem: the intra-atomic exchange energy J (2.7 eV), the Jahn-Teller energy gain E0 (<.25 eV), and the one-electron bandwidth W (>3 eV for layered compounds and up to 4.5 eV for cubic compounds). W comparable to or larger than J implies the near- electrons may not be completely polarized, and E0 small compared to W implies a Jahn-Teller polaron may be unlikely.

    10:30 AM *V7.6

    Spin-tunneling and tunnel magnetoresistance (TMR) attract much attention at the moment because of their promise for magnetic sensors and memory devices. Basic ideas and outstanding questions will be discussed in relation to conventional magnetics and magnetic half-metallic oxides. Direct tunneling, which is mainly responsible for the observed behavior of conventional junctions, will be compared with impurity-assisted and resonant spin tunneling. It is shown that the assisted or resonant events tend to reduce a magnitude of TMR, but dramatically increase current through the structure. Model applies to half-metallic systems with 100% spin polarization, where resistance can change by a few 1000 percent even with imperfect junctions, as illustrated for CrO2-based systems with matching oxides. Relevance of these results for magnetic oxides with colossal MR will be discussed.

    11:00 AM *V7.7
    FORMATION OF FERROMAGNETIC/FERROELECTRIC SUPERLATTICES BY A LASER MBE AND THEIR ELECTRIC & MAGNETIC PROPERTIES. Hitoshi Tabata, Kenji Ueda and Tomoji Kawai, Institute of Scientific and Industrial Research, Osaka University, Osaka, JAPAN

    Transition metal oxides have a lot of interesting potential tor the functional electric devices. Early transition metal oxides such as PZT and BaTiO3, show ferroelectric and dielectric properties cortesponding to their band insulative characters. They are also expected to large piezoelectric and electro-strictive effect. Magnetic character is interesting on the middle transition metal oxide group (Mn Fe Co etc.) with increasing the number of spins.
    Until now the materials researches tend to be performed independently on magnet and ferroelectric field. We have constructed superlattices with a combination of ferroelectric/ferromagnetic materials by a laser MBE technique. An ideal hetero-epitaxy can be obtained owing to the similar lattice parameter of ferroelectric BaTiO3, PZT and antiferro- or ferro-magnetic (La,Sr)MnO3, LaCrO3, LaFeO3, BiMnO3 and BiFeO3.
    We can introduce the lattice stress easily by applying voltage for the piezoelectric compounds. The CMR effect of (La,Sr)MnO3 layer is strongly affected by the lattice stress due to the piezoelectric properties of PZT layer. In the FET-type devices consisting of ferromagnetic /ferroelectric heterostrusture show interesting phenomena. Electromagnetic properties of ferromagnetic layer can be controlled by changing the lattice distortion and induced charge via changing of applied voltage for the ferroelectric layer.

    11:30 AM V7.8
    LOW-ENERGY k-DEPENDENT ELECTRONIC STRUCTURE OF THE LAYERED MAGNETORESISTIVE OXIDE La1.2Sr1.8Mn2O7. T. Saitoh, D.S. Dessau, C.-H. Park, Z.-X. Shen, P. Villella, N. Hamada, Y. Moritomo, and Y. Tokura, University of Tokyo, Department of Applied Physics, Tokyo, JAPAN.

    We have studied the k-dependent electronic structure of the layered colossal magnetoresistive oxide La1.2Sr1 .8Mn207 using high-resolution angle-resolved photoemission spectroscopy. We found dispersive energy bands as a function of the crystal momentum k near the Fermi level (EF). The overall dispersion relation is basically in agreement with the band-structure calculations yet close to EF there is a significant deviation from the predicted dispersions. Instead of clear Fermi-surface (FS) crossings, we observe a depression of the features as the FS is approached as if there is a ``soft'' or ``pseudo'' gap in the excitation spectrum. The pseudogap continuously opens with temperature and does not show further significant opening above , corresponding to the metal-insulator transition. There is minimal k dependence to the pseudogap, indicating that it most likely originates from a local effect such as a lattice distortion.

    11:45 AM V7.9
    ELECTRICAL AND MAGNETIC PROPERTIES OF FERROMAGNETIC-ANTIFERROMAGNETIC BI-LAYERS OF PEROVSKITE MANGANESE OXIDES. M. Rajeswari*, A. Goyal, R. Shreekala, J.Y. Gu, K. Ghosh, P. Fournier, R. Ramesh*, T. Venkatesan*, (*NSF MRSEC on Oxides and Surfaces), Center for Superconductivity Research, University of Maryland, College Park, MD; Y. Ijiri, J.A. Borchers, J.W. Lynn*, Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD.

    We are investigating interlayer coupling effects in ferromagnetic-antiferromagnetic bilayers of the perovskite manganese oxide Lal-xCaxMnO3 and several other members of the family popularly known as colossal magnetoresistive oxides. Inter-layer magnetic interactions in these bilayers are of interest for utilizing exchange biasing effects in spin valve structures as well as for suppressing the noise associated with magnetic domain fluctuations in magnetoresistive sensors. Exchange biasing effects are very sensitive to several factors such as the nature of the ferromagnetic-antiferromagnetic interface, thickness of the layers etc. In order to minimize possible degradation of the interface due to interdiffusion and interracial reactions, we chose the antiferromagnetic layer to have essentially the same composition as the ferromagnctic layer except for the variation in the dopant concentration x. We will discuss the growth optimization by pulsed laser deposition and the magnetic and magnto-transport properties of such bi-layers.

    Chair: R. Ramesh
    Wednesday Afternoon, December 3, 1997
    Providence/Orleans (M)

    1:30 PM *V8.1
    SUB-200 Oe GIANT MAGNETORESISTANCE IN MANGANITE TUNNEL JUNCTIONS*. Gang Xiao, Dept of Physics, Brown University, Providence, RI.

    Metallic rnanganite oxidfes La1-xDxMnO3 (D=Sr, Ca, etc.), display ``colossal'' magnetoresistance (CMR) near their magnetic phase transition temperatures (Tc) when subject to a Tesla-scale magnetic field. This phenomenal effect is the result of the strong interplay inherent in this class of materials among electronic structure, magnetic ordering, and lattice dynamics. Though fundamentally interesting the CMR effect achieved at large fields poses severe technical challenges to applications in magnetoelectronic devices, where low field sensitivity is crucial. Among the objectives of our research effort involving manganite materials is to reduce the field scale of MR by designing and fabricating tunnel junctions and other structures rich in magnetic domain walls. The junction electrodes were made of doped rnanganite epitaxial films, and the insulating barrier of SrTiO3. The interfacial expitaxy has been imaged by using high resolution transmission electron microscopy. We have used self-aligned lithographic process to pattern the junctions to micron scale in size. Large MR values exceeding 80% at low fields of a few tens of Oe have been observed. The mechanism of the spin-dependent transport is due to the spin-polarized tunneling between the half-metallic electrodes, in which the spins of the conduction electrons are nearly fully polarized. We will present results of field and temperature dependence of MR in these structures and discuss the electronic structure of the manganite inferred from tunneling measurement. Results of large MR at low fields due to the grain-boundary effect will also be presented.

    2:00 PM V8.2
    COLOSSAL MAGNETORESISTANCE EFFECTS AT LOW FIELDS IN MANGANITE TUNNEL SPIN VALVES. M. Viret(1), M. Drouet(2), J. P. Contour(2), J. Nassar(2), C. Fermon(1) and A. Fert(2) (1) CEA Saclay, Service de physique de lÈtat condensè, Gif-sur-Yvette, FRANCE (2) UMR CNRS/Thomson-CSF, LCR, Orsay, FRANCE.

    All-oxide tunnel spin valves in which electrodes of mixed valence manganites (La_0.7Sr_0.3MnO_3) are separated by a thin insulating layer have been synthesised. The structures were prepared by Pulsed Laser Deposition and several tunnel barriers were used including PrBa_2(CuGa)_3O_7, CeO_2 and SrTiO_3. The latter gives the largest magnetoresistive effects where the trilayers' resistances are increased by factors greater than 5 at 4.2 K and at fields below 100 Gauss. The inferred band polarisation at the Fermi level is in excess of 80%. This supports the view that in the manganites, the bands crossing the Fermi surface are mainly 3d bands with full spin polarisation. Analysis of the temperature variation of the barrier resistivity shows that the dramatic loss of the magnetoresistive effect above 150K is due to a reduced oxygen content of the interface between La_0.7Sr_0.3MnO_3 and the barrier. Specific oxydisation during deposition leads to improvements in the temperature behaviour of the device.

    2:15 PM V8.3

    Ferromagnet-insulator-ferromagnet tunneling junctions using a ramp-edge geometry based on doped manganite ferromagnetic electrodes and a SrTiO3 (STO) insulator have been successfully fabricated. As expected for the spin-dependent tunneling, the junction magnetoresistance (JMR) is observed to be dependent on the relative orientation of the magnetization in the electrodes. In (La0.7Sr0.3)MnO3/STO/(La0.7Sr0.3)MnO3 ramp-edge junctions, a junction magnetoresistance as large as 23% is observed at low fields (< 300 Oe) and low temperatures (T< 100K). I-V characteristics and the temperature dependence of junction resistance are indicative of a high quality tunneling barrier. The spin-dependent tunneling characteristics are observed even at room temperature. These junctions also withstand temperature cycling and are stable in ambient conditions. The maximum JMR at high temperature is believed to be limited by the choice of top and bottom electrodes and the magnetization process in the electrodes, rather than by the insulating barrier. The effect of insulating barrier thickness and the optimum tunneling barrier will be discussed. The results using different barrier layer including SrRuO3 and other electrodes will also be discussed.

    2:30 PM V8.4
    GROWTH OF ALL OXIDE BASED SPIN VALVES. M.C. Robson, Y. Gim, S.B. Ogale, T. Venkatesan, and R. Ramesh, Center for Superconductivity Research, University of Maryland, College Park, MD; M. Johnson, Naval Research Laboratory, Washington, DC.

    We will present our work on the development of oxide based spin valves. Spin valves consist of two ferromagnetic layers of differing coercive fields that are separated by a non-magnetic metallic spacer. When a current is applied across the spin valve, spin polarized electrons travel from one ferromagnetic layer to the other through the metallic spacer. If the metallic layer thickness is less than the spin depth, , most of the electrons will reach the second ferromagnetic layer with their spin orientation undisturbed. Therefore, the resistance drop across the spin valve will depend on the relative orientation of the magnetizations of the ferromagnetic layers (either parallel or antiparallel). The resistance of the antiparallel state (Ra) will have a higher value than that of the parallel state (Rp). This leads to a magnetoresistance effect, (Ra - Rp)/Rp. The appearance of the two resistance states is a result of the different coercive fields of the ferromagnetic layers (Hc1 < Hc2). When the applied field is greater than |Hc2| or less than |Hc1| the ferromagnetic layers are magnetized parallel to each other, whereas between |Hc1| and |H c2| the ferromagnetic layers are magnetized antiparallel. Therefore, we will describe our fabrication of such a spin valve using La0.7Sr0.3MnO3 as the ferromagnetic material and La0.5Sr0.5CoO3 or SrRuO3 as the non-magnetic metallic spacer. The spin valve has been grown by in-situ pulsed laser deposition on LaAlO3 substrate. We have deposited La0.7Sr0.3MnO3 at two different deposition temperatures in order to achieve two different coercive fields between 20 and 80 Oe. X-ray diffraction patterns show predominantly c-axis orientation of all layers in the structure. TEM images show sharp interfaces with no interdiffusion. Preliminary magnetoresistance data will also be presented. This work is supported by DARPA and MRSEC.

    3:15 PM *V8.5
    BOLOMETRIC AND MAGNETORESISTIVE APPLICATIONS OF PEROVSKITE MANGANESE OXIDE THIN FILMS. M. Rajeswari, NSF MRSEC on Oxide and Surfaces, Center for Superconductivity Research, University of Maryland, College Park, MD.

    Perovskite rare-earth manganese oxides have recently been a focus of active research due to the potential for magnetoresistive applications related to the colossal magnetoresistance effect. Another less widely known application of these materials is related to the insulator-metal transition in zero fields. The sharp resistive drop close to this transition makes these materials potential candidates for uncooled bolometric detector applications. In this talk I will present some of the recent work which address issues related to both the above-mentioned applications. For the infrared detector applications, the large 1/f noise present in these materials has been a major concern. I will discuss the anomalous characteristics and possible origin of the l/f noise as well as the correlation of the noise magnitude with the material microstructure, oxygen content and the magnetic homogeneity. I will also address several approaches to reduce the noise magnitudes. Another remarkable observation in this context is the significance of the temperature coefficient of resistance as an indicator of the structural quality and magnetic homogeneity of the sample. From the perspective of magnetoresistive sensor applications, the focus will bc on the contribution of inter-granular transport to the magnetoresistance. Recent work has demonstrated that the behavior of magnetoresistance in granular bulk materials and polycrystalline thin films is qualitatively different from that of the intrinsic magnetoresistance in single crystal and epitaxial films. This difference has been indicated to be related to tunneling transport across grain boundaries. This tunneling magnetoresistance remains large over a wide temperature range and may potentially be exploited for applications.

    3:45 PM V8.6
    GROWTH AND CHARACTERIZATION OF La1-xCaxMnO3/La2-xSrxCuO4 MULTILAYERS. J. Fompeyrine1,2 and J.-P. Locquet2. 1 Institute for Inorganic Chemistry, Universit'at Bern, Bern, SWITZERLAND; 2 IBM Research Division, Zurich Research Laboratory, Rueschlikon, SWITZERLAND.

    Since a few years, there is a great interest in the colossal magnetoresistance (CMR) oxides with chemical composition La1-xCaxMnO3 (LCMO). It was also shown that the large magnetic fields required to obtain the CMR can be significantly reduced by using heterostructures. Further, the magnetic and transport properties turned out to be extremely stress sensitive. Here we report the properties of heterostructures where the manganite layer is sandwiched between the cuprate La2-xSrxCuO4 (LSCO). Since the transport properties and in-plane lattice parameters change vastly as a function of the doping for both compounds, such heterostructures provide a good model system to tune their properties. The first results of the tri- and multilayers suggest a good chemical compatibility between the two compounds with a limited interdiffusion. The in-plane transport properties are generally dominated by the metallic (superconducting) nature of the LSCO layer (Sr = 0.15). Therefore, to extract the intrinsic properties of a LCMO layer, the out-of-plane transport properties must be measured. We obtained them from microscopic devices, prepared using focused ion beam lithography. Since such measurements require a significant interlayer resistance, our first efforts have been focussed on semiconducting manganite layers.

    4:00 PM V8.7
    FABRICATION OF HIGH TEMPERATURE SUPERCONDUCTOR-COLOSSAL MAGNETORESISTOR SPIN INJECTION DEVICES. J. Kim, R. M. Stroud, R. C. Y. Auyeung, C. R. Eddy, D. Koller, M. S. Osofsky, R. J. Soulen Jr., J. S. Horwitz, and D. B. Chrisey, Naval Research Laboratory, Surface Modification Branch, Washington, DC.

    High temperature superconductor/insulator/ferromagnetic oxide devices have been fabricated for the study of spin polarized transport in high Tc materials. YBa2Cu3O /SrTiO3 /La0.7Sr0.3MnO3 trilayers (YBCO/STO/LSMO) were grown on SrTiO3 substrates by pulsed laser deposition. The YBCO and LSMO films were patterned into perpendicular bow-ties by standard photolithographic etching techniques, resulting in a junction area of 140 m x 140 m. Films were typically grown at 700C - 800C in 50mT - 350mT of O2, at a laser fluence of 1.8 - 2.2 J/cm2. The high quality of the trilayer films is demonstrated by x-ray diffraction (XRD) and transport measurements of the individual layers. The XRD data showed epitaxial growth in the (001)YBCO(100)STO(100)LSMO directions. The typical rocking curve widths of the LSMO (200) and YBCO (005) peaks were 0.06 and 0.3, respectively. The films were annealed in flowing oxygen to remove oxygen deficiency and other defects, as evidenced by increased lattice parameters or high room temperature resistivity (). Post-anneal characterization of the YBCO showed c = 11.69, -cm, T 88K and ; LSMO showed a = 3.86, -cm, and . Optimization of device parameters such as film thickness and junction area for the maximum spin injection effect will be discussed.

    4:15 PM V8.8
    NOVEL HIGH-Tc TRANSISTORS WITH MANGANITE OXIDES. Z.W. Dong*, R. Ramesh,* Mark Johnson, ** Z.Y. Chen, S.P. Pai, Y.G. Zhao, R.P. Sharma, C.J. Lobb, R.L. Greene, T. Venkatesan*, Center for Superconductivity Research, Department of Physics, University of Maryland, College Park, MD; *Also at NSF MRSEC on Oxide Thin Films, Probes and Surfaces, University of Maryland; **Naval Research Laboratory, Washington, DC.

    In the last few years, a large effort has been devoted to the development of transistor-like three terminal high Tc superconductor (HTS) devices, and one viable approach is the quasiparticle injection device (QPID). The injection of quasiparticles into a superconductor creates a local nonequilibrium state which suppresses the superconducting order parameter and depresses the critical current density. HTS has a lower carrier density and should be more sensitive to quasiparticle injection, but the gain of HTS QPIDs has been low. However, the injected quasiparticles can be spin-polarized by using a ferromagnetic manganite gate layer. A current gain greater than one has been attained for spin polarized injection devices, which is 10 to 30 times larger than the gain of spin-unpolarized injection devices. Such large effects could be useful in a variety of active HTS/colossal magnetoresistance heterostructure based devices. In order to study spin-polarized and unpolarized quasiparticle injection effects in HTS, Au/YBa2Cu3O7/LaAlO3/Nd0.7Sr0.3MnO3 (NSMO) and Au/YBa2Cu3O7/LaAlO3/LaNiO3 (LNO) heterostructures were made by pulsed laser deposition on (100) LaAlO3 (LAO) substrates. The effect of injection of spin polarized quasiparticles from a ferromagnetic NSMO gate layer was compared to that of unpolarized quasiparticles from a nonmagnetic metallic LNO gate. A detailed comparison between those two types of QPIDs and a systematic investigation of the LAO barrier thickness dependence of injection effects have been carried out. It showed that in order to obtain a considerable current gain, certain range of LAO thickness is required.

    4:30 PM V8.9

    A new use for magnetite was discovered this Spring at ATR Electronics, Inc. a high tech venture in Minnesota. Magnetite, as found in the final slurries feeding the formation of taconite, is an excellent magnetic material for constructing boost inductors, filter chokes, and final stage smoothing filters found in modern power supplies. No prior art exists surrounding the use of magnetite as a power magnetic component material. Research effort on this material has been established at the University of Minnesota at the Theoretical Physics Research Laboratory under Dr. Goldman, Chairman of the Physics Department. The impact of this discovery is a dramatic reduction in the cost of power converters and the increase in energy conversion efficiency. Patents have been filed.

    4:45 PM V8.10
    EXCHANGE COUPLING AND RELATED INTERFACE EFFECTS IN EPITAXIAL Fe3O4/LSMO AND Fe3O4/LSMO HETEROSTRUCTURES GROWN ON LaA1O3. K. Ghosh, S.B. Ogale, R. Ramesh, T. Venkatesan and R.L. Greene, Center for Superconductivity Research, University of Maryland, College Park, MD.

    Exchange coupling between the CMR manganite La2/3Sr1/3MnO3(LSMO) and Ferrite Fe3O4 in bilayer and trilayer configurations with SrTiO3(STO) has been investigated. For this study the heterostructure of Fe3O4/STO/LSMO was grown epitaxially on (001) LaAlO3 substrate by pulsed laser deposition. The magnetization for different thicknesses of the STO layer was measured by a SQUID magnetometer. Field cooled hysteresis measurements at low temperature (5K) show a large anisotropy (100 Oe) in the M-H loop in the case of the bilayer structure which suggests large exchange coupling between Fe3O4 and LSMO. When an STO layer of thickness 100 A is incorporated between the Fe3O4 and LSMO layers, the anisotropy in the M-H loop vanishes and we observe a double loop in the M-H curve reflecting two distinct coercive fields, is seen. The exchange coupling effect on LSMO by Fe3O4 overlayer for different thicknesses of the insulating layer of STO will be discussed in detail.

    Chair: Michael F. Hundley
    Thursday Morning, December 4, 1997
    Providence/Orleans (M)

    8:30 AM *V9.1
    UNDERSTANDING AND UTILIZING CMR MATERIALS. L.F. Cohen Blackett Laboratory, Imperial College, London, UNITED KINGDOM.

    There has been a revival of interest in the metallic manganite oxide materials in recent years as they may prove useful for applications. Two related magneto resistive (MR) effects are observed in these materials. The first is the so called colossal MR behaviour which is observed close to the ferromagnetic transition temperature (Tc) and is associated with a metal semiconductor transition. The second as yet less studied effect, increases as the temperature is lowered away from Tc. Activities related to optimising these materials for utilisation are focused towards maximising Tc and enhancing the low field MR behaviour. It appears that understanding the role played by microstructure in particular grain boundaries may hold the key to utilisation. A review of the progress in these areas will be reported.

    9:00 AM V9.2
    GRAIN BOUNDARY EFFECTS ON THE TRANSPORT PROPERTIES OF La0.7Sr0.3MnO THIN FILMS. J.Y. Gu, R. Shreekala, M. Rajeswari, T. Venkatesan, and R. Ramesh, Center for Superconductivity Research, University of Maryland, College Park, MD; C. Kwon, Los Alamos National Laboratory, Los Alamos, NM.

    We have systematically studied the effects of grain boundaries on the transport properties of manganese oxide thin films. To introduce grain boundaries in La0.7Sr0.3MnO (LSMO) films, we have grown LSMO using different heterostructures. For example, LSMO films were fabricated on SiO2/Si(001) substrates using a Bi4Ti3O12 template layer or on top of SrTiO3/YBa2Cu3O7 grown on MgO substrates. For comparison with these heterostructures, we have also grown epitaxial and highly in-plane oriented LSMO on BTO/LaAlO3 substrates. The films in this study have been made by in-situ pulsed laser deposition. X-ray -2 and -scan measurements show that the LSMO layer on Bi4Ti3O12/SiO2/Si is c-axis textured with random in-plane orientations. From the x-ray -scan and planar TEM measurements, the LSMO layer in the LSMO/SrTiO3/YBa2Cu3O7/MgO heterostructure is found to have 45 grain boundaries between [100] and [110]. This is expected because YBa2Cu3O7 grown on MgO is well known to have such grain boundaries and the LSMO/SrTiO3 layers follow the in-plane orientation of the underlying YBa2Cu3O7. The electrical and magnetic properties of the LSMO layer for each heterostructure were investigated. The magnetic properties of these films show the same behavior. On the other hand, the transport properties, such as resistivity and magnetoresistance, show very different behavior. This indicates that the in-plane grain boundaries affect the transport properties of manganese oxide thin films. For example, the resistivity behavior as a function of temperature for the LSMO/Bi4Ti3O12/SiO2/Si films is found to be dominated by grain boundary effects. Low field sensitive magnetoresistance at room temperature of these films suggests spin tunneling through the grain boundaries. This work is supported by ONR and MRSEC.

    9:15 AM V9.3
    OBSERVATION OF GROWTH-RELATED MAGNETIC STRUCTURES IN La0.67Sr0.33MnO3. M.E. Hawley, G.W. Brown, C. Kwon, M.F. Hundley, and Q.X. Jia, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM.

    Ambient observation of magnetic structures by magnetic force microscopy (MFM) in La0.67Sr_0.33MnO_3films has not yet been clearly correlated with stresses induced by the kinetic or thermodynamic growth processes or the compressive (LaAlO_3) or tensile (SrTiO_3) nature of the substrate lattice-mismatch. Although domain-like magnetic structures have been seen in some as-grown films and related to substrate-induced stress and film thickness, no magnetic structure has been seen for other films grown under different kinetic conditions on the same pair of substrates. In this study we have identified a set of growth temperatures, O_2 pressures, repetition rates, and cooling rates for our pulsed-laser grown films, using the above substrates, to determine the relationship between growth and stress-induced magnetic structures. Results from scanning tunneling, atomic force, and MFM microscopies, temperature-dependent transport and magnetization measurements and grain size versus film coercivity will be presented to show the relationship between growth and magnetic properties.

    9:30 AM V9.4
    INFLUENCE OF INTERFACIAL STRAIN AND ANNEALING ON STRUCTURE AND PROPERTIES OF EPITAXIAL La0.67Ba0.33MnO3 FILMS. L. Ryen, E. Olsson, Department of Physics, Chalmers University of Technology, Göteborg, SWEDEN; C. Kwon, R. Ramesh, Center for Superconductivity Research, Department of Physics, University of Maryland, College Park, MD.

    The magnetic properties of mixed-valence manganites have been show to depend on oxygen stoichiometry, elastic lattice strain, and grain boundaries. This work is a transmission electron microscopy investigation, with emphasis on the effect of interfacial strain, oxygen annealing and other crystal defects on the magnetic behavior. One as grown and one annealed epitaxial La0.67Ba0.33MnO3 (LBMO) single layer films have been characterized. Both films were laser deposited on (110)rhombohedral LaAlO3 (LAO) substrates. The microstructures of the films have been compared to an as grown LBMO film, deposited with a SrTiO3 (STO) buffer layer. All films grew epitaxially with the full orientation relationships (001)LBMO//(110)LAOr and (100)LBMO//(101)LAOr for the single layer films, (001)LBMO//(001)STO//(110)LAO and (100)LBMO//(100)STO//(101)LAOr for the sample with STO buffer layer. Misfit dislocations were present in the single layer samples. The single layer films also exhibited a columnar morphology. The annealing affected the number density of interfacial dislocations and column boundaries. The as grown single layer film had a remnant 0.8% strain, while the annealed film was fully relaxed with less grain boundaries. Mn3O4 precipitates were observed in the as grown samples, but not in the annealed film. The STO buffer layer improved the crystal quality of the LBMO, compared to the single layer films. Results from electron energy-loss spectroscopy concerning the manganese oxidation state and the effect of compositional deviations on the lattice parameters, will be presented.

    9:45 AM V9.5
    STRAIN EFFECT ON THE MAGNETIC AND ELECTRICAL PROPERTIES OF EPITAXIAL MAGNETIC METALLIC OXIDE SrRuO3 THIN FILMS. Q. Gan, R.A. Rao, and C.B. Eom, Department of Mechanical Engineering & Materials Science, Duke University, Durham, NC; M. Lee, Department of Physics, The University of Virginia, Charlottesville, VA.

    We have investigated the strain effect on the magnetic and electrical properties of epitaxial magnetic perovskite SrRuO3 thin films. The single domain epitaxial SrRuO3 thin films were grown on vicinal (001) SrTiO3 substrates with 8º miscut toward [010]. The strain state was studied by x-ray diffraction, which indicated that the as-grown SrRuO3 thin films were subjected to bi-axial compressive strain in the plane. By etching the substrates preferentially, we obtained free-standing SrRuO3 thin films. X-ray diffraction showed that the lattice parameters of the free-standing thin films are the same as that of the bulk SrRuO3, i.e., the free-standing thin films are strain relaxed. The SQUID magnetic and electrical transport measurements of the as-grown and free-standing thin films showed a significant increase of the Curie temperature from 150K to 160K before and after strain relaxation. The changes in the magnetization and electrical properties will also be discussed. We correlate these property changes to the structural distortion which had resulted from the strain in the epitaxial thin films.

    10:30 AM *V9.6
    SPIN DYNAMICS OF MAGNETORESISTIVE OXIDES. Robert H. Heffner, Los Alamos National Laboratory, Los Alamos, NM.

    Manganites with the perovskite structure, such as (La, A)MnO3, A = Ca, Ba, and Sr, have been studied extensively in the last two years as examples of systems exhibiting strong coupling between spin, charge and lattice degrees-of-freedom 1. One manifestation of this coupling is a large negative magnetoresistance () which occurs near the ferromagnetic/metal-insulator transition TC 2. Transport measurements are consistent with small-polaron hopping above TC 3 and neutron PDF measurements 4 give evidence for local structural distortions just below TC. These results have been modeled theoretically with a combination of double-exchange between Mn spins and polaronic lattice distortions 5. There have been comparatively few microscopic experiments elucidating the spin dynamics in these materials, however. In this talk we review muon spin relaxation experiments in the ferromagnetic state of the perovskites 6 and layered manganites, giving evidence for polaronic degrees-of-freedom below TC. The results, which are not typical of conventional ferromagnets, are discussed in terms of models of spin-lattice-charge coupling. The spin dynamics of additional classes of magnetoresistive ferromagnetic oxides (such as the pyrochlore structures Tl2Mn2O7 7 and the mono-oxide EuO) will be discussed as contrasting examples.

    11:00 AM V9.7
    THICKNESS DEPENDENCE OF TRANSPORT IN La0.7Ca0.3MnO3 THIN FILMS. A. Goyal, M. Rajeswari, R. Shreekala, K. Ghosh, and T. Venkatesan, Center for Superconductivity Research, University of Maryland, College Park, MD.

    We have studied the changes in transport behavior of La0.7Ca0.3MnO3 LCMO thin films with thickness variation. Thickness dependence is of interest from several view points. From a fundamental perspective, it allows us to probe the system as it approaches unit cell limit and also to address questions of dimensionality. Other important aspects are the effect of lattice-mismatch induced stress and interfacial effects. We have grown LCMO thin films on LAO by pulsed laser deposition (PLD) technique, with thicknesses varying from 125 to 2500 . We find that the film properties change significantly as the thickness is reduced below 1000 . We have studied the resistivity, the temperature coefficient of resistance (TCR) and the lattice parameter as a function of thickness. It is interesting to note that upon subjecting thinner films to heat treatment most of the film properties approach that of thicker (2000 ) films. We will present the results of our experiments designed to distinguish intrinsic effects from stress and interface related effects.

    11:15 AM V9.8
    SPUTTERED THIN FILMS OF La0.67Ca0.33MnO; ANOTHER LOOK AT THE THICKNESS DEPENDENCE OF THE PHYSICAL PROPERTIES. S. Freisem, J. Aarts, R. Hendrikx, Kamerlingh Onnes Laboratory, Leiden University, Leiden, NETHERLANDS; H. W. Zandbergen, Laboratory of Material Science, Delft University of Technology, Delft, NETHERLANDS.

    The physical properties of manganese perovskites, especially those related to Colossal MagnetoResistance, are very sensitive to lattice distortions. Thin films on different substrates appear to adapt well to the underlying mismatched lattice and are usually strained, resulting in Curie temperatures Tc, resistance peak temperatures Tp and Magnetoresistance Ratio's MR different from bulk values. Given the sensitivity to strain, it is not surprising that literature values for these parameters show wide spreads, since the film morphology is mostly determined by type and temperature of deposition, substrate, thickness, etc. We have therefore undertaken a systematic study of transport and magnetic properties, combined with X-ray diffraction and TEM measurements, on one material, La0.67Ca0.33MnO, deposited by high-pressure oxygen magnetron sputtering at 840 , on SrTiO3 and LaAlO3, for thicknesses between 15 nm and 200 nm.
    We find that in this range the lattice parameters have not yet relaxed, with especially smooth growth on SrTiO3, that for all films 175 and that all resistances R(T) above Tc show the same activated behaviour. However, Tp, usually somewhat below Tc, and MR can show surprisingly different behaviour with occasionally very low Tp and consequently high MR, which appears to be linked to the saturation magnetization Ms. Generally, the thinnest films show values around 2.5 per Mn atom (measured in fields up to 5 T, and to be compared to 3 - 4 per Mn atom for the bulk material), but this decreases strongly with thickness, indicating increasing amounts of antiferromagnetically coupled Mn, even though the films do not appear seriously defective. Since values for Ms in films are usually not reported, we note that this may be a general feature of thin films, and we will discuss how it may affect the transport properties, including history effects.

    11:30 AM V9.9
    THE EFFECT OF RADIATION INDUCED DISORDER ON La1-xCaxMnyO THIN FILMS. R. M. Stroud, V. M. Browning, W. W. Fuller-Mora, J. Kim, D. L. Knies, K. S. Grabowski, M. S. Osofosky, J. S. Horwitz and D. B. Chrisey, Naval Research Laboratory, Washington, DC.

    The effect of disorder on the resistivity and magnetization of pulsed laser deposited La1-xCaxMnyO thin films (1000 - 3000 ) was studied. Films deposited on LaAlO3 substrates were annealed to ensure full oxidation and minimize growth defects, and then ion irradiated to introduce specific defect concentrations. High resolution transmission electron microscopy studies showed that before irradiation the annealed films were well-ordered, except for strain at the substrate-film interface. Damage levels of 0.006 to 0.024 displacements per atom (dpa), as calculated using TRIM, were produced with a range of incident ions (10 MeV Si, 6 MeV I, 180 KeV He) and fluences (0.5 - 2.0 * 1013 ions/cm2, 1.1 - 4.4 * 1014 ions/cm2, 0.6 - 2.4 * 1015 ions/cm2). For the same overall displacement damage concentration, the type and distribution of disorder depends on the incident ion: at 0.006 dpa, the average spacing between incident ion tracks is 45 for I, 9.5 for Si and 4 for He; the defect type tends toward dense cascades for the heavier ion, I, and point defects for the lighter ion, He. The resistivity of the annealed films peaked at T = 230K to 260K, depending on composition. With increasing ion induced disorder, the peak resistivity temperature decreased: for a 2600 film with nominal composition La0.7Ca0.3MnO3 and damage level of 0.018 dpa, Tp decreased from 250K to 95K. A corresponding decrease in Tc and the saturation magnetization was also observed. The relative effects of disorder produced by the Si, I and He ions, on well-ordered, O deficient and Mn deficient films will be discussed.

    11:45 AM V9.10
    HALL EFFECT OF La1-xCaxMnO3 EPITAXTAL THIN FILM ON (100) AND (110) SrTiO3 SUBSTRATES. Q.Y. Chen, X.T. Cui, W.N. Kang, Z.J. Qu, and W.K. Chu, Texas Center for Superconductivity and Department of Physics, University of Houston, Houston, TX.

    We have investigated the temperature and field dependent Hall effect of La1-xCaxMnO3 thin films grown on (100) and (110) SrTiO3 substrates. The epitaxial films were grown by laser ablation using a cascaded deposition and annealing procedure. The samples are of high quality judged from xray diffraction. Rutherford backscattering spectroscopy, and ion channeling analysis. The Curie temperature was about 250K for the (lOO)-oriented thin films. and was lower for the (110) films, raging from 160K 200K based on our preliminary work. We have found that while the Hall resistivity Indicated holelike behavior for a wide range of temperature and magnetic field, below 1OOK there was a transition from being holelike to electronlike. Such transition became progressively evident as the field strength was increased. In this presentation we will describe the scaling of Hall resistivity with magnetoresistivity and its relevance to the intrinsic material properties, especially the charge carrier scattering mechanism in relation to the local magnetization.

    Chair: Robert H. Heffner
    Thursday Afternoon, December 4, 1997
    Providence/Orleans (M)

    1:30 PM V10.1
    THERMOGRAVIMETRIC STUDY OF THE OXYGEN NONSTOICHIOMETRY IN A SERIES OF DOPED AND UNDOPED Ni-Zn FERRITES. Erich M. Gundlach, Patrick K. Gallagher, The Ohio State Univ., Dept. of Chemistry, Columbus, OH.

    Oxygen non-stoichiometry plays an important role in the magnetic, electrical, and mechanical properties of ferrite materials. The processing of ferrites possessing the desired oxygen non-stoichiometry requires knowledge of its relationship to temperature and oxygen partial pressure; however, these relationships are known only for a limited number of Ni-Zn ferrite compositions. The PO2 vs. temperature phase diagrams for a series of Ni-Zn ferrites were determined by thermogravimetric analysis under atmospheres of varying PO2, 2x10-4 to 1 atm, in the temperature range 800C to 1400C. In addition, the effects of titania, silica, and calcia doping upon these phase equilibria were studied. Isostoichiometric lines are shown on plots of log PO2 vs. 1/T.

    1:45 PM V10.2
    EFFECT OF OXYGEN NON-STOICHIOMETRY ON THE STRUCTURAL, MAGNETIC AND MAGNETOTRANSPORT PROPERTIES OF La0.85Sr0.15MnO SINGLE CRYSTALS. P. Berthet, J. Berthon, A.M. de Léon-Guevarra and A. Revcolevschi, Laboratoire de Chimie des Solides, Orsay, FRANCE; A. Anane, C. Dupas, K. Le Dang, L-P. Renard, and P. Veillet, Institut d'Electronique Fondamentale, Université Paris-Sud, Orsay, FRANCE.

    The oxygen non-stoichiometry is expected to play an important role in the electronic and magnetic properties of colossal magnetoresistance manganites with a composition close to the metal-insulator phase boundary. We report here a detailed study of single crystals of La0.85Sr0.15MnO with various oxygen vacancies from = 0 to - 0.055. The b and c lattice parameters were found to increase with d whereas the a parameter remains practically constant. As expected, the oxygen vacancies reduce the hole concentration and therefore the Curie temperature and the electrical conductivity. However these effects are somewhat different from those observed in the hole-doped La1-xSrxMnO3 systems, owing to different lattice distortion. The lattice change of the sample is illustrated by a strong shift in the hyperfine field of Mn4+ ions, detected with the NMR method. The oxygen deficiency and all the above properties resulting from it were shown to be reversible.

    2:00 PM *V10.3
    VOLUME-BASED THERMODYNAMIC CONSIDERATIONS FOR THE MAGNETORESISTANCE AND METAL-INSULATOR TRANSITION OF CMR OXIDES. J. J. Neumeier, Florida Atlantic University, Department of Physics, Boca Raton, FL; A. L. Cornelius, Los Alamos National Laboratory, Materials Science and Technology Division, Los Alamos, NM.

    Decreasing the volume of CMR materials, by hydrostatic pressure, can reduce the electrical resistivity by orders of magnitude in some cases [1]. The volume sensitivity of is well-illustrated when samples at the compositional metal-insulator boundary are studied. For example, a ferromagnetic specimen of La0.79Ca0.21MnO3, which is an insulator at low temperature and ambient pressure ((5 K) = 100 -cm), exhibits a metal-insulator transition at a pressure of only 3.4 kbar. From thermodynamics we know that at the ferromagnetic to paramagnetic phase transition, a change in slope of the volume vs temperature V(T) occurs. In a CMR material which exhibits a metal-insulator transition, the decrease of V(T) below TC generates a self pressure which is larger than 5 kbar. Such a self pressure would be negligible in conventional ferromagnetic metals where the influence of pressure on the electrical resistivity is small, however, in CMR materials the self pressure cannot be neglected. To investigate the influence of self pressure on the electrical properties, namely the metal-insulator transition and magnetoresistance, we have conducted measurements of the electrical resistivity under pressure, the thermal expansion as a function of temperature and magnetic field, and the magnetostriction. Our results suggest that the metal-insulator transition may be attributed to the thermodynamically generated self pressure and that a significant portion of the magnetoresistance may result from the negative magnetostriction.

    3:00 PM V10.4
    INFLUENCE OF REDUCED OXYGEN PARTIAL PRESSURE ON RAMAN PHONON SPECTROSCOPY, OUT OF PLANE LATTICE PARAMETERS AND PHYSICAL PROPERTIES. N. Malde, S. de Silva, A.K.M. Hossain and L.F. Cohen, Blackett Laboratory, Imperial College, London, UNITED KINGDOM; T. Kodenkandath and J.D. MacManus Driscoll, Materials Department, Imperial College, London, UNITED KINGDOM; N.D. Mathur, M.G. Blamire, J.E. Evetts, Material Department, Cambridge University, Cambridge, UNITED KINGDOM.

    Optimization of so called colossal magneto-resistance materials to produce large MR at high temperatures is a key issue. As with all functional metal oxide materials it is important to address the sensitivity of the material to oxygen content. This was the main objective of this study and it was achieved by using a coulometric titration technique to anneal films in various reduced oxygen atmospheres. The influence of annealing at reduced oxygen pressure on the physical and structural properties of several La0.7Ca0.3MnO3 thin films has been determined. The c axis lattice parameters and some of the Raman modes are rather insensitive to oxygation condition. However the temperature at which the peak magneto-resistance occurs and other Raman modes change continuously as a function of decreasing oxygen partial pressure.

    3:15 PM V10.5
    RAMAN INVESTIGATION OF THE LAYERED MANGANESE PEROVSKITE LaSrMnO. D.B. Romero, V.B. Podobedov, A. Weber, J.P. Rice, National Institute of Standards and Technology, Gaithersburg, MD; J.F. Mitchell, Argonne National Laboratory, Argonne, IL; R.P. Sharma, H.D. Drew, University of Maryland, College Park, MD.

    A detailed polarization and temperature dependence study of the Raman scattering from the layered manganese perovskite LaSrMnO reveals the anisotropic electronic and lattice properties of this compound that manifest signatures of the transition from the paramagnetic-insulating to the ferromagnetic-metallic state. The anomalous behavior of two phonons associated with the manganese and planar oxygen atoms provide evidence of a highly distorted MnO octahedron in the metallic state and a local structural transition induced at the onset of ferromagnetic ordering. The scattering of the Mn e-electron leads to a broad electronic background in the spectra. The low-frequency electronic scattering within the MnO plane is strongly temperature dependent indicating a suppression below T and an enhancement above T. On the other hand, the out-of-plane spectra show that the electronic scattering is suppressed at all temperatures. These electronic and lattice effects point to the importance of disorder-induced localization and spin-lattice interactions in understanding the transport and magnetic properties of the quasi two-dimensional manganites.

    3:30 PM V10.6
    ANALYSIS OF VIBRONIC MODES IN La-Ca-Mn-O EPITAXIALLY GROWN THIN FILMS BY RAMAN SCATTERING. H.-U. Habermeier, P.X. Zhang, T. Zhou, and T. Ruf, MPI für Festkörperforschung, Stuttgart, GERMANY.

    Thin films of the system La1-xCaxMnO3 system are deposited on SrTiO3 substrates using an optimized pulsed laser deposition process. The films are epitaxially grown as demonstrated by XRD pole figure and high resolution cross sectional TEM analysis. Raman scattering by lattice vibrations or other elemental excitations is used to analyze detailed information on local structures and related interactions. The Raman lines in the 200 cm-1 to 800 cm-1 region could be identified as vibrations of the oxygen in the octahedra surrounding the Mn ions using oxygen isotope substituted polycrystalline samples. A thorough analysis of the position and temperature dependence of the Raman lines of epitaxially grown La1-xCaxMnO3 thin films with x = 0.0, 0.1, and 0.33 reveal that the microscopic distortion of the oxygen octahedra play a critical role in determining the vibrational modes. Upon cooling the films across the magnetic ordering temperature, some of the Raman peaks show a temperature dependent red shift. The close correlation of this red shift the temperature dependent scattering intensity and the symmetry analysis of the scattering process is used to analyse the scattering mechanism. Furthermore, informations on spin ordering, spin-phonon and spin electron interactions can be obtained.

    3:45 PM V10.7
    HIGH FREQUENCY MAGNETO-ELECTRODYNAMICS OF Sr-DOPED MANGANITE SINGLE CRYSTALS. B. Revcolevschi, H. Srikanth, Z. Zhai, D.P. Choudhury, S. Sridhar, Northeastern University, Dept. of Physics, Boston, MA; L. Pinsard, A. Revcolevschi, Universite Paris, Laboratoire de Chimie des Solides, Orsay, FRANCE.

    In addition to exhibiting colossal magnetoresistance (CMR), the mixed manganese perovskite oxides have several interesting properties. The close correlation between electronic, magnetic and structural properties in this system has been well established. In a bid to elucidate the high frequency characteristics and the inter dependence of these effects, we have studied the metallic La1-xSrxMnO3 (x=0.175) system. A self-resonant tunnel diode oscillator (TDO) technique is employed to investigate the radio frequency response of the La-Sr-Mn-O single crystals. Our method has a unique advantage in that the influence of transport and magnetic phenomena can be examined in the same experiment. We observe a gigantic switching behavior at the ferromagnetic transition at 300K. This has significant application potential in high frequency devices. Structural transition from rhombhohedral-orthorhombic phases is also seen as the temperature is lowered around 220K. The magnetic field dependence of individual transitions and the response at microwave frequencies will also be presented.

    Thursday Afternoon, December 4, 1997
    4:00 P.M.
    Providence/Orleans (M)

    INFLUENCE OF STRAIN AND STRUCTURAL HOMOGENEITY ON MAGNETOTRANSPORT IN LCMO THIN FILMS. K.A. Thomas1, P.S.I.P.N. de Silva2, L.F. Cohen2, M. Rajeswari3, A. Goyal3, T. Venkatesan3, N.D. Mathur4, M.G. Blamire4, J.E. Evetts4, R.Hiskes5 and J.L. MacManus-Driscoll1. 1Dept. of Materials, Imperial College, London, UK; 2Blackett Laboratory, Imperial College, London, UK; 3Dept. of Physics and Electrical Engineering, Univ. of Maryland, MD; 4Dept. of Materials Science, Univ. of Cambridge, Cambridge, UK; 5Hewlett-Packard, Palo Alto, CA.

    As-grown La0.7Ca0.3MnO3 (LCMO) thin films made by pulsed laser deposition (PLD) and metal-organic chemical vapour deposition (MOCVD) on LaAIO3 were subjected to a series of short, controlled anneals to allow grain growth and concomitant relaxation of the strain induced during epitaxial growth process. After each anneal, the film microstructure were characterized to determine average grain size, lattice constants, crystalline mosaic spread and were correlated with the magnetotransport properties. The stain analysis was carried out from x-ray data using the integral breadth method. The oxygen content was not changed in the films from the first to the subsequent anneals. The influence of annealing on the PLD films was more pronounced than for the MOCVD films because the as-grown PLD films were more crystalline with better initial epitaxy but with a higher degree of non-uniform strain. The relaxation of interfacial strain was therefore more pronounced for the PLD films upon annealing. For both sets of films, the influence of annealing was to increase both Tc and peak magnetoresistance. The increase in these parameters has been shown to arise due to a decrease in non-uniform strain. This is the first time this effect has been observed and quantified in LCMO epitaxial films.

    MAGNETOTHERMAL CONDUCTIVITY IN MANGANESE PEROVSKITE OXIDES. Baoxing Chen, Jihui Yang, Siqing Hu, Ctirad Uher, Department of Physics, University of Michigan, Ann Arbor, MI.

    We have investigated the thermal conductivity, thermopower, and resistivity of bulk manganese-based perovskite oxide samples from 2K to 350K and under a magnetic field as high as 9T. Large changes in the magnetothermal conductivity were found around the magnetic transition temperature and they accompany large magnetoresistivity and magnetothmopower found previously. The temperature dependence of the thermal conductivity displays a striking dip, while the resistivity and the thermopower peak near the transition temperatures. Both the dips and the peaks shift toward high temperature upon the increase of magnetic field. The temperature and field dependences of the thermal conductivity are attributed to the scattering of the phonons by spin fluctuations.

    ANISOTROPIC STRAINS AND MAGNETORESISTANCE OF La0.7Ca0.3MnO3. T. Y. Koo, S. H. Park, K.-B. Lee, and Y. H. Jeong, Department of Physics, Pohang University of Science and Technology, Pohang, SOUTH KOREA.

    Thin films of perovskite manganite La0.7Ca0.3MnO3 were grown epitaxially on SrTiO3(100), MgO(100) and LaAlO3(100) substrates by the pulsed laser deposition method. Microscopic structures of these thin film samples as well as a bulk sample were fully determined by x-ray diffraction measurements. The unit cells of the three films have different shapes, i.e., contracted tetragonal, cubic, and elongated tetragonal for SrTiO3, MgO, and LaAlO3 cases, respectively, while the unit cell of the bulk is cubic. It is found that the samples with cubic unit cell show smaller peak magnetoresistance at low fields ( 1 T) than the noncubic ones do. The present result demonstrates that the magnetoresistance of La0.7Ca0.3MnO3 at low fields can be controlled by lattice distortion via externally imposed strains.


    Two kinds of local magnetization inhomogeneities are investigated in order to realise switchable magnetoresistance in thin epitaxial manganite films: the lateral inhomogeneity in bicrystal grain boundaries and the vertical inhomogeneity in coupled magnetic film systems. Epitaxial Lax(Sr,Ca)1-xMnO films have been sputtered on monocrystalline and bicrystalline substrates and their magnetoresistance measured as a function of magnetic field and temperature. In the bicrystalline films the intrinsic magnetoresistance near the Curie temperature is separated from the grain boundary resistance. In the film systems different types of magnetic coupling are demonstrated and their influence on magnetoresistance studied.

    OF ION IMPLANTATION ON MAGNETORESISTANCE IN La0.67Ca0.33MnO3 THIN FILMS. S. de Silva, N. Malde, A.K.M. Hossain and L.F. Cohen, Blackett Laboratory, Imperial College, London, UNITED KINGDOM; T. Kodenkandath, R. Chater and J.D. MacManus Driscoll, Materials Department, Imperial College, London, UNITED KINGDOM; T. Tate, Department of Electrical Engineering, Imperial College, London, UNITED KINGDOM; N.D. Mathur, M.G. Blamire, J.E. Evetts, Material Department, Cambridge University, Cambridge, UNITED KINGDOM.

    We report polarized Raman spectra of LaMMnO crystals doped with Sr or Ca (x=0, 0.1, 0.2 and 0.3) in the temperature range from 5 K to 423 K. Small distortion of the nearly cubic structure of doped LaMMnO single crystals results in the structure of Raman spectra as that composed of two parts with different contribution depending on the value of doping. The first part is assigned to the distorted non-cubic perovskite lattice. The second part is mainly due to the density of vibrational states and is classified as the second-order Raman scattering. It is shown that the reduction of the JT distortion in LaMMnO doped crystals has the same affect on Raman spectra as the change of symmetry of crystal due to the temperature-induced phase transition. The dependence of the external mode frequency was found to be opposite to that expected from the difference in the atomic weight of Sr and La. This dependence may be used for Raman diagnostics of the LaMMnO crystals. The abrupt frequency shift of the B mode in LaMnO crystal, assigned to a paramagnetic-canted antiferromagnetic phase transition near T =140 K, was explained within a polaron effect.