About the 1996 Electronic Materials Conference: Thursday Morning Sessions (June 27)

Session K: Magnetoresistive Oxides

8:20AM, K1 *Invited

"Magnetoresistive and Magnetostructural Phase Transitions in Perovskite Manganites:" Y. TOKURA, A. Asamitsu, H. Kuwahara, Y. Tomioka, Y. Moritomo, M. Kasai, Joint Research Center for Atom Technology (JRCAT-ATP/JRCAT-NAIR), Tsukuba 350, Japan

Ample examples of magnetic field-induced phase transitions ^(1-4) in the manganites are presented from a view point of new magnetoelectronic and magnetostructural functionality. To enhance the magnetoresistance in a reduced magnetic field, we can utilize the competing instabilities, i.e. the charge-ordering accompanying the collective electron- lattice coupling and the double-exchange interaction, for the 50% hole-doped compounds or the slightly deviated hole concentrations from 50%.

For the same purpose, another approach was also successfully taken; that is the exploration of layered perovskite manganites with a reduced electronic dimensionality.⁵ The hole-doped manganite with MnO bilayer-sheets shows a much enhanced field-sensitivity, though the Tc is lowered to ca. 130K, as compared with the corresponding pseudo-cubic perovskite.

Recent attempts to transfer such functionality in single crystals to the film samples are also reported.

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO.), Japan.

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¹A. Asamitsu et al, Nature 373, 407 (1995)
²Y. Tomioka et al., Phys.Rev.Lett. 74, 5108 (1995)
³H. Kuwahara et al, Science 270, 961 (1995)
⁴H. Kuwahara et atl., Science (in press)

9:00AM, K2 *Invited

"Study of CMR Films' Microstructure as a Function of PVD Growth Conditions and Post-Anneal:" M.E. HAWLEY, C.D. Adams, P.N. Arendt, E.L. Brosha⁵, F.H. Garzon, R.J. Houlton, M.F. Hundley, R.H. Heffner, X.D. Wu, Center for Materials Science, MS K765, Los Alamos National Laboratory, Los Alamos, NM 87545; Superconductivity Technology Center, MS 763, Los Alamos National Laboratory, Los Alamos, NM 87545; MST-7, MS E549; ⁴MST-10, MS K764, Los Alamos National Laboratory, Los Alamos, NM 87545; MST-11, MS D549, Material Science & Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545

We find the magnetic and electrical transport properties of annealed manganite films, La0.67Ca0.33 MnO3 and La0.67 Sr0.33 MnO3 essentially the same as those for bulk polycrystalline and single crystal material. In the metallic state the intrinsic magnetization decreases as T² while the intrinsic resistivity increases proportional to T2. We identify three regions of magnetoresistance in these materials. The largest effect is likely due to magnetic disorder scattering near the curie temperature. We find small polaron hopping behavior in the high temperature resistivity as well as a phase transition at 750K in La0.67Ca0.33 MnO3. The limiting low and high temperature resistivities place a limit on the maximum possible magnetoresistance of these materials. We have also investigated in detail the magnetic field dependence of the longitudinal and transverse magnetoresistance above and below Tc. The quadratic and linear dependence of the magnetoresistance above and below Tc respectively can be explained using fundamental symmetry considerations. In very low fields, we find a much more complicated behavior. The related perovskite, Gd0.67 Ca0.33 MnO3, is ferrimagnetic and shows polaron hopping conductivity with no transition to a metal at the Curie temperature.

Included in this family encompassed by the family of complex metal are the new colossal magneto-resistive perovskites with potential applications in advanced high density magnetic data storage devices based on single or multilayer thin films units of these materials fabricated by vapor phase deposition (PVD) methods. Realization of this potential requires solving basic thin film materials problems requiring understanding and controlling the growth of these materials and understanding the inter-relationship between structure and properties. Toward this end, we have grown La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 on LaAlO3 and SrTiO3 single crystal substrates by pulsed laser and RF sputter deposition at temperatures from 500deg.C to 900deg.C with and without a post-deposition anneal at over 900deg.C for about 10 hours. The evolution of the microstructure of these films was studied by scanning probe microscopies (SPM), x-ray diffraction, and transmission electron microscopy (TEM). The SPM characterization showed that at the lower end of the growth temperature range, the as-grown films were polygranular with grain size increasing with temperature. The 500deg.C as-grown films appeared to be amorphous while the 750deg.C film grains were layered with terrace steps often one unit cell high. In contrast, films grown at 900deg.C consisted of coalesced islands with some 3-D surface crystals. After annealing, all films coalesced into very large extended unit layers. The change in microstructure was reflected in improved transport properties of the coalesced films over their unannealed granular precursors. Previous reported work on the growth of La0.84 Sr0.16 MnO3 and La0.8Sr0.2CoO3 demonstrated the sensitivity of the microstructure to substrate and deposition conditions.

9:40AM, K3

"Intrinsic Properties of Colossal Magnetoresistive Manganites:" G. JEFFREY SNYDER, Ron Hiskes, Steve DiCarolis, M.R. Beasley¹, T. H. Geballe,Department of Applied Physics, Stanford University, Stanford, CA 94305-4090; Hewlett-Packard, 3500 Deer Creek Rd., Palo Alto, CA 94303-0867

We find the magnetic and electrical transport properties of annealed manganite films, La0.67Ca0.33 MnO3 and La0.67 Sr0.33 MnO3 essentially the same as those for bulk polycrystalline and single crystal material. In the metallic state the intrinsic magnetization decreases as T² while the intrinsic resistivity increases proportional to T2. We identify three regions of magnetoresistance in these materials. The largest effect is likely due to magnetic disorder scattering near the curie temperature. We find small polaron hopping behavior in the high temperature resistivity as well as a phase transition at 750K in La0.67Ca0.33 MnO3. The limiting low and high temperature resistivities place a limit on the maximum possible magnetoresistance of these materials. We have also investigated in detail the magnetic field dependence of the longitudinal and transverse magnetoresistance above and below Tc. The quadratic and linear dependence of the magnetoresistance above and below Tc respectively can be explained using fundamental symmetry considerations. In very low fields, we find a much more complicated behavior. The related perovskite, Gd0.67 Ca0.33 MnO3, is ferrimagnetic and shows polaron hopping conductivity with no transition to a metal at the Curie temperature.

10:20AM, K4*Invited

"Fabrication and Characterization of "Colossal" Magnetorestive Oxide Thin Films:" C. KWON, Center for Superconductivity Research, Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742

Colossal magnetoresistance (CMR) of the order of 1O⁶% or more has been obtained in epitaxial (AxB1-x)MnO3-y (A = La, Nd, Sm; B = Ba, Sr, Ca) thin films grown by pulsed laser deposition. This magnetoresistance phenomena has been observed in a wide frequency range from dc to microwave and visible. The transport and magnetic properties of CMR films depend strongly on the sample preparation conditions during the deposition. The oxygen stoichiometry is likely responsible for the dependence between the sample preparation conditions and the magnetic properties. We have also used Ferromagnetic Resonance (FMR) to evaluate the magnetic homogeneity of as grown films. Trilayer samples of SrTiO3 /(La0.7Ba0.3)MnO3/SrTiO3 show a uniform magnetization compared to the single LBMO layers where the perpendicular magnetization has a parabolic shape with a spatial variation of ~ 20%. The strain between the substrate and the (La0.7Ba0.3)MnO3 layer may cause this spatial variation of the magnetization in a single layer sample. The study of SrTiO3 /(La0.7Ca0.3)MnO3 superlattices shows the importance of the interfacial stress in determining their properties. In this presentation we will also discuss our progress to date on the studies to enhance the field and temperature dependencies of the MR properties and various other measurements performed on the epitaxial CMR films.

This work is supported by Office of Naval Research under Grant No. ONR-N0001495 105470NR. In collaboration with M. C. Robson, K.-C. Kim, R. Ramesh, Z. Trajanovic, C. H. Chen, M. Rajeswari, R. P. Sharma, T. Venkatesan, S. Lofland, S. M. Bhagat, M. Quijada, H. D. Drew, and R. L Greene.

11:00AM, K5 *Invited

"A Solution Chemical Route to La1-xSrxMnO3 Thin Films Exhibiting Giant Magnetoresistance:" A.R. MODAK, Silicon Systems, Inc., 2300 Delaware Ave., Santa Cruz, CA 95060; Kannan M. Krishnan, Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720

La1-x MxMnO3 (where M = Sr, Ba or Ca) thin films exhibiting very high magnetoresistance are being actively investigated because of their potential impact on the performance of magnetic field sensing and data storage devices based on MR technology. To date, laser ablation has been the preferred film deposition technique for fabricating these thin films in spite of such limitations as small deposition area and high substrate temperature. Hence, alternative thin film deposition techniques such as chemical vapor deposition, sputtering and sol-gel processing are also being pursued.

We have developed a solution chemical process for depositing La1-x SrxMnO3 thin films that can also be applied to Ca and Ba based compositions by appropriate modification of precursors. This process offers advantages of overall simplicity, compatibility with IC planar technology, low processing temperature, excellent compositional control and ability to produce a diverse range of structures. The process starts with organometallic precursors of La, Sr and Mn which undergo a series of steps of complexation, followed by hydrolysis and condensation to yield the (LaSrMn) stock solution. The complex solution is then deposited on Si(100) and LaAlO3(l00) substrates by spin coating followed by heat treatment at 700deg.C to yield La1-xSrxMnO3 (x = 0 - 1) thin films. Epitaxial (100) oriented thin films were obtained on LaAlO3 (100) substrates and films on Si(100) were polycrystalline. Single phase thin films were obtained for compositions up to 40% Sr, after which two phase microstructure were typical, consistent with results from bulk experiments. Large MR values up to 600% for epitaxial films and 130% for polycrystalline films were observed. Detailed microstructural characteristics (XRD and TEM) of these films and their relationship to their magnetic and magnetoresistance properties will be discussed.

11:40AM, K6

"Exchange Coupling in Ferrite Heterostructures:" Y. SUZUKI, R.B. van Dover, E.M. Gyorgy, Bell Laboratories, 600 Mountain Ave., Murray Hill, NJ 07974; Julia M. Phillips, Sandia National Labs, PO Box 5800, Albuquerque, NM 87185-0601

Transition metal oxides exhibit a wide range of electronic and magnetic properties and are promising for an integrated oxide-based technology. Magnetic oxide thin films with high saturation magnetization and Curie temperature, combined with minimal eddy current, magnetostrictive, hysteresis and dielectric losses, are of recent interest in view of high frequency applications. In particular, spinal-structure ferrites are very promising. Spinel ferrite thin films have been grown on a variety of substrates but with mediocre magnetic properties. We have grown high quality single crystalline spinel ferrite thin films and heterostructures by introducing a buffer layer between the spinel ferrite and conventional oxide substrates. Ferrite films grown on SrTiO3 and MgA12O4 buffered with CoCr2O4 exhibit excellent crystallinity and bulk saturation magnetization values. Analyses based on X-ray diffraction, Rutherford backscattering, atomic force microscopy and transmission electron microscopy provide a consistent picture of the structural properties of these films. Good magnetic properties are observed only in films with high structural quality. In (Mn, Zn)Fe2O4 /CoFe204 ferrite bilayers, we observe at room temperature strong exchange coupling between the layers that can be described in terms of a one dimensional model of the coherent rotation of spins. The exchange coupling constant of the interface, Aint, that is estimated from the exchange biased (Mn, Zn)Fe2O4 loops is comparable to estimated exchange coupling constants within each individual layer, AMZF and ACF. This result contrasts with exchange coupling in polycrystalline Ni0.8Fe0.2/Mn0.5Fe0.5 Ni0 81Fe0 19/Ni0 53Mn0 47 bilayers where Aint is at best two orders of magnitude smaller than estimates of ANiFe. In these metallic bilayers, the weak exchange coupling is attributed to imperfect interfaces. In our system, the strong exchange coupling indicates the high structural quality of the interface.