AVS2000 Session MI-MoA: Thin Films and Multilayers

Monday, October 2, 2000 2:00 PM in Room 206

Monday Afternoon

Start	Invited?	Item
2:00 PM		MI-MoA-1 Positive Exchange Bias Model: Fe/FeF₂ and Fe/MnF₂ Bilayers M. Kiwi, J. Mejía-López, R.D. Portugal, R. Ramírez (Pontificia Universidad Católica, Chile) Positive exchange bias (PEB) is a phenomenon that was recently observed experimentally while normal (or negative) exchange bias (NEB) had been discovered more than forty years ago. It is remarkable that both phenomena are observed in the same samples subject to different cooling fields. Here we put forward a model to explain PEB and its crossover to NEB in Fe/FeF₂ and Fe/MnF₂ bilayers. Our model incorporates recent experimental information, such as the perpendicular coupling between the ferromagnet (FM) and the antiferromagnet (AFM) and that compensated AFM interfaces show the highest NEB shifts. A freezing of the canted AFM interface spins is proposed, and thus while hysteresis loops are performed the energy is stored in an incomplete domain wall in the FM. Recent polarized neutron reflectometry experiments provide strong support for this picture.¹ The energy minima at each point of the magnetization curve are obtained exactly, with arbitrary precision, by a new analytical formulation which is introduced in this work. The results extracted from the model are in qualitative and quantitative agreement with available experimental facts. ¹ M R Fitzsimmons et al., Phys. Rev. Lett. 84, 3986 (2000).
2:20 PM		MI-MoA-2 Asymetry of Magnetization Reversal in the Fe/(Fe,Mn)F₂ Exchange Bias System as a Function of Crystallinity A. Hoffmann, M.R. Fitzsimmons (Los Alamos National Laboratory); C. Leighton, K. Liu, I.K. Schuller (University of California, San Diego); J. Nogués (Universitat Autònoma de Barcelona, Spain); C.F. Majkrzak, J.A. Dura (National Institute of Standards and Technology); H. Fritzsche (Hahn-Meitner Institut, Berlin, Germany) Using polarized neutron reflectometry we have recently shown that the magnetization reversal can be different on both sides of the hysteresis loop for polycrystalline Fe exchange coupled to epitaxial twinned FeF₂ or MnF₂.¹ Namely, with decreasing field (+M_s -> -M_s) we observe a rotation of the magnetization indicated by the presence of spin-flip scattering, while for increasing field (-M_s -> +M_s) the data suggest magnetization reversal by domain wall motion. However for epitaxial twinned films of FeF₂ and MnF₂, this difference of magnetization reversal depends on the direction of the cooling field with respect to the crystallographic orientation. In order to clarify the role of frustration between twinned antiferromagnetic domains, we examined the asymmetry of the magnetization reversal also in samples with either single-crystal (untwinned) or (110) textured polycrystalline FeF₂ films. In untwinned and textured samples we detected no significant asymmetry in the magnetization reversal. This suggests that the twinning of the antiferromagnet is essential for the asymmetry in the magnetization reversal to occur. This can be understood by a "45 degree coupling" of the Fe to the FeF₂ due to a twin-driven frustration. As a result there can be a unidirectional anisotropy due to exchange bias along the easy axis of the ferromagnet, which in turn then may give rise to the asymmetry in the magnetization reversal. This work was supported by U.S. Department of Energy, BES-DMS under Contract No. W-7405-Eng-36, grant DE-FG03-87ER-45332, and funds from the University of California Collaborative University and Laboratory Assisted Research. ¹ ¹ M.R. Fitzsimmons, P. Yashar, C. Leighton, I.K. Schuller, J. Nogués, C.F. Maikrzak, and J.A. Dura, Phys. Rev. Lett. vol. 84, 3986 (2000).
2:40 PM		MI-MoA-3 Growth and Magnetic Properties of Ultrathin Fe on Pd(110)¹ B. Roldan Cuenya, J. Pearson, C. Yu, D. Li, S.D. Bader (Argonne National Laboratory) While it is known that initial metal growth on Pd(110) surfaces is highly anisotropic and tends to form nanoscale wires, the magnetic properties of such systems are less studied. We have investigated the growth and magnetic properties of 0-3 ML Fe on a stepped Pd(110) with SMOKE, RHEED, and LEED in order to address the correlation among morphology, structure, and magnetic properties at low dimensions. The Fe films, grown at 70 °C, are flat and epitaxial up to 1.5 ML, where 3-dimensional growth starts. RHEED oscillations with 1-ML period were observed on the (1,0) and (2,0) streaks while an oscillation with a period of 0.5-ML exists on the (0,0) streak. The initial growth is pseudomorphic. The in-plane row spacing along the [110] direction starts to relax at ~ 0.5 ML and decreases by ~ 5% at 3 ML. The submonolayer Fe films remain ferromagnetic down to ~ 0.35 ML, below which the Curie temperature (T_C) may become lower than our low-temperature limit of 25K. The T_C initially increases sharply with coverage and starts to saturate at 0.8-1 ML to ~210K. The magnetic easy axis is perpendicular to the surface for 0.4-0.7ML of Fe and in-plane for Fe thicknesses > 2 ML. Between 0.7-1.2 ML, there appear to be mixed magnetic phases as indicated by the co-existence of both polar and longitudinal Kerr signals, and an increase in coercivity. ¹ Supported by DOE BES-MS #W-31-109-ENG-38 and ANL LDRD#00-001N. ² J.-P, Bucher, E. Hahn, P. Fernandez, C. Massobrio, and K. Kern, Europhysics Lett., 27, 473 (1994).
3:00 PM		MI-MoA-4 Real Space Study of Ultrathin Fe Films on Cu(100) A. Biedermann, M. Schmid, P. Varga (Vienna University of Technology, Austria) Ultrathin iron films deposited on Cu(100) represent a prototype system for the exploration of novel magnetic structures and thin film magnetism. We present the first atomically resolved survey of the surface/ thin film structures using scanning tunneling microscopy (STM). The clean thin films deposited at room temperature are known to be pseudomorphic (fcc) between 2 and 10 monolayer film thickness, however, with notable deviations from the ideal structure. We have identified (1) a novel "striped-bcc" phase at 4-5 monolayer film thickness and (2) the structure of the initial bcc grains at 6-8 monolayer film thickness. Locally, both the "striped-bcc" phase and the initial bcc-Fe grains show a strained bcc configuration, identifying the tendency of the Fe films to form a bcc-lattice as a common driving force for the observed structures. The appearence of the "striped-bcc" phase coincides with an increased ferromagnetism at 4-5 monolayer thickness seen by several other groups.
3:20 PM		MI-MoA-5 Spin Reorientation Transition in Magnetically Coupled Fe/Cu/Ni/Cu(001) System H.J. Choi, W.L. Ling, J.H. Wolfe (University of California, Berkeley); S. Anders, A. Scholl (Lawrence Berkeley National Laboratory); F. Nolting, H. Ohldag (Stanford Synchrotron Radiation Laboratory); U. Bevensiepen, R. Kawakami, Z. Qiu (University of California, Berkeley) It is well known that low temperature grown Fe film on Cu(001) exhibits spin reorientation transition (SRT) at a critical thickness where the magnetic remanence is greatly suppressed within a pseudo-gap region. Subsequent experiments showed that the loss of the macroscopic magnetization is due to the formation of magnetic stripe domains. It is generally believed that the understanding of the strip domains will greatly advance our knowledge on the magnetic long-range order in two-dimensional Heisenberg system. With this motivation, we investigated the SRT of Fe film in Fe/Cu/Ni/Cu(001) system where the Fe layer is magnetically coupled to the perpendicular magnetization of Ni with the interlayer coupling strength being controlled by the Cu thickness. With in situ surface magneto-optic Kerr effect measurement, we show that the Fe-Ni interlayer coupling results in an alternating alignment of the Fe magnetization with Cu thickness for Fe film thinner than the SRT thickness d_R, but has no effect for Fe film thicker than d_R. The SRT thickness d_R, defined as the onset of in-plane magnetization, was found to be independent of the Fe-Ni interlayer coupling. Within the SRT pseudo-gap region, however, the longitudinal magnetic remanence exhibits oscillatory behavior with Cu thickness with a periodicity exactly half of that of the oscillatory interlayer coupling. This result shows that the strip domains are severely modified by the strength of the Fe-Ni interlayer coupling. To provide more detailed information, element specific domain imaging was taken in this system using photoemission electron microscope at the Advanced Light Source of Lawrence Berkeley National Laboratory.
3:40 PM	Invited	MI-MoA-6 Magnetic Anisotropies in Ultra-Thin Films: The Spin-Reorientation Transition in Fe(110)/W(110) E. Vescovo, H.-J. Kim (Brookhaven National Laboratory) Reorientation of the easy axis of magnetization are not unusual in ultra-thin films. They are generally understood as the result of a delicate energy balance between three dimensional anisotropy terms - which favor the bulk easy-axis direction - and two-dimensional surface and interface terms - which can favor a different direction. Bulk contributions are proportional to the film thickness. Correspondingly, in thin films, there can be a critical thickness (t_R) at which the two contributions balance exactly. In all those cases, a switch of the magnetization has to be expected when crossing t_R. Fe(110) films epitaxially grown on W(110) display a spin reorientation phase transitions as a function of film thickness.¹ This transition --- from the in-plane [110] direction to the [100] direction --- is characterized by a t_R of about 50 monolayers. Here we report on new spin- and angle-resolved photoemission experiments on this system. We show how t_R can be greatly varied by directly manipulating the Fe(110) surface. Furthermore, we explore the temperature dependence of the anisotropy balance and provide compelling evidence demonstrating that this in-plane spin reorientation transition can also be induced by varying the temperature, at constant thickness. ¹ ¹ H.J. Elmers and U. Gradmann, Appl. Phys. A 51, 255 (1998)
4:20 PM		MI-MoA-8 Fermi Surfaces and Magnetic Behavior of Thin FeNi Alloy Films M. Hochstrasser, N.A.R. Gilman, R.F. Willis (The Pennsylvania State University); F.O. Schumann, J.G. Tobin (Lawrence Livermore National Laboratory) We report angle-resolved and spin-resolved photoemission measurements of changes in the electronic structure with changing composition of pseudomorphic films of FeNi magnetic alloys grown epitaxially on Cu(100). With x-ray magnetic linear dichroism angle dependent (XMLDAD) 3p core-level photoemission the evolution of the elemental magnetic moments was monitored. In addition, changes occuring in the spin-polarized valence bands were observed with spin-polarized photoemission, together with changes in the k-distribution of states at the Fermi energy. A <110> projection of the Fermi surface shows a delocalized "dogbone" feature due to sp-states and more localized "hotspots" corresponding to the emergence of minority spin d-states. Hybridization between the s p- and d-states occurs at thesse locations on the dogbone indicative to a strong nesting of wavevectors of excitations spanning the Fermi surface. The sp-dogbone states spin polarize with increasing average magnetic moment. Both elemental moments, observe d in XMLDAD, grow with increasing Fe concentration up to a maximum at the Fe concentration of 55%, that on the Fe increasing at a faster rate than the Ni moment. Beyond this point, the Fe moment shows a rapid decline to a "low-spin" value, of the order of that of the Ni monent, which tracks the behavior of the Fe moment but to a smaller degree. Spin-resolved valence band photoemission measurements show first an increase in the exchange splitting of 3d-states, followed by a decline, essentially tracking th e core-level dichrosim. The magnetic instability observed abovethe invar concentration (Fe > 65%) is characterized further by a diffuseness in the spectral distribution and an increased lifetime broadening of mainly minority-spin states, indicative of magnetic non-collinear disorder.
4:40 PM		MI-MoA-9 Highly Spin-Polarized Chromium Dioxide Thin Films Prepared by Chemical Vapor Deposition from Chromyl Chloride W.J. DeSisto (University of Maine); P.R. Broussard (Covenant College); T.F. Ambrose, B.E. Nadgorny, M.S. Osofsky (Naval Research Laboratory) Highly spin-polarized materials, and in particular thin films, are central to many magneto-electronic devices. An efficient and controlled chemical vapor deposition (CVD) process for depositing highly spin-polarized, metastable chromium dioxide (CrO₂) on (100) TiO₂ substrates has been developed using chromyl chloride (CrO₂Cl₂) as a precursor. This precursor is a liquid at room temperature with a vapor pressure adequate for CVD using conventional precursor handling equipment. The films were shiny, black, and approximately 200 nm thick. The spin polarization, as measured by the Point Contact Andreev Reflection (PCAR) technique, was 81± 3%. X-ray diffraction θ/2θ scans indicated the films grew completely (100) oriented, in registry with the (100) oriented TiO₂ substrate. X-ray diffraction φ-scans on the CrO₂ (110) reflection indicated the expected two-fold symmetry, with no evidence of misaligned material. The resistivity at room temperature was 240 microΩcm and decreased to 10 microΩcm at 5K, consistent with metallic behavior. The films were ferromagnetic with a Curie temperature of 395 K and a coercivity of approximately 100 Oe at 298 K. This deposition technique enhances the possibility of fabricating a GMR and/or a tunnel junction device based on CrO₂, and thus opens up new opportunities in magneto-electronics.
5:00 PM		MI-MoA-10 Effect of Composition and Microstructure on Temperature Coefficient of Resistance of Polycrystalline La1-xCaxMnO3 Thin Films C.-H. Lai, C.-F. Hsu (National Tsing Hua University, Taiwan, R. O. C.); Y.-C. Chin, C.-T. Jiang (Chung-Shan Institute of Science and Technology, Taiwan) Perovskite La1-xCaxMnO3 has drawn much attention on account of the colossal magnetosistance. Due to the sharp resistivity drop around the insulator-metal transition temperature (Tp), epitaxial La1-xCaxMnO3 thin films have been demonstrated to be a promising candidate for IR detector (bolometric) application. In this work, polycrystalline La1-xCaxMnO3 films were deposited by using rf sputtering on Si/SiO2 substrates, and the dependence of the temperature coefficient of resistance (TCR) on the film composition and the structure was investigated. When the temperature is higher than Curie temperature Tc, the resistivity of our polycrystalline La1-xCaxMnO3 films follow the "small palaron model", that is, the resistivity can be expressed as R=BTexp(Ea/kT). Consequently, increasing the activation energy Ea increases the TCR value. By adjusting the Ar/O2 flux during depositions or changing the atmosphere of post-annealing, oxygen content can be manipulated. Because precise oxygen content is difficult to measure, the lattice constant of the films was used for the indicator of relative oxygen contents. When oxygen content increased, the Ea and TCR decreased accompanied with the increase in Tp. Ea and TCR value also decreased with increasing Ca content. ESCA results showed that the ratio Mn4+/Mn3+ increased with increasing oxygen (or Ca) content. Since the carrier transportation of La1-xCaxMnO3 is mainly by hopping along Mn4+-O-Mn3+, the increase in the ratio Mn4+/Mn3+ may imply that hopping probability increases, resulting in smaller resistance and Ea. The surface roughness can significantly increase the resistance but the TCR value is about the same. The resistance reduced with the grain growth but the TCR value maintained constant. The TCR value can reach 3%/K at room temperature.