AVS1997 Session MI-MoM: Structure and Magnetism of Surfaces and Interfaces

Monday, October 20, 1997 8:20 AM in Room J3

Monday Morning

Start	Invited?	Item
8:20 AM	Invited	MI-MoM-1 Oscillatory Exchange Coupling in Magnetic Multilayers. M.D. Stiles (National Institute of Standards & Technology) When magnetic layers are separated by a non-magnetic spacer layer, the magnetizations can couple together by an indirect exchange interaction that is mediated by the conduction electrons of the spacer layer. As the thickness of the spacer layer is varied, this coupling oscillates between ferromagnetic and antiferromagnetic. In multilayers in which the magnetic and the non-magnetic materials are well lattice matched to each other, the coupling exhibits a superposition of oscillations with different periods. This oscillatory exchange coupling can be understood as arising from quantum well resonances set up in the spacer layer material by spin-dependent reflection from the interfaces. In this talk I will present a model of how these quantum well resonances give rise to the oscillatory exchange coupling, and describe the predictions that the model makes. Comparing the predictions with experimental results then highlights the importance of disorder in these systems. I will then describe the effect of disorder, particularly at interfaces, on the oscillatory exchange coupling.
9:00 AM		MI-MoM-3 Fe/Rh Multilayers: Magnetic and Structural Characterization M.A. Tomaz, D.C. Ingram, G.R. Harp (Ohio University); D. Lederman, E. Mayo (West Virginia University); W.L. O'Brien (Synchrotron Radiation Center, University of Wisconsin, Madison) We have measured the layer averaged magnetic moments of both Fe and Rh in a number of sputtered Fe/Rh (100) multilayer thin films by x-ray magnetic circular dichroism (XMCD). Additionally we have characterized the structure of these films using x-ray diffractometry. We report the observation of two distinct regimes in these films. The first is characterized by Rh moments of at least 1 µ_B, Fe moments enhanced as much as 30% above bulk, and a bct crystal structure. The second regime is distinguished by the absence of both Fe and Rh moments and an fct crystal lattice. The demarcation between the two regions is sharp and indicated by the collapse of both Fe and Rh moments, and the coexistence of both bct and fct phases. The point at which both crystalline phases first coexist, we term the critical thickness. The critical thickness occurs approximately at the point where the Rh layer thickness is one half the Fe layer thickness in the films studied. We attribute the change in magnetic behavior to the structural transformation.
9:20 AM	Invited	MI-MoM-4 Magnetic Anisotropy and Atomic Structure of the Fe/GaAs Interface* E.M. Kneedler, B.T. Jonker, P.M. Thibado, L.J. Whitman, R.J. Wagner (Naval Research Laboratory) The interest in magnetic metal/semiconductor heterostructures is fueled largely by the potential for novel spin-sensitive devices. Fe/GaAs is particularly attractive because of the relatively high-quality epitaxial growth. Previous studies which utilized thermally desorbed or sputter/annealed GaAs substrates have revealed a uniaxial component to the in-plane magnetic anisotropy. However, an understanding of its source remains elusive: the complexity of the system has made it difficult to establish whether this uniaxial component is intrinsic or extrinsic. Typical challenges include reproducibly preparing a well-characterized substrate and obtaining an accurate atomic-scale picture of the interface. In an effort to elucidate the role of the interface in mediating this uniaxial component, we have studied the effect of the initial GaAs surface by utilizing different surface reconstructions [e.g. (2x4)-As, c(4x4)-As, (4x6)-Ga], placing special emphasis on preparation and characterization of the starting surface. We have performed MOKE measurements in situ in conjunction with PED, STM and XPS to relate the magnetic character to the structural, morphological, and compositional properties of the film. Initial growth and nucleation is driven by the preferential adsorption of Fe on As-dimer sites, resulting in nucleation patterns characteristic of the initial substrate reconstruction. Little if any Fe is magnetically dead, although a strong reduction in Curie temperature is observed. The similar uniaxial behavior of MOKE data for all reconstructions implies that the interface tends to assume a final structure independent of the initial substrate surface reconstruction. * Supported by ONR
10:00 AM		MI-MoM-6 Structural and Magnetic Phases of Ultrathin Fe Wedges on Diamond (100)* D. Li, D.J. Keavney, J.E. Pearson (Argonne National Laboratory); W. Keune (University of Duisburg, Germany); S.D. Bader (Argonne National Laboratory) Fcc Fe is predicted to have ferro- and antiferromagnetic phases depending on atomic volume.¹ Epitaxy of Fe on different substrates permits fcc-like phase stabilization with different in-plane lattice constants. Diamond (100) has a lattice constant of 3.57 Å, as compared to the extrapolated room-temperature fcc Fe value of 3.59 Å, or the 3.61 Å value for well-studied Cu(100) substrates. Thus, it has recently been used as a substrate to grow metastable fcc Fe films.² In this work, we investigate wedge-shaped Fe films of 0 - 1.6 nm on diamond(100) by means of RHEED and the surface magneto-optical Kerr effect (SMOKE) in order to systematically identify the structural and magnetic phases. The films were grown at room temperature and subsequently annealed to 400 °C. Both the substrate and films < 4 ML thick show 1x1 streaks in the RHEED patterns, indicating a flat, well-ordered fcc structure. Films > 4 ML thick exhibit a 3D RHEED pattern with relaxed lattice spacing that is consistent with a transformation to the bcc structure. The structural phase transition is apparent from the thickness dependence of both the spacing and the width of the streaks/spots in the RHEED patterns taken along the wedge. SMOKE measurements were carried out between 120 - 300 K. No magnetic signal was observed in either longitudinal or polar measurements for Fe thicknesses < 4 ML. For > 4.5 ML Fe, the films are ferromagnetic with in-plane easy axis, and the saturation magnetization (at 130 K) increases linearly with Fe thickness. The Curie temperature increases from 120 to 300 K within 1 ML of the onset of magnetism along the wedge. These results indicate that for < 4 ML, metastable, epitaxial fcc Fe films grow on C(100), but are not ferromagnetic above 120 K. Thicker films are ferromagnetic, but are rough and are bcc. * Supported by DOE BES-MS #W-31-109-ENG-38. ¹Moruzzi et al., Phys. Rev. B 34, 1784 (1986). ²Pappas et al., Appl. Phys. Lett., 64, 28 (1994).
10:20 AM		MI-MoM-7 Exploring Step-Induced Magnetic Anisotropies in Ultrathin Films R.K. Kawakami, H.J. Choi, E.J. Escorcia-Aparicio, Z.Q. Qiu (University of California, Berkeley) Magnetocrystalline anisotropy originates from the spin-orbit interaction, thus, must respect the symmetry of the lattice. To understand how the lattice symmetry-breaking induces the magnetic anisotropy, we carried an investigation on ultrathin Fe films grown on stepped Ag(001) surface. For steps parallel to the [110] direction of the Ag, we found that the steps induce an uniaxial anisotropy with the easy magnetization axis parallel to the step edges, and this step-induced anisotropy shifts the spin reorientation transition to a thicker thickness. To exlore the relation between the induced anisotropy and the step-density, we polished a curved Ag(001) substrate to vary the vicinal angle continuously fron 0 to ~10 degrees. Surface Magneto-Optic Kerr Effect (SMOKE) technique was used to measure the magnetic hysteresis loops for Fe film grown on the curved substrate. We found that for a 25 monolayer film, the step-induced uniaxial anisotropy varies as the square of the step-density. We also found that the step-induced anisotropy depends weakly on the film thickness, suggesting that the anisotropy is not localized at the step edges. Possible origins of the step-induced anisotropy, including the strain effect, will be discussed.
10:40 AM		MI-MoM-8 Co on Stepped Cu(100) Surfaces: A Comparison of Experimental Data with Monte Carlo Growth and Micromagnetics Simulations. S.T. Coyle (Arizona State University); J.L. Blue (National Institute for Standards & Technology); G.G. Hembree, M.R. Scheinfein (Arizona State University) Monolayer thick Co films grown epitaxially on single-crystal (bulk) Cu(100) were investigated using in-situ nanometer resolution ultrahigh-vacuum scanning electron microscopy, Auger electron spectroscopy and the surface magneto-optic Kerr effect. Dramatic roughening of step edges with distinctive crystallographic faceting was observed beginning at low coverages (<0.25 monolayer). The facetted step nanostructure was found to be dependent on step edge orientation, terrace width and kink density. Monte Carlo simulations of the growth of Co/Cu(100) in the presence of step edges were performed[1]. The effect of relevant model parameters on film morphology, including step edge roughening and interdiffusion, is discussed and compared to data extracted from secondary electron micrographs. Micromagnetic simulations were also performed to investigate the effect of step and kink anisotropies on long and short-range magnetic ordering. A possible explanation is given for an observed out-of-plane component of the magnetization. This work is supported by the Office of Naval Research under grant No. N00014-93-1-0099. 1. James L. Blue, Isabel Beichl, and Francis Sullivan, Phys. Rev. E statistical Phys., Plasma Phys. 51, R867 (1995)
11:00 AM	Invited	MI-MoM-9 Direct Observation of Interface Roughness Dependence of Interfacial Magnetism using Diffuse X-ray Resonant Magnetic Scattering J.F. MacKay, M.G. Lagally (University of Wisconsin, Madison) Interfacial roughness plays an important role in the magnetoresistance of magnetic multilayers, yet the relationship between roughness and the magnetic properties at interfaces is not well understood. Using synchrotron produced circularly polarized x-rays tuned to the appropriate elements L-edge, x-ray diffuse intensity scans for a fixed angle of incidence are made for sputter deposited magnetic thin films. For smooth films, the diffuse intensity from x-ray resonant magnetic scattering implies a longer length scale than for charge scattering, and the diffuse resonant magnetic scattering intensity is reduced compared to charge scattering. We conclude that long-wavelength roughness contributes more to magnetic scattering than does short-wavelength roughness. For latterally highly correlated roughness, resonant magnetic scattering mirrors charge scattering, hence the magnetic behavior of the interface is the same as the bulk film.
11:40 AM		MI-MoM-11 Correlating Changes in Magnetic Roughness with Variations in the Helicity Dependent Soft X-ray Resonant Diffuse Scattering J.W. Freeland, V. Chakarian (Naval Research Laboratory); C.-C. Kao (Brookhaven National Laboratory); K. Bussmann, Y.U. Idzerda (Naval Research Laboratory) A helicity dependent resonant diffuse scattering study of CoFe/Cu/SiN₃/Si thin films grown with different Cu buffer layer thicknesses is presented. Altering the Cu buffer layer thickness varies the RMS roughness of the surface prior to magnetic film deposition. Atomic force microscopy measurements have determined that the film with a thin Cu base (40 Å) has a 3-4 Å RMS roughness while a thick base (1500 Å) has an RMS roughness a factor of 10 larger. The chemical and magnetic roughness can be determined from the specular and off-specular (diffuse) scattering measured resonantly at the Co L₃ absorption edge. While the specular scattering contains information about layer thicknesses and the chemical and magnetic RMS roughness, the off specular scattering contains information about the in-plane correlation of the roughness. These magnetic roughness determinations have then been correlated with changes in the magnetic properties, such element specific magnetic hysteresis loops measured in reflection, which show an increase in saturation and coercive fields with increasing roughness.