Papers

AVS1996 Session MI+NS-WeM: Magnetic Anisotropy and Structure

Wednesday, October 16, 1996 8:20 AM in Room 106A/B

Wednesday Morning

Start	Invited?	Item
8:20 AM		MI+NS-WeM-1 Magnetization Reversal in Rough Ultrathin Films A. Zangwill, A. Moschel (Georgia Institute of Technology); M. Stiles (National Institute of Standards & Technology); L. Tang (Imperial College, United Kingdom) Inevitably, even the best ultrathin epitaxial magnetic films exhibit a finite density of monolayer-height steps associated either with the perimeter of two-dimensional islands that nucleate during growth or with the edges of vicinal terraces created by an intentional miscut. On four-fold symmetric surfaces where shape and surface anisotropy compel individual moments to lie in the average surface plane, the steps induce a local two-fold in-plane anisotropy that can be much stronger than the intrinsic four-fold anisotropy of the flat surface. In that situation, the steps nucleate non-uniform reversal processes and alter the hysteresis significantly from coherent rotation models. In this talk, I report numerical and analytic studies of this problem using a two-dimensional classical X-Y model with spatially inhomogeneous anistropy. An explanation is offered for the very small values of the coercive field found in single crystal ultrathin films (Brown's paradox) and several qualitative trends are found that agree well with existing experiments.
9:00 AM		MI+NS-WeM-3 Microstructure and Magnetic Anisotropy of Ultrathin Co Films Studied by SPLEEM M. Altman (Hong Kong University of Science and Technology); E. Bauer (Arizona State University) Perpendicular magnetic anisotropy (PMA) in magnetic thin films is recognized to have its origins in the symmetry breaking caused by the presence of surfaces and interfaces. However, surfaces and interfaces are well known to have a significant number of defects, predominantly consisting of atomic steps, which may have an important impact upon the magnetic properties. For this reason, it is crucial to assess the surface and interface microstructure simultaneously with the thin film magnetic properties. In the present work, we have examined c-axis oriented Co thin films using spin polarized low energy electron microscopy (SPLEEM). The crossover from out-of-plane to in-plane magnetization was studied as a function of Co film thickness on Au buffer layers of various thickness on the W(110) surface. PMA was found to be significantly enhanced in Co on thin compared to thicker Au buffers layers. We attribute this effect to the surface magnetoelastic contribution to the magnetic energy. More interestingly, we have observed a distinct correlation of magnetic domain structure with interface step morphology. This is the first direct experimental observation of magnetic domain wall pinning at atomic interface steps. A depinning transition of domain walls from steps in 2-d magnets will be discussed. Future work must take care to consider this newly identified influence upon thin film magnetic properties.
9:20 AM		MI+NS-WeM-4 Perpendicular Magnetization Process in Epitaxial Cu/Ni/Cu/Si(001) R. O'Handley (Massachusetts Institute of Technology); H. Hug (University of Basel, Switzerland); G. Bochi, D. Paul (Massachusetts Institute of Technology); H. Guntherodt (University of Basel, Switzerland) Epitaxial Cu/Ni/Cu (001) films exhibit perpendicular anisotropy over an exceptionally wide thickness range (30\Ao\ < Ni < 145\Ao\). High resolution magnetic force microscopy (MFM) in perpendicular fields up to 500 Oe has been used to reveal new details of the magnetization process; here we focus on two features: 1) the forces of attraction and repulsion between different segments of domain walls and 2) a bimodal distribution of magnetization response times to perpendicular applied fields. As high energy domains are reduced in area by a perpendicular applied field, the domain patterns (whose wall orientations in zero field have no correlation with the easy in-plane <110> directions) evolve to a serpentine pattern. Some lengths of the serpentine domains collapse completely while others shrink in width with opposite walls failing to annihilate each other even in fields up to 500 Oe. These "hard domains" show a preference for alignment with the easy <110> directions. We believe the annihilation and hard domain behavior to be due to the combination of opposing short-range exchange and long-range dipole interactions between lengths of domain wall having the same or opposite chirality; chirality along a domain wall changes at a Bloch line. The magnetization process in films of 75 to 100 \Ao\ Ni is revealed in the MFM images to have an instantaneous field response and a slower time response (in constant field) over a period of several minutes or, in the 75 /Ao/ Ni film, several hours. The instantaneous response is due to the motion of glissile domain wall segments; the after effect appears to be due to the thermally activated motion of Bloch lines along the hard wall segments.
9:40 AM		MI+NS-WeM-5 Spin Reorientation and Stripe Domain Phases V. Pokrovsky (Texas A&M University) A review of recent investigations of ultrathin ferromagnetic films thermodynamics is presented. In these films the exchange interaction establishes big ferromagnetic clusters, whose magnetization direction is determined by interplay of weaker forces: the surface-induced uniaxial anisotropy and dipolar forces. The change of balance between these interactions with temperature and thickness leads to the Reorientation Phase transition (RPT) from perpendicular to parallel magnetization (1). In a vicinity of the RPT, domains of opposite perpendicular magnetization (stripes) enter the film (2). They form either a 2d crystal with the algebraic positional order, or an Ising nematic liquid with spontaneosly broken tetragonal symmetry, or the tetragonal liquid (3). Liquid stripe phases were observed experimentally (4) and in numerical simulation (5). Stripe phases are extremely sensitive to the magnetic field perpendicular to the film (2), (3). This prediction was verified experimentally (7). The large highly anisotropic susceptibility offers an opportunity for applications. The in-plane magnetization phase has a peculiar dynamics (8). The long-range dipolar forces in this phase suppress long spin waves. Instead an anomalous, anisotropic spin diffusion modes occurs. (1) The experimental observation: D. Pappas et al.,Phys. Rev. Lett. 64, 3179 (1990) and Z.Q. Qiu et al., Phys. Rev. Lett. 70, 1006 (1993); for a theory see D. Pescia and V. Pokrovsky, Phys. Rev. Lett. 65, 2599 (1990). (2) A. Kashuba and V. Pokrovsky, Phys. Rev. Lett. 70, 3155 (1993). (3) Ar. Abanov et al., Phys. Rev. B 51, 1023 (1995). (4) R. Allenspach and A. Bischof, Phys.Rev. Lett. 69, 3385 (1992). (5) I. Booth et al., Phys. Rev. Lett. 75, 950 (1995). (6) A. Berger and H. Hopster, Phys. Rev. Lett. [\bf 76], 519 (1996). (7) A.B. Kashuba et al., submitted to Phys. Rev. Lett., 1996.
10:20 AM		MI+NS-WeM-7 Interface-Induced Morphology and Magnetic Anisotropy of Fe Films on GaAs(001)-(2x4) and c(4x4) E. Kneedler, B. Jonker, P. Thibado, R. Wagner, B. Shanabrook, L. Whitman (Naval Research Laboratory) We have conducted a study of epitaxial Fe films grown on MBE-prepared GaAs(001)-(2x4) and -c(4x4) substrates, with film thickness ranging from 0.1 to 50 monolayers (ML). The correlation between the morphology of the Fe film and in-plane magnetic anisotropy was probed with photo-electron diffraction (PED), scanning tunnelling microscopy (STM), and in-situ magneto-optical Kerr effect (MOKE). PED is sensitive to the early growth mode of the Fe. The initial growth on the (2x4) substrate is determined to be predominantly layer-by-layer with 1-2 ML of roughness. Submonolayer growth on the c(4x4) substrate is similar, with perhaps slightly more roughening. STM clearly shows initial adsorption and nucleation of Fe occuring on As dimer sites for both substrates. In spite of these similarities, the Fe films on the two substrates are quite different. STM images show elongation of Fe islands along the dimer rows of the (2x4) substrate. In contrast, islands on the c(4x4) are essentially isotropic and larger in size, with a correspondingly lower number density. Longitudinal MOKE enables a direct measurement of the in-plane anisotropy as a function of film thickness by measuring magnetization hysteresis loops along easy, hard and intermediate magnetic axes. The contrast in uniaxial anisotropies measured for Fe films will be discussed in terms of the different film morphologies induced by the two surface reconstructions.
10:40 AM		MI+NS-WeM-8 Magnetic Properties of Iron-Iron Oxide Films Grown on Cu(001) and Ag(001) L. Scipioni, B. Sinkovic, W. Zhu (New York University) Thin films (t < 20 \Ao\) of iron oxide and mixed oxide/Fe-metal films have been grown on Cu(001) and Ag(001) substrates by reactive evaporation.. Films with excess of Fe (Fe:O > 1) grow along (111) on Cu and along (100) on Ag substrates. A predominant oxide species is FeO as determined by XPS, UPS and LEED. The in-situ MOKE detects ferromagnetic response only for films with Fe:O > 1, which was attributed to metallic Fe by MCD measurements. The angle-dependent XPS data indicate that the metallic Fe forms at the oxide/Cu(001) interface. This interfacial Fe film exhibits strong perpendicular anisotropy and temperature dependence with coercivity (H\sub c\) changes of 100-fold and more. The low coercivity state is present at room temperature and at low temperatures (~100 K), while high H\sub c\ is measured for intermediate temperatures. The (H\sub c\)\sub max\ state occurs close to the Neel temperature of FeO and varies with the thickness of interfacial Fe layer. This suggest that such unusual magnetic behavior of the interfacial Fe film is strongly influenced by the presence of FeO overlayer. Films with excess of Fe (Fe:O > 1) grown on Ag(001) show weak longitudinal anisotropy and increasing coercivity with decreasing temperature. The mixed oxide/Fe-metal films grown on both, Cu and Ag substrates show substantial reduction in the Curie temperature from the bulk Fe-metal value.
11:00 AM		MI+NS-WeM-9 Correlation between the Magnetic and Structural Properties of Thin Sputtered Fe\sub x\Co\sub 1-x\ Films Studied by FMR J. Pflaum (Ruhr-Universit\um a\t Bochum, Germany); F. Schreiber (Princeton University); J. Heise (Ruhr-Universit\um a\t Bochum, Germany); Z. Frait (Czech Academy of Sciences); H. Zabel, J. Pelzl (Ruhr-Universit\um a\t Bochum, Germany) A comprehensive study of the correlation between the structural behavior and the magnetic characteristics of thin Fe\sub x\Co\sub 1-x\ alloy films is given within this contribution. The films were prepared by rf-sputtering. To study the influence of the substrate on the magnetic film properties MgO, Al\sub 2\O\sub 3\ and glass substrates were used. The concentration of the samples covers the complete range from pure Fe to pure Co and the thicknesses amount to 25 to 1500\Ao\. By extensively X-ray diffraction we yielded information about the structural phase transition in the region of x=0.15. The correlated magnetic qualities are analyzed by Ferromagnetic Resonance Spectroscopy (FMR) at frequencies of 9 to 92GHz. The concentration dependence of magnetization and magnetocrystalline anisotropy of the films is shifted towards the Fe-rich side in comparison to the corresponding bulk systems. The g-factor, g, as a measure for the relative contribution of the orbital moment to the magnetic moment is found to be almost concentration independent in the composition range of 1\>=\x\>=\0.5. From the FMR linewidth the damping behavior of the alloy films is investigated. We compare the results to a microscopic theory for the relationship between the Gilbert damping factor, G, and the g-factor which is intermediated by the spin-orbit coupling. For thick Fe\sub 55\Co\sub 45\ films standing spinwave spectra are shown. An analysis taking into account the relatively high electric conductivity of the material (i.e. including Maxwell equations) is presented and compared to models which use the insulator approximation. This work was supported by the Deutsche Sonderforschungsgemeinschaft (SFB 166) and also supported by the VW-Stiftung.