AVS1996 Session MI-WeA: Magneto-optics and Anisotropy

Wednesday, October 16, 1996 2:00 PM in Room 106A/B

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		MI-WeA-1 Magnetic X-ray Linear Dichroism of Ultrathin Fe-Ni Alloy Films K. Goodman, J. Tobin (Lawrence Livermore National Laboratory); F. Schumman, R. Willis (The Pennsylvania State University); J. Denlinger, E. Rotenberg, A. Warwick (Lawrence Berkeley National Laboratory) We have studied the magnetic structure of ultrathin Fe-Ni alloy films as a function of Fe concentration by measuring the linear dichroism of the 3p-core levels in angle-resolved photoemission spectroscopy. The alloy films, grown by molecular-beam epitaxy on Cu(001) surfaces, were fcc and approximately four monolayers thick. The intensity of the Fe dichroism varied with Fe concentration, with larger dichroisms at lower Fe concentrations. The implication of these results to an ultrathin film analogue of the bulk Invar effect in Fe-Ni alloys will be discussed. These measurements were performed at the Spectromicroscopy Facility (Beamline 7) of the Advanced Light Source. The Spectromicroscopy Facility and the Advanced Light Source were constructed and are operated under the support of the U.S. Department of Energy. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.
2:20 PM		MI-WeA-2 Magnetic Anisotropy and Soft X-ray Dichroism of Pseudomorphic Alloy Films G. Mankey, K. Subramanian, R. Stockbauer, R. Kurtz (Louisiana State University) We report measurements of the structural, magnetic and electronic properties of ultra thin alloy films of Fe /sub x/Ni/sub (1-x)/ deposited on Cu(100). Our unique combination of experimental techniques allows us to extract information crucial for a more complete understanding of magnetic anisotropy in thin magnetic films. Our goal is to relate the magnetic anisotropy to thin film structure, valence band filling, exchange splitting and the strength of the spin-orbit coupling. Film structure and thickness is determined from x-ray photoelectron diffraction. The element-specific ferromagnetic response of both the Fe and Ni alloy constituents are characterized with core level magnetic dichroism. The core-level measurements are correlated with valence band photoemission and dichroism studies which allow us to extract a measure of the band filling, exchange splitting and the spin-orbit coupling. The angular dependence of magnetic effects in the valence band emission is evaluated using a display-type Ellipsoidal Mirror Analyzer and compared with predictions based on symmetry arguments and band structure calculations. The data presented here are obtained using both linearly and circularly polarized synchrotron radiation from the Plane Grating Monochromator beam line at the Center for Advanced Microstructures and Devices synchrotron.
2:40 PM	Invited	MI-WeA-3 Second Order Magneto-optic Effects in Anisotropic Thin Films R. Osgood III (Argonne National Laboratory) The magneto-optic Kerr effect has been shown to give important information about the anisotropy of thin films. The signal detected in a magneto-optic Kerr effect experiment is often assumed to be proportional to the magnetization, which is indeed true for films with no in-plane anisotropy. Yet, due to recent advances in growth techniques, in-plane anisotropy is common in films epitaxially bonded to a substrate. Second order magneto-optic effects, where the signal contains a term proportional to the magnetization squared, can manifest themselves in such films. These second order effects can provide interesting insights into the magnetization reversal process, and also describe the dielectric tensor of a ferromagnetic metal. The second order magneto-optic effect is connected with the asymmetry in the magneto-optic hysteresis loop, which arises as a result of the coherent rotation of the magnetization vector. Such asymmetries have often appeared in the literature and we explain them. The second order magneto-optic effect is analogous to regular magnetoresistance, just as the first order magneto-optic Kerr effect is analogous to the Hall effect, except that the magneto-optic effects occur at much higher frequencies. We show typical measurements of the second order magneto-optic effect in Co, Fe, and NiFe thin films and discuss how these second order effects give quantitative information about the microscopic parameters of the ferromagnetic metal, as described by a classical model. This work is sponsored by USDOE-BES W-31-109-ENG-38 at Argonne National Laboratory and by NSF DMR-9100292-01 at Stanford University.
3:20 PM		MI-WeA-5 Modeling and Verification of Polar Magneto-optic Kerr Response D. Glenn, D. Thompson, X. Gao, J. Woollam (University of Nebraska, Lincoln) The polar magneto-optic Kerr effects of TbFeCo and SiC thin film structures, grown by sputter deposition, have been studied to achieve a structure with an optimum response as a function of wavelength. Instrumentation has been developed to accurately measure polar Kerr rotation and ellipticity over a spectral range of 0.73 eV to 5.0 eV. Mathematical modeling based on Maxwell's equations has been developed that is capable of predicting the polar Kerr rotation and ellipticity of multilayer systems. This modeling was used to predict the polar Kerr response for TbFeCo and SiC thin film structures as a function of wavelength and thickness of the individual layers. The optical constants of TbFeCo and SiC are necessary for modeling and were determined by in-situ and ex-situ spectroscopic ellipsometry (SE). The TbFeCo optical constants were determined from in-situ real-time SE analysis during deposition. The SiC was sputter deposited and the optical constants determined from ex-situ SE analysis on three separate samples. Modeling was used to predict the structure and composition of a series of samples that would exhibit the wavelength and thickness dependence for optimum polar Kerr response and figure of merit. A series of samples was made with the structure of SiC/TbFeCo/SiC on Si. The polar Kerr response was measured for this series of samples and was compared to predictions from the model. We show that the mathematical modeling accurately predicts the polar Kerr response./super */ Research supported by NSF grants OSR-9255225 and DMR-9222976.
3:40 PM		MI-WeA-6 Modeling the In-Plane Anisotropy in Epitaxial Co/Cu E. Smith (University of Michigan); R. Naik, R. Lukaszew (Wayne State University); R. Clarke (University of Michigan) We have developed a new model to compute theoretical hysteresis curves in order to probe the origins of the in-plane anisotropy energy in magnetic thin films. The model uses a free energy expression, dependent on the magnetocrystalline and Zeeman terms, to locate the stationary points of the magnetization as a function of applied magnetic field orientation and strength in the plane of the film. The model is therefore based on a coherent rotation scheme with the extension that we allow the magnetization to 'hop' between local energy minima. A key result is that the magnetization is very sensitive to the formation of a metastable state when the applied field is directed along the predicted in-plane magnetic hard axis. This metastable state gives rise to sharp spikes in the coercivity vs. in-plane angle which are observed experimentally. We obtain excellent quantitative agreement between the predictions of our model and experimental data for Co/Cu <001> samples measured using the Magneto-Optic Kerr Effect (MOKE).
4:00 PM		MI-WeA-7 High Coercivity Co/Pt Multilayers for Quantitative Magnetic Field Imaging D. Weller (IBM Almaden Research Center); J. Heidmann (IBM Storage Systems Division); W. McChesney (IBM Almaden Research Center) We have developed high coercivity Co/Pt multilayer films for quantitative evalutation of large magnetic fields as i.e. produced during the write process by a magnetic head. The magnetic films typically consist of a sequence of substrate/ SiN\sub x\-buffer/Pt/10x(1.2nm Pt/.25-.8nm Co)/SiN\sup x\ cap. The metal layers are electron beam evaporated and the transparent SiN\sup x\ layers are sputtered. These multilayers have a high perpendicular magnetic remanence with effective anisotropy fields ranging up to 60 kOe and coercivities of order H\sup C\=5-8 kOe. They are grown at an elevated deposition temperature of about 200 \super o\C, which is key to achieving large coercivities. When deposited directly onto the air bearing surface of a magnetic write head we can use these structures as quantitative magnetic field sensors. Here the in-plane field component of the write head leads to a reversible rotation of the magnetization away from the perpendicular easy axis of the sensor film. This rotation can be directly mapped with a polar Kerr microscope and converted into an in-plane head field map, as will be demonstrated.
4:20 PM		MI-WeA-8 Magnetic and Structural Properties of Nanostructured Pt/Co Multilayers P. Carcia, R. McLean, M. Reilly (Dupont Research and Development); P. Grundy (University of Salford, United Kingdom) Pt/Co multilayers have potential application as new magneto-optical recording media at blue wavelength, where they have a larger figure-of-merit than rare-earth, transition-metal alloy media. Recently, we showed that sputtering Pt/Co multilayers on ultra-thin, etched Pt underlayers (< 30A thick) maximizes their magnetic anisotropy energy and coercivity. This paper explores the relationship between magnetic properties of these multilayers and their nanoscale structure. By X-ray diffraction, we found that multilayers with optimum magnetic properties were highly textured with long range coherence of their artificially modulated structure. And previous studies have correlated structural improvements, such as these, with a reduction in media noise. Scanning tunneling microscopy revealed that growth of Pt/Co multilayers on etched Pt increased their grain size, and that etching Pt created a filamentary structure with an apparent higher nucleation density of Pt atomic clusters. This feature, and not improved Pt underlayer flatness, appeared to be responsible for better multilayer structural and magnetic properties.
4:40 PM		MI-WeA-9 Magnetic Switching in Ultra Short Magnetic Field Pulses C. Back (Stanford University); D. Weller (IBM Almaden Research Center); J. Heidmann (IBM Storage Systems Division); D. Mauri (IBM Almaden Research Center); E. Garwin (Stanford University); H. Siegmann (Swiss Federal Institute of Technology, Switzerland) We have investigated the magnetic switching behavior of a series of perpendicularly magnetized Co/Pt multilayer films in picosecond magnetic field pulses by imaging the resulting magnetization pattern using Kerr microscopy. The unique ultra short pulses with magnetic field amplitudes of several tesla were obtained in the final focus test beam of the Stanford Linear Accelerator Center as described before [1]. The initial magnetization reversal is governed by spin precession which can be described by the Ginzburg-Landau-Lifshitz equation which predicts a minimal switching field amplitude, required to reverse the magnetization at a fixed pulse length of the applied field [2, 3]. We have systematically varied this switching condition by changing the magnetic anisotropy field of our samples. A comparison between the experimentally observed switching fields and the theoretical results is made. [1] Siegmann et al. J. Magn. Magn. Mater. 151, L8 (1995). [2] L. Landau and E. Lifshitz, Physik. Z. Sowietunion 8, 153 (1935). [3] L. He and W. D. Doyle, J. Appl. Phys. 79, 6489 (1996).