AVS2001 Session MI-ThP: Magnetic Thin Films & Surfaces Poster Session

Thursday, November 1, 2001 5:30 PM in Room 134/135
Thursday Afternoon

Time Period ThP Sessions | Topic MI Sessions | Time Periods | Topics | AVS2001 Schedule

MI-ThP-1 Magnetic Spectroscopy at the Elliptically Polarizing Undulator Beamline 4.0.2 at the Advanced Light Source
E. Arenholz, A.T. Young (Advanced Light Source)
Beamline 4.0.2 is the first undulator beamline at the Advanced Light Source equipped with a Sasaki-type elliptically polarizing undulator (EPU). The EPU allows full control of the polarization of the x rays. Variable linear polarization from linear horizontal to linear vertical as well as 100% circular polarization are possible. The undulator in combination with a plane-grating-variable-included-angle monochromator is designed to provide high flux photon beams from 50 eV to 2000 eV, fully covering the L3,2 edges of important magnetic transition metals (Fe, Co, Ni, ...) and also the M5,4 edges of magnetic rare earth elements of interest (Gd, Tb, ...). We will present first experimental results that illustrate the ability of the beamline to detect small dichroism effects in dilute systems and materials which show only weak magnetic effects (<0.3%), giving us confidence in the suitability of the set up to study novel magnetic systems with high precision.
MI-ThP-2 X-ray Magnetic Linear Dichroism of Fe-Ni Alloys on Cu(111)
T.F. Johnson, S. Chiang, Y. Sato (University of California, Davis); D.A. Arena (Lawrence Livermore National Laboratory); S.A. Morton (University of Missouri-Rolla); M. Hochstrasser, J.G. Tobin (Lawrence Livermore National Laboratory); J.D. Shine, J.A. Giacomo, G.E. Thayer, D.P. Land, X.D. Zhu (University of California, Davis); G.D. Waddill (University of Missouri-Rolla)
We have prepared FexNi1-x multilayers on Cu(111) in order to learn how to control the structure and magnetism of these thin alloy films, which are relevant to the giant magnetoresistance (GMR) effect used in magnetic disk drive heads. Using the Spectromicroscopy Facility (7.0.1.2) on Undulator Beamline 7.0 at the Advanced Light Source, we have measured X-ray magnetic linear dichroism (XMLD) signals from both Fe and Ni 3p lines for fourteen different thin Ni-Fe alloy films on Cu(111), with Fe concentration ranging from 9% to 84% and for a variety of film thicknesses. The Curie temperature for all of these samples was in the range 200K to 500K. For many of these films, the Curie temperature was considerably lower than was previously seen for similar films deposited on Cu(100). For a particular Fe concentration x, the Curie temperature increases with alloy film thickness. For a specific film thickness, the Curie temperature has a maximum near x=0.4. We have also measured the Fe and Ni asymmetries as a function of Fe concentration.
MI-ThP-4 Growth of Mn on Fe(001): Surface Alloy Formation and Multilayer Growth
T. Yamada, M.M.J. Bischoff, A.J. Quinn (University of Nijmegen, The Netherlands); T. Mizoguchi (Gakushuin University, Japan); H. van Kempen (University of Nijmegen, The Netherlands)
A complicated relationship is usually found between the magnetic configuration of a magnetic thin film, the crystallographic structure and the electronic structure. Mn films on Fe(001) are the ultimate example of a system where all these properties are interwoven. Conflicting results are, e.g., reported on the magnetic properties, which suggest a strong dependence on impurities, intermixing, and growth mode. Scanning tunneling microscopy (STM) is the ideal technique to tackle this problem, since it allows studying both the atomic structure in the conventional constant current mode and the electronic structure in the spectroscopic mode. In this contribution, it will be shown that for deposition of submonolayers at temperature s above 400K, Mn atoms are place exchanged with Fe substrate atoms. Locally a c(2x2) MnFe surface alloy is formed. Spectroscopy measurements will be presented for incorporated Mn atoms, pure Mn islands, and the local c(2x2) MnFe alloy structures which all show characteristic features in the dI/dV spectrum. For growth of thicker Mn films at 400K, intermixed Fe atoms can still be observed until the third layer. Analysis of the step heights gives evidence that the structure relaxes after the second layer. From the fourth layer upon, spectroscopy measurements reveal a feature in the dI/dV spectrum which strength oscillates with layer thickness and therefore seems to be related with the reported antiferromagnetic coupling of the Mn layers. The spin-polarized nature of these surface states can be used to study the surface magnetism on a local scale in spin-polarized tunneling experiments. Experiments with Fe covered tungsten tips will be discussed.
MI-ThP-6 Epitaxial Growth of Ferromagnetic Ni2MnIn Thin Films on InAs (001)
J.Q. Xie, J.W. Dong, J. Lu, S. McKernan, C.J. Palmstrom (University of Minnesota)
There has been growing interest in ferromagnetic/semiconductor hererostructures for the development of spintronic devices which utilize the carrier's spin as well as its charge. InAs is the semiconductor of choice because of its high electron mobility and the ease to form an ohmic contact to it. Although no elemental ferromagnet is lattice matched to InAs, the lattice mismatch between the Heusler alloy Ni2MnIn and InAs is only 0.2%. In bulk, Ni2MnIn is reported to have a cubic (L21) crystal structure with a lattice constant a0 = 6.069 Å and a Curie temperature ~ 314 K. Recent theoretical studies showed that the minority spins are situated at the Γ point in Ni2MnIn and the majority spins are far away from the Γ point. Therefore, the band structure alignment between Ni2MnIn and InAs would enhance the injection of the minority spins, suggesting that Ni2MnIn may be a good choice for spin injection as a ferromagnetic contact. In this talk, we report on the epitaxial growth of Ni2MnIn thin films on InAs (001) by the molecular beam epitaxy technique. Both in situ reflection high energy electron diffraction and ex situ x-ray diffraction, Rutherford backscattering spectrometry, and transmission electron microscopy measurements indicate the high-quality epitaxial growth of Ni2MnIn films on InAs (001). The films have a Curie temperature ~ 170 K and a saturation magnetization ~ 420 emu/cm3. The lower Curie temperature, compared to that of bulk Ni2MnIn, is believed to result from the growth of Ni2MnIn in the B2 structure. Composition has a dramatic effect on the Curie temperature. For Ni2MnIn1.7, a Curie temperature of ~ 290 K was observed. If ordered films can be grown, significantly higher Curie temperatures may be expected.
MI-ThP-7 On the Origin of the Thickness-dependent Dimensional-crossover in Ultrathin Magnetic Films
N.A.R. Gilman (Penn State University); M. Hochstrasser (Lawrence Berkeley National Laboratory); R. Zhang, R.F. Willis (Penn State University)
We report experimental results that show that the order parameter β, which determines the long-range (spin) ordering in magnetic thin films (M=Mo(1-T/Tc)beta), changes abruptly due to a crossover in dimensionality at different thicknesses in Ni(100), Ni(110) and Ni(111) films. We argue that the different critical thicknesses arise from finite-size quantization energies of the (spin) excitations, which are dependent on the magnitude of associated wavevectors spanning different crystallographic directions of the fcc Fermi surface. Experimental data on nickel alloys support this view.
MI-ThP-8 Fabrication of CoCrTa Magnetic Film by RF-sputtering
Y. Ohta (Fukui National College of Technology, Japan); K. Ohashi, T. Tsumori (Shin-Etu Chemical Co., Ltd., Japan)
CoCrTa longitudinal media was prepared by sputtering onto Si substrates at substrate temperature of 523 K by RF magnetron sputtering system. The films were fabricated at the substrate temperature of 523K and Argon gas pressure of 3.5 mTorr during sputtering. The film was deposited on surface Si and glass substrates. CoCrTa thin films of several thicknesses were prepared by sputtering. The microstructures and particle size were investigated by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The crystal structure was checked by X-ray diffractometry (XRD) and the magnetic properties were measured by a vibrating sample magnetorometer (VSM). The underlayer thickness dependence of magnetic properties of CoCrTa deposited on Si and glass substrates were investigated. It was known that thickness of underlayer has significant influence the crystallographic texture and magnetic properties of the magnetic layer. CoCrTa layer deposited on the Si substrate revealed small size grain and smooth surface than ones deposited on the glass substrate.
MI-ThP-9 Ion-beam Modification of the Physical Properties and the Structure of Fe/Si Multilayered Films
Y.P. Lee, J.S. Park (Hanyang University, Korea); Y.V. Kudryavtsev (Institute of Metal Physics, Ukraine); J. Dubowik, B. Szymanski (Institute of Molecular Physics, Poland); J.Y. Rhee (Hoseo University, Korea); G.S. Chang (Yonsei University, Korea)
The influence of ion-beam mixing (IBM) on the strucure, and the magnetic, magneto-optical and optical properties of Fe/Si multilayered films (MLF) was investigated. The IBM was performed with Ar+ ions of an energy of 80 keV and a dose of 1.5 X 1016 Ar+/cm2. It was shown that the IBM destroys the layered structure of the MLF down to a depth of about 110 - 150 nm and leads to the formation of a new phase which is characterized to possess a perfect crystalline structure, a low coercivity and a Curie temperature of about 550 K. It is suggested that the phase formed by the IBM is a metastable Fe2Si silicide with a B2 type of structure. An annealing of the ion-beam mixed Fe/Si MLF at 720 K destroys further the undisturbed layered structure at the bottom and also leads to a decomposition of the Fe2Si phase into a metastable magnetically-hard Fe5Si3 silicide and, presumably, Fe3Si.
MI-ThP-10 Suface Magnetic Phase Diagram for a Semi-infinite Ferromagnet
D.P. Pappas (National Institute of Standards and Technology, Boulder); A.P. Popov (Moscow State Engineering Physics Institute, Russia)
The phase diagram for the orientation of the surface region is calculated in the parameter space defined by the surface and bulk anisotropy for semi-infinite systems and thin films. A discrete layer-by-layer approach is developed and compared to the continuum approach. We also consider the 1.5 atomic layer Fe on Gd system, and find that it is a good physical realization of the model. We find that suface magnetic canting always occurs when the magnitude of the surface anistropy is comparable with the interlayer exchange interaction.
MI-ThP-11 Low Field Magnetoresistance through Grain Boundaries in Double Exchange Compounds
D. García, B. Alascio (Instituto Balseiro, Argentina)
To model transport through the grain boundary of a manganite bicrystal we study the conductance of a spin chain doped with itinerant electrons which are strongly Hund coupled to the spin at each site. We induce a domain wall in the chain and propose a single site pinning mechanism. At large pinning we find that the magnetoresistance corresponds to that of two uniformly magnetized domains. Assuming uniaxial anisotropy within each grain we use the above results to calculate the magnetoresistance through the grain boundary and obtain remarkable agreement with experiment. Further, our study solves an apparent contradiction between experiments by different authors.
MI-ThP-12 Formation of Barriers for Magnetic Tunneling: Ion Embedding vs Diffusion
S.O. Demokritov, B.F.P. Roos, P.A. Beck, B. Hillebrands (University Kaiserslautern, Germany)
A novel type of oxidation technique, the ionized atom beam oxidation, was used to prepare ultrathin insulating aluminum oxide barriers for magnetic tunnel junctions. Thanks to high chemical reactivity of atomic oxygen combined with very low energy (30-50 eV) of the ions the oxide grows fast, homogeneous, and amorphous. The limited oxidation depth reduces the possibility of an overoxidation of the underlaying magnetic electrode. By means of in-situ techniques for monitoring the oxide growth during the oxidation, it was possible to identify two mechanisms which dominate the oxidation at different stages of the process. During the initial stage an ion embedding mechanism controls the oxidation. This mechanism describes the penetration of kinetic O-ions into the target metal layer until they are stopped on their way through the film by elastic and inelastic processes. The ions form chemical bindings with the surrounding metal atoms at their stopping place. The oxidation depth defined by this mechanism depends on the energy of the ions and reaches 1-2 nm. As the oxide layer grows, the incoming O-ions find less and less leftover metal atoms near their stopping place and a diffusion process starts to effect the oxidation. The further oxidation growth is determined by an electrical field controlled diffusion of metal and oxygen atoms. Monte-Carlo-simulations based on the developed "ion embedding with diffusion" model completely describe the formation of thin oxide barriers for magnetic tunnel junctions.
MI-ThP-13 Non-monotonic Magnetic Surface Anisotropy of Epitaxial Fe Films Grown on Vicinal Substrates
S.O. Demokritov, M. Rickart, J. Jorzick, B. Hillebrands (University Kaiserslautern, Germany)
The presented work is devoted to the study of the influence of atomic steps at the interface between a magnetic film and a substrate on the magnetic surface anisotropy of the film. Step induced anisotropy contributions appear because of the broken translational invariance perpendicular to the steps. The Fe films were prepared on vicinal MgO(001) substrates with Ag(001) and Au(001) buffers using a molecular beam epitaxy UHV system. Substrates with two different miscut orientations (along [100] and [110]) were used. In-situ characterization was performed by LEED, RHEED and SPM, and chemical analysis by Auger electron spectroscopy. Magnetic anisotropy contributions have been determined from the hysteresis loops and from frequencies of spin waves measured by Brillouin light scattering. In addition to the four-fold anisotropy intrinsic for (001) Fe films the vicinal films demonstrate an uniaxial anisotropy. The strength of the uniaxial anisotropy depends on the miscut angle, the miscut orientation, and the buffer material. Fe films prepared on Au buffers with the miscut orientation along the [100]-direction show an additional uniaxial anisotropy with the easy axis perpendicular to the steps. For the Fe films prepared on Au buffers with the miscut orientation along the [110]-direction a non-monotonic dependence of the anisotropy strength as a function of the miscut angle is observed: the orientation of the magnetic easy axis switches from parallel to perpendicular to the steps, crossing zero value between 1.5 and 2 degrees of the miscut angle. Fe films prepared on Ag buffers with the miscut orientation along the [100] direction shows the same orientation of the uniaxial anisotropy easy axis, as those prepared on Au, however the anisotropy strength is much higher than that observed on Fe/Au system. The properties of the observed uniaxial anisotropies are analyzed on the basis of the Neel pair-bonding model.
Time Period ThP Sessions | Topic MI Sessions | Time Periods | Topics | AVS2001 Schedule