AVS1997 Session MI-TuP: Magnetic Interfaces and Nanostructures Poster Session
Tuesday, October 21, 1997 5:30 PM in Room Exhibit Hall 1
Tuesday Afternoon
Time Period TuP Sessions | Topic MI Sessions | Time Periods | Topics | AVS1997 Schedule
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MI-TuP-1 Magnetic Moment Measurements of Pt and Fe in Fe/Pt Multilayers Using XMCD
W.J. Antel, Jr., M.M. Schwickert (Ohio University); W.L. O'Brien (University of Wisconson, Madison); G.R. Harp (Ohio University) The magnetic moments of Pt and Fe in a Fe/Pt multilayer are studied as a function of Pt thickness. The moments are measured using x-ray magnetic circular dichroism at the N-edge of Pt and at the L-edge of Fe. Comparison between the Pt multilayer moments and the Pt moment in a Pt75-Co25 alloy indicate that the multilayer moments are approximately 1/4 the alloy moment. L-edge measurements of the Fe moments are correlated with a large increase in the Kerr rotation in these films for Pt thicknesses < 5 Å. |
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MI-TuP-2 Low Remanence Domain State in Epitaxial FePt(001) L10 Films with Large Perpendicular Magnetic Anisotropy
J.-U. Thiele, D.K. Weller, A. Moser, M.E. Best, M.F. Toney, T.H. Westmore, R.F.C. Farrow (IBM Almaden Research Center) Structural and magnetic properties of epitaxial Fe50Pt50 (001) oriented alloy films, grown by single target dc-magnetron sputtering onto Pt seeded single crystalline MgO (100) substrates, have been studied as a function of seed layer thickness and substrate temperature. A maximum degree of long-range chemical ordering of S=0.95, as quantitatively determined with X-ray diffraction (XRD), is obtained at 500 °C substrate temperature. The epitaxial quality is mainly controlled by the seed layer thickness. The magnetic behaviour is very sensitive to the degree of misorientation, as we show by systematic polar and transversal Kerr hysteresis loop measurements. Of special interest is the observation of a low remanence domain state in the highest quality L10 FePt(001) films as observed with magnetic force microscopy. These films show similarly large perpendicular magnetic anisotropy constants as have been reported in co-evaporated MBE1 and sequentially layered and post-annealed sputtered films2. Interestingly, by adding structural imperfections (regions with deviating orientation, e.g. (111), one can remove this domain state and obtain almost 100% remanence.
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MI-TuP-3 Larger Short-Range Compositional Anisotropy in the Evaporated Co-Pd Alloy than in Pd/Co Multilayer Films
S.K. Kim, J.B. Kortright (Lawrence Berkeley National Laboratory); V.A. Chernov (Siberian Synchrotron Radiation Center of Budker Institute of Nuclear Physics, Russia); Y.M. Koo (Pohang Univeristy of Science and Technology, South Korea) A short-range compositional anisotropy (SRCA) near the interfaces in Pd/Co multilayer can be expected to be larger than in Co-Pd alloy films because the interfaces impose strong local asymmetry to the multilayers. However, we have found experimentally a larger SRCA in Co-Pd alloys than in Pd/Co multilayers that posses a long-range compositional modulation, by investigating polarized Extended-X-ray-Absorption-Fine-Structure (EXAFS) spectra at above Co K-edge. The ordering effect (directional preference of surrounding atomic species) in the alloy films is found to depend strongly on their composition. The multilayers have an interfacial mixing region in which SRCA is smaller than in the coevaporated alloy films. A model of the average Co atomic configuration in alloy and multilayer films will be described according to the determined SRCA and the composition to relate its structure with out-of-plane magnetization observed in these alloy films. Local structural effects such as chemical anisotropy should be carefully considered to identify macroscopic magnetic phenomena observed in these films. |
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MI-TuP-4 The Effect of Surface Mobility on the Formation of Platelets in CoPt3
A.L. Shapiro (University of California, San Diego); O. Vajk (Reed College); B.B. Maranville, F. Hellman (University of California, San Diego) We have investigated the effect of surface mobility on growth-surface-driven clustering and perpendicular magnetic anisotropy (PMA) in CoPt3 alloy films. Previous work1 has shown that when CoPt3 is deposited by electron-beam evaporation at temperatures below the threshold of bulk atomic mobility (400 ° C) the Co atoms cluster on the growth surface and are "frozen in" by succeeding layers. This is evidenced in the enhanced Curie temperature and large value of the perpendicular magnetic anisotropy (6x106 erg/ cc) found in these samples. The PMA is explained by the Co clustering into platelets with internal interfaces perpendicular to the growth direction. Clustering and anisotropy disappear at deposition temperatures lower than 250 ° C. Surface atomic mobility is responsible for increasing the amount of clustering and PMA up to the limit of the onset of bulk atomic mobility. We have modified surface atomic mobility by varying the deposition rate by several decades and by introducing a small percentage of Si to the CoPt3 alloy during deposition. Slowing down the deposition rate at 250 ° C, where the clustering and PMA first begin to appear, noticeably changes the anisotropy of the sample. Silicon inhibits clustering by limiting surface mobility, as evidenced by smaller grain sizes in polycrystalline samples, but the value of the PMA decreases only by 30 percent. In contrast, bulk annealing above 400 ° C destroyes PMA very rapidly while changing the amount of clustering only slightly. A possible explanation for the annealing result is that atomic mobility in the bulk first destroyes interfacial quality of the clusters before actually significantly mixing the separated regions. 1 P.W. Rooney et al.,Phys.Rev.Lett.,1995, 75(10),1843 This work was supported by DOE grant no. DE-FG03-95ER45529 and by the University of California Campus Laboratory Collaboration. Thanks to the Center for Magnetic Recording Research at UCSD. |
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MI-TuP-5 Co/Rh Superlattices: Perpendicular Anisotropy and Magneto-Optical Properties.
X. Ying (Royal Institute of Technology, Sweden (Currently at Hitachi Maxell Ltd., Japan)); J.J. Xu (Intel Corporation); K.V. Rao (Royal Institute of Technology, Sweden); H. Awano (Hitachi Maxell Ltd., Japan); T. Katayama (Electrotechnical Laboratory, Japan) We report the observation of perpendicular anisotropy in Co/Rh superlattices for a fixed Rh thickness of 9Å with Co layer thickness less than 7Å. The superlattices, fabricated by using MBE, are found to be epitaxially grown on fcc Rh(111) buffer layers . Both small and large angle XRD and RHEED characterization techniques have been used to study the quality as well as the orientation of the films. Polar Kerr hysteretic loop measurements suggest pependicular anisotropy in films with Co layer thicknesses less than 7Å. From SQUID magnetic measurements we determine the origin of the perpendicular anisotropy to arise mainly from intersurface anisotropy Ks of the order of 130 milli-ergs/square cm.. Investigations of the temperature dependence of the perpendicular anisotropy reveal that above 183° C the easy axis for the magnetization becomes 'in planar'. This transition temperature is also found to be dependent on the Co layer thickness as expected. The Kerr rotation angle of Co/Rh at room temperature and in the 400nm wavelength region is found to be much smaller than that observed for Co/Pt and Co/Pd. However, the wavelength dependence is similar in the sense that it increases with decreasing wavelength down to 300nm. As far as we know there is no other reported study in literature on the magneto-optical properties of Co/Rh superlattices. |
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MI-TuP-6 Complete X-Ray Magneto-Optical Description of Fe Near its L2,3 Absorption Edges
J.B. Kortright, S.K. Kim (Lawrence Berkeley National Laboratory) A complete optical description of the magneto-optical (MO) response of materials in the x-ray range requires knowledge of the spectral dependence of the full dielectric tensor, or equivalently of the complex refractive indices for circularly polarized components of opposite helicity, near core absorption edges. This complete description is important because basic optical properties such as reflectivity and penetration depth depend on both refractive and absorptive effects contained in this complete description. Magnetic circular dichroism (MCD) directly measures the absorptive MO response, while Faraday magneto-optical rotation (MOR) directly measures the refractive MO response, and together these measures provide this complete description. Refractive and absorptive responses are related by Kramers-Kronig dispersion relations, so that measurement of only one is required, in principle, to obtain a complete description. We consider the utility of the dispersion relations to obtain the refractive from the absorptive response, and vice versa, for the case of polycrystalline Fe films near the Fe L2,3 levels using only a limited energy range near the edges. We find that results are sensitive to the precise energy range of the dispersion integrals, and that close agreement can be obtained between the measured MCD(MOR) and the dispersion transformation of the measured MOR(MCD) with careful choice of energy range. Examples of numerical application of the dispersion relations are presented, as are the limitations of these analyses in providing this complete MO description. Implications of the need for this complete MO description for Fe and Fe-containing films will also be discussed. |
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MI-TuP-7 Can Soft X-Ray Magnetic Resonant Scattering be Used for Element-Specific Magnetometry of Multilayers?
V. Chakarian, J.W. Freeland, Y.U. Idzerda (Naval Research Laboratory) Element-specific magnetic hysteresis measurements using soft x-ray magnetic resonant scattering (SX-RMS) are presented to determine the feasiblity of scattering-based element-specific magnetometry of magnetic multilayers. In SX-RMS, the reflectivity of circular polarized soft x-rays are measured for photon spin vector parallel and antiparallel to the magnetization direction as a function of either energy or incidence angle. By measuring the magnetic field dependence of the reflected intensity, magnetic hysteresis loops of the sample can be determined, in a manner analogous to the corresponding measurements in absorption [1,2]. The measurements were done on a Co/Cr/Co multilayer in which an anti-ferromagnetic ordering of the two Co layers can be realized. It is shown that, unlike what is seen in absorption, the hysteresis measurements of magnetic multilayers obtained via SX-RMS are not equivalent to the magnetization. Hysteresis loops obtained via SX-RMS at different angles of incidence show very large deviations from the 'true' hysteretic behavior, as measured either in absorption mode or by standard magnetometry methods. This inequivalence is due to the changes in the polarization of the diffracted x-rays as they traverse intervening magnetic layers. This change of polarization is strongly affected by the photon incidence angle and energy, as well as the detailed magnetic structure of the intervening layers, clearly demonstrated by measurements at different angles of incidence yielding different hysteresis loops. [1] V. Chakarian et al., Appl. Phys. Lett., 66, 3368 (1995). [2] V. Chakarian et al., Phys. Rev. B., 53, 11313 (1996). |
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MI-TuP-8 Angle-resolved Photoemission Spectroscopy of NiCo Magnetic Ultrathin Films
T.R. Cummins, G.D. Waddill (University of Missouri, Rolla); S.R. Mishra, D.P. Pappas (Virginia Commonwealth University); P. Bedrossian, J.G. Tobin (Lawrence Livermore National Laboratory) Magnetic ultrathin films of differing compositions of NiCo grown on Cu (001) have been studied using core and valence level photoemission spectroscopy. The measurements were performed at the Spectromicroscopy Facility (Beamline 7) of the Advanced Light Source [1] using a newly developed spectrometer being optimised for magnetic overlayer preparation and magnetically sensitive spectroscopies, including x-ray dichroism [1] and true spin resolution. Spin integrated photoemission spectra illustrate the filling behavior of the near Fermi level region to be dominated by Ni/Co derived d bands. The presence of exchange interactions are apparent through splitting of the 3s core levels. Magnetic interactions observed in our spectra are correlated with our previous studies in this alloy system using Magnetic x-ray Linear Dichroism in photoelectron spectroscopy and Magnetic x-ray circular dichroism in core level absorption spectroscopy [2]. A progress report on the development of the spectrometer will also be presented, with particular focus on encouraging initial performance and the prospects for spin sensitivity. 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. The Advanced Light Source and Spectromicroscopy Facility were built and are supported by the U. S. Department of Energy. This work is based on research supported by the National Science Foundation under grant No. DDMR-9458004. Additional support from Research Corporation under grant No. CC3778 and the Jeffress Trust, No. J338 is also acknowledged. [1] J. G. Tobin, K. W. Goodman, G. J. Mankey, R. F. Willis, J. D. Denlinger, E. Rotenberg, and A. Warwick, J. Appl. Phys. 79, 5926 (1996) and J. Vac. Sci. Tech. B 14, 3171 (1996). [2] S. Z. Wu, F. O. Schumann, R. F. Willis, K. W. Goodman, J. G. Tobin, and R. Carr, J. Vac. Sci. Tech. A 15, July/Aug., 1997. |
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MI-TuP-9 X-ray Magnetic-Circular-Dichroism Study of Ni/Fe Multilayers
T. Lin, M.M. Schwickert, M.A. Tomaz (Ohio University); W.L. O'Brien (University of Wisconsin, Madison); G.R. Harp (Ohio University) Element specific magnetic moments in Ni/Fe mutilayers are studied using x-ray magnetic circular dichroism (XMCD). Additionally, the lattice structure in Ni/Fe mutilayers is also studied by using x-ray diffraction. Multilayers are deposited with a Ni wedge (0-10Å) and an Fe layer (7.5Å), repeated 20 times, to explore the magnetic moment and the structure dependence of the thickness of Ni. Up to 10Å (~6 ML), Ni stays in bcc structure, compared to the fcc structure in bulk Ni. The magnetic moment of Ni at about 1 ML thickness can reach about two times of that in bulk Ni. As the thickness of the Ni wedge increases, the Ni magnetic moment decreases. |
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MI-TuP-10 A Comparison of the Absorption and Dichroism Spectra at the L and M Edges of Pd, Rh, Ru, and Mo.
G.R. Harp, M.A. Tomaz (Ohio University); E. Hallin (Canadian Synchrotron Radiation Facility); T.K. Sham (University of Western Ontario, Canada); M.M. Schwickert, T. Lin (Ohio University) We present a survey of x-ray magnetic circular dichroism (XMCD) spectra from 4d elements (Pd, Rh, Ru, and Mo) in magnetic multilayers and alloys. Measurements were made at the M-edge and L-edge for each element to compare the relative merits of the different edges for magnetic dichroism. Both sets of data were obtained at the Synchrotron Radiation Center in Stoughton, WI. The L edge spectra were acquired on the Canadian double crystal monochromator using InSb grating crystals, while the M edge spectra were acquired on a 10 meter toriodal grating monochromator. We compare the two sets of results and find that for Pd, L-edge spectroscopy is superior for XMCD studies, while for Mo, the M-edge is superior. We find that for Rh and Ru, both the L and M edges are suitable, though the L edges may be preferable due to a greater potential for the application of sum rules. We also discuss the relative merits and challenges associated with XMCD experiments at the L and M edges for these elements focusing on the degree of polarization and photon flux. |
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MI-TuP-11 Unexpected Magnetic Phases in Dilute Mn Alloy Ultrathin Films
S. Banerjee, W.L. O'Brien, B.P. Tonner (University of Wisconsin, Milwaukee) Antiferromagnetic manganese based alloys are one of the materials used as exchange bias layers in spin-valve structures. As part of a search for replacements to the Fe-Mn system, we have been studying the magnetic properties of dilute Mn-Co and Mn-Ni alloys, by using x-ray magnetic circular dichroism. Beginning nearly a quarter of a century ago1, the magnetic phase diagram of Mn-Co has been controversial, even for the simplest issue of whether the Mn and Co moments are parallel or anti-parallel. In contrast to the latest 'accepted' result from neutron scattering2, we find that Mn is coupled ferromagnetically to the Co moments. Similarly, ultrathin films of Mn-Ni are completely ferromagnetic, not antiferromagnetic as for the bulk alloys. We discuss the potential influence of nanostructure, surfaces, and epitaxy on the differences found here, with the 'textbook' results.
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MI-TuP-12 Spin-Polarized Photoemission from Shallow Core Levels in Localized Materials
E. Vescovo (Brookhaven National Laboratory); O. Rader (Tokyo University, Japan); G. van der Laan (Daresbury Laboratory, United Kingdom); C. Carbone (Forschungszentrum Juelich, Germany) In ferromagnetic materials the exchange field induced by the polarized outer shell electrons splits the core level excitation spectra into a set of magnetic sublevels. The exchange interaction and its coupling to the Coulomb and spin-orbit interaction give rise to orbital and spin polarization effects in core level photoemission spectra. These effects, which can be analyzed within a common framework, are increasingly exploited to probe experimentally the magnetic structure on an atomic scale. We present spin resolved photoemission measurements from the 5p core levels of Gd and Tb metal. In contrast to the 3d transition-metal 3p spectra, the rare-earth 5p spin-resolved spectra show a detailed fine structure that cannot be explained within an one-electron model. Spin-orbit interaction, Coulomb and exchange interaction have to be treated on equal footing. A remarkable agreement is obtained between experiment and theory when the spin-polarization of the 5p emission is analyzed on the basis of atomic multiplet calculations in intermediate coupling. The results emphasize strongly the potential of spin analysis in core level studies. |
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MI-TuP-13 State Resolved Magnetic Circular Dichroism in Core-Level Photoelectron Angular Distributions from Gd(0001)
J. Morais (Lawrence Berkeley National Laboratory); R. Denecke (Lawrence Berkeley National Lab & Univ. of California, Davis); J.G. Menchero (University of California, Berkeley); R.X. Ynzunza (Lawrence Berkeley National Lab & Univ. of California, Davis); J. Liesegang (La Trobe University, Australia); C.S. Fadley (Lawrence Berkeley National Laboratory) We have studied magnetic circular dichroism in photoelectron angular distributions (MCDAD) for Gd 4d and 4f core-level emission from in-plane magnetized Gd(0001) films. The films were 100ML of Gd on a W(110) substrate, deposited at room temperature and annealed at 700K to form smooth Gd(0001). The MCDAD measurements were performed on beamline 9.3.2 at the Advanced Light Source by keeping the light helicity constant in left circular polarization and inverting the magnetization direction with a rotation of the sample by 180° around the surface normal. The MCDAD asymmetries were obtained as normalized differences between spectra measured with the two magnetization orientations. Their angular dependence was determined by changing the direction of the magnetization axis relative to the plane containing light incidence and electron emission. In addition, our energy resolution enabled us to study the MCDAD for the individual states in the Gd 4d multiplet for the first time. We observe a strong variation of MCD with angle which represents a combined influence of both free-atom effects (in agreement with prior calculations by Van der Laan et al.) and non-magnetic and magnetic solid-state photoelectron diffraction effects. The diffraction processes have been studied in a state-specific way via single-scattering and multiple scattering cluster calculations which include spin-orbit splitting and exchange splitting in the initial state. Work supported by ONR (Contract N00014-94-1-0162), DOE, BES, Mat. Sci. Div. (Contract DE-AC03-76SF00098), CNPq (Brazil), and DFG (Germany). |
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MI-TuP-14 The Electronic Structure of Gadolinium Grown on Mo(112)
C. Waldfried, T. McAvoy, D.A. Welipitiya, P.A. Dowben (University of Nebraska, Lincoln) In growing Gd on a corrugated (112) surface of molybdenum we were able to obtain strained layers of Gd(0001)[1]. We studied the magnetic ordering and electronic structure of strained thin films of Gd using the techniques of spin polarized PES, IPES, LEED and MOKE. Gadolinium overlayers order at a coverage of 2/3 monolayers in a p(2x3) LEED pattern. For thicker films the lattice mismatch between the naturally hexagonal closed packed (hcp) Gd films and the Mo(112) surface results in incommensurate strained Gd thin films with substantial misfit dislocations. The strain of the thin Gd films is reflected by a reduced Brillouin zone size by approximately 20%. The induced strain severely alters the band structure of the conventional Gd 5d/6s bulk bands, which disperse in the opposite direction than the relaxed Gd(0001). The surface state is shifted away form the Fermi level which may alter the wave vector dependent magnetic befavior. The strained Gd films order ferromagnetically, with enhanced exchange splitting at the Brillouin zone edge. [1] Carlo Waldfried, D.N. McIlroy, C.W. Hutchings and P.A. Dowben, Phys. Rev. B 54 (1996) |
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MI-TuP-15 Variable Temperature Scanning Tunneling Spectroscopy of Gd(0001) Surface States
M. Bode, M. Getzlaff, S. Heinze, R. Wiesendanger (University of Hamburg, Germany) We have investigated the thickness- and temperature-dependent splitting of dz2-like surface states of Gd(0001) films grown on W(110) substrates by means of scanning tunneling spectroscopy (STS). These surface states are held to be responsible for the extraordinary surface magnetic properties of Gd(0001)1,2. Close to the ground state (T = 20K) we measured binding energies of -225 meV and 425 meV for the occupied and unoccupied Gd(0001) surface state, respectively, i.e. a splitting of 650 meV. The splitting reduces with increasing temperature. Even for T = 350 K a splitting of 300 meV remains. Our results exclude a pure spin--mixing behaviour. A Stoner-like fit of the data results in a surface Curie-temperature of about TCS = 400 K. Due to the spatial selectivity of STS we could determine the thickness-dependent splitting on unhomogeneous sample surfaces. The splitting is reduced by approximately 100meV for the bilayer mainly caused by a shift of the binding energy of the occupied surface state. For coverages Θ ≥ 3ML the binding energy was found to be equal to fully relaxed Gd(0001) within the error bars.
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MI-TuP-16 A Study of the Surface Magnetism of Gd(0001) Grown on Y(0001)
T.S. Sherwood, S.R. Mishra (Virginia Commonwealth University); A.P. Popov (Moscow Engineering Physics Institute, Russia); D.P. Pappas (Virginia Commonwealth University) A study of the surface magnetism of Gd(0001) grown on Y(0001) will be presented. The growth parameters and annealing temperature needed to observe an enhanced surface Curie temperature (TCS) will be discussed. The effects of sputter induced disorder and depostion of thin films of Fe on the Gd surface will be discussed. For thick films (>30 monolayers, ML) TCS can be reversibly quenched by a light sputter and subsequent anneal. A reversible spin flop transition from in-plane to perpendicular magnetization at the surface is observed from ultra-thin (1.5 - 4 ML) Fe films deposited on the Gd surface. This transition is unusual because the overlayers have a higher TC than the bulk Gd and the Fe forms a disordered overlayer on the Gd(0001) surface. The spin flop is attributed to the competition between the intrinsic perpendicular anisotropy due to intermixing at the Fe-Gd interface and the exchange interaction with the bulk Gd. The spin flop is observed to occur well below 293 K, i.e. below the bulk Gd TC. |
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MI-TuP-17 Growth Mechanisms and Electronic Structures of MnSb Ultra Thin Films on GaAs Substrates.
K. Ono, M. Shuzo, H. Fujioka (University of Tokyo, Japan); H. Akinaga, S. Miyanishi (Joint Research Center for Atom Technology, Japan); Y. Watanabe (NTT, Japan); M. Oshima (University of Tokyo, Japan) MnSb is a ferromagnetic metal and has attracted much attention for their interesting physical properties such as a large magneto-optic effect, a large magnetic anisotropy in the direction of the easy magnetization axis and a large magneto-resistance. Very recently, epitaxial Mn-based magnetic compounds such as MnSb, MnAs, MnGa and MnAl have been successfully grown on semiconductor substrates using low temperature molecular beam epitaxy (LT-MBE). A hetero-junction between semiconductors and ferromagnetic metals has attracted much interest as a new type of magnetoelectronic devices utilizing not only a charge but a spin of carriers. In the epitaxial growth of such ferromagnetic metals on semiconductors, we are confronted by the difficulty that there is a strong reactivity at their interface. In order to apply these films to realistic devices, the information and the control of these surfaces and interfaces are very important. In this report, we present the initial growth mechanism of MnSb thin films on GaAs substrates studied by photoemission spectroscopy using synchrotron radiation. MnSb films were grown on (100) and (111)B GaAs substrates at 300 C by LT-MBE. The beam pressure ratio of Sb to Mn was kept about 2. The growth rate was 0.3 A / s. RHEED, XRD and cross-sectional TEM results show the MnSb films of very high quality. Photoemission measurements were performed at BL-1A of the Photon Factory. After the growth of MnSb films, samples were transferred to the photoemission measurements chamber without breaking the UHV. We found that the Ga 3d peak intensity remains not changed with increasing the MnSb overlayer thickness. However the As 3d peak intensity decreases gradually. These results suggest the Ga atoms are segregated at the top surface during the growth, acting as a surfactant. The chemical state of this surfactant Ga is assigned as a Ga-Mn bonding state, not a Ga-Sb state. The Ga-surfactants decrease the surface energy and contribute to the layer-by-layer growth. |
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MI-TuP-19 Enhanced Carrier Lifetimes and Reduced Defect Levels at the Fe / GaAs(001) 2x4 Interface
B.T. Jonker, O.J. Glembocki, R.T. Holm, R.J. Wagner (Naval Research Laboratory) Spin injection into a semiconductor is expected to utilize a ferromagnetic material as the electrical contact. One system of potential interest consists of Fe contacts to a GaAs-based device structure. We have previously shown using photoreflectance spectroscopy that semiconductor carrier lifetimes are significantly improved at the Fe/GaAs(001) 2x4-As interface relative to the simple oxide terminated or sulfur passivated surface. We report here the results of dc bias- and frequency-dependent electroreflectance spectroscopy measurements performed to assess the density, energy position and temporal response of potential defect states within the band gap. On oxide terminated surfaces, such states appear at characteristic energies and serve to pin the Fermi level, as well as severely limit both carrier lifetimes and the frequency response. Surface barrier heights and carrier lifetimes were determined from the period and damping envelope of the Franz-Keldysh oscillations which dominate the reflectance spectrum above the band gap. We find that the electronic quality of the interface is highly dependent upon GaAs MBE growth conditions and preparation of the final surface, even for a given GaAs surface reconstruction. Bias dependent measurements show that the Fermi level can be swept across the band gap, resulting in a linear dependence of electric field on bias voltage, in contrast to the highly non-linear behavior observed under slight forward bias for a metal gate/oxide terminated surface. We further find that the frequency response is significantly improved for the Fe/GaAs-2x4 sample, and conclude that Fe termination of the GaAs-2x4 surface results in a significant reduction of defect states relative to the oxide- or sulfur-terminated surfaces. We discuss these results in relation to current models for the metal/semiconductor interface. Supported by the Office of Naval Research. |
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MI-TuP-20 Magnetic Properties of Ultrathin Fe Films Grown on Ge(100) and Sulphur Passivated Ge(100) Surface
P. Ma, P. Norton (University of Western Ontario, Canada) Ultrathin bcc Fe films have been successfully grown on Ge(100) and S-passivated Ge(100) surfaces. On the Ge(100) surface, it is found that a small amount of Ge either intermixed with the Fe film or segregated to the surface. On the S-passivated surface, there is essentially no intermixing or segregation. Different magnetic behaviours were observed for the Fe films grown on clean and S-passivated Ge(100) surfaces with our in-situ MOKE system. Hysteresis loop simulations based on the simple coherent model suggests that in-plane uniaxial anisotropy exists in these films. Following the in-situ studies, we also studied the gold capped Fe films with ex-situ MOKE and FMR. It is found that both in-plane four-fold anisotropy and in-plane uniaxial anisotropy are strongly dependent on the film thickness. For very thin film, the in-plane uniaxial anisotropy is relatively large. |
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MI-TuP-21 Short-Period Quantum-Well States in Cu/Co/Cu(100)
P. Segovia (Universidad Autonoma de Madrid, Spain); J.E. Ortega (Universidad del Pais Vasco, Spain); E.G. Michel (Universidad Autonoma de Madrid, Spain) The discovery of spin-polarized quantum-well (QW) states at the Fermi level in thin noble-metal films, suggested a direct connection between these states and the oscillatory magnetic coupling (ferro-antiferro) between magnetic layers in superlattices. The QW model established a variety of coupling periodicities, related to the peculiar shape of the spacer Fermi surface. In the Cu/Co(100)/Cu(100) system, the two extreme points of the Cu Fermi surface along the 100 direction (i.e. the belly and the neck), give rise to two different magnetic coupling periods of 2.6 ML and 5.7 ML, respectively. In spite that long-periodicity belly-related QW-states were found a few years ago, QW-states at the neck were never observed, projecting some shades on the QW-model. In our high-resolution photoemission experiments both neck and belly-related QW states were observed, and the conditions for their appearance were characterizaed. Pseudomorphic Co(100) films were epitaxially grown on Cu(100). On top of these films, thin Cu layers were deposited, and their electronic states close to the Fermi level were investigated using angle-resolved photoemission spectroscopy. Neck-related stated were observed exhibiting the periodicity expected in the QW-theory. Their properties were analyzed in detail, and the results obtained prove the connection between QW-states and magnetic coupling. The role of the interface quality in order to confine the states was also investigated in a systematic search of the best growth conditions. QW states were observed only for abrupt interfaces. |
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MI-TuP-22 Probing the Quantum Well States In Ultrathin Cu Layers Using Photoemission
R.K. Kawakami (University of California, Berkeley); E. Rotenberg (Lawrence Berkeley National Laboratory); E.J. Escorcia-Aparicio, H.J. Choi (University of California, Berkeley); T.R. Cummins, J.G. Tobin (Lawrence Livermore National Laboratory); N.V. Smith (Lawrence Berkeley National Laboratory); Z.Q. Qiu (University of California, Berkeley) Quantum well (QW) states in magnetic artificial structures have been shown to play an essential role in the oscillatory magnetic coupling and the giant magnetoresistance. The confinement of the conduction electrons in the nonmagnetic layer results in an oscillation in the density-of- states at various energy levels as a function of film thickness. In this talk, we present our photoemission results on quantum well states in ultrathin Cu(100) films. The experiment was performed at Beamline 7 of the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory (LBNL). The fine spot size (~50-100 micron) and high intensity of the photon source enable us to probe the QW states systematically as a function of the film thickness. In particular, double wedged Cu/Co/Ni(8ML)/Co(20ML)/Cu(100) films were investigated systematically as a function of the Cu and Co film thicknesses. The density-of-states at the Fermi level in the Cu layer shows an oscillatory behavior with a periodicity of ~5.6ML, independent of the Co spacer layer thickness. The oscillation amplitude, however, is modified periodically by the presence of the Ni layer across the Co spacer layer. This result demonstrates the interference effect of the QW states in the Cu and Ni layers. Some theoretical predictions based on QW-type models will be discussed. |
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MI-TuP-23 Preparation and Characterization of Spin Polarized Tunneling Junctions with AlN Barrier
J.J. Sun, P.P. Freitas, T.S. Plaskett, R.C. Sousa, S. Cardoso, T.T.P. Galvão (INESC, Portugal) Spin polarized tunnel junctions with AlN barrier were first reported by Moodera et al.1, showing negative resistance in the order of hundreds of mΩ. We have recently reported some preliminary results on AlN tunnel junctions with junction resistance (Rj) in the kΩ range2, which do not show negative resistance. In this paper, a more detailed study was made on tunnel junctions with the structure glass/Ta(Cu)/NiFe(100 Å)/AlN(15-40 Å)/Co(120 Å). The AlN layers were prepared by dc reactive magnetron sputtering in an Ar- N2 mixture at room temperature. The junctions prepared by contact mask had an area of 0.25 mm2 and a Rj ranging from tens of Ω to kΩ. The I-V characteristic and the temperature dependence of the current are in good agreement with the tunneling theories3,4. The barrier height and corresponding effective barrier thickness were derived to be 0.8 to 2.1 eV and 16 to 26 Å, respectively, depending on the nominal thickness of the AlN barrier. The junctions with AlN nominal thickness between 20 and 30 Å showed the magnetoresistance (MR) of 1-2 % at room temperature, reaching 5.2 % at 17 K. When the thickness of the AlN layer was increased to more than 30 Å, the I-V curve of the sample still showed tunneling characteristics, but the MR signal was lost. The MR and Rj of junctions showed a stronger dependence on temperature than those with an Al2O3 barrier. Both MR and Rj increased about 1 to 3 times when the temperature was reduced from room temperature to 17 K. For some junctions, MR was obtained only after annealing. One of these annealed junctions showed 2.9 % MR at room temperature, which increased to 6.9 % at 17 K. In general, the value of MR was less than that expected by spin polarization theory. This may be explained in terms of trapping sites or defects in AlN barrier. The quality of AlN barrier and the surface smoothness of the base electrodes need to be improved.
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MI-TuP-24 Magnetic Nanostructures Produced by Electron Beam Patterning of Transition Metal Fluorides.
D.G. Streblechenko, M.R. Scheinfein (Arizona State University) Research on magnetic nanostructures is stimulated by interest in producing devices such as magnetic memory elements, magnetic tunneling devices and in-plane GMR structures. We have developed transition metal (TM) fluoride electron beam sensitive resists suitable for in-situ fabrication of arbitrarily shaped nanometer scale magnetic structures. 20 nm thick TM fluoride films are prepared by thermal evaporation onto thin carbon films at a rate of about 2 nm/sec in a vacuum of 3x10 -8torr. Nanopatterns are written directly into the TM fluoride film in a VG HB501 scanning transmission electron microscope operating at E=100 keV; I=0.2 nA; and P<5x10-9 torr. A 0.5 nm diameter electron probe is used in the direct-write process. Electron energy loss spectroscopy measurements indicate that as fluorine is freed, the TM coalesces. Electron micrographs of exposed patterns show that the resist resolution is on the order of nanometers. Exposure of broad areas leads to coalescent TM layers which cap the remaining fluoride and decrease the rate of fluorine removal. The cross-section for the removal of a fluorine atom and the cross-section for the same process in the presence of an arbitrarily thick capping layer were measured. Typically, an electron dose of 500 C/cm2 at 100 keV will remove 90% of the fluorine from a 20 nm thick CoF2 film. In-situ electron holography is used to measure the magnetic flux in and around the synthesized magnetic nanostructures. The controlled sizes and shapes facilitate the characterization of magnetic properties such as particle-particle coupling strength, and the temperature dependent magnetization. This work was supported by the US Navy grant ONR # 00014-95-1-0891, microscopy was performed in the Center for High-Resolution Microscopy at Arizona State University supported by NSF grant DMR93-14326. |
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MI-TuP-25 Properties of Closely Spaced Arrays of Magnetic Elements
K.J. Kirk, J.N. Chapman (University of Glasgow, United Kingdom); C.D.W. Wilkinson (Glasgow University, United Kingdom) We have investigated the remanent states and switching behaviour of arrays of closely packed magnetic elements.The patterns were made on electron transparent substrates using electron beam lithography and lift-off techniques and examined by magnetic imaging in a transmission electron microscope (TEM). The remanent states of acicular elements with width 200nm or less, and length 5-10 x width are predominantly single domain with stray field at the ends. The coercivity increases strongly with decreasing width and also depends on the shape of the ends. Pointed ends were found to suppress the formation of flux closure domains usually found at the ends of rectangular elements and led to an approximate doubling of the coercivity. In a closely spaced array, the magnetostatic interaction between neighbouring elements acts to lower the effective coercivity of the elements which switch first. As the elements in an array are made smaller, it is interesting to consider whether they offer attractive properties as discrete storage media. In particular comparison can be made between continuous media with substantially de-coupled grains (e.g. CoCrPt alloys), where statistical considerations limit the ultimate minimium bit size, and patterned bits in exchange-coupled films, (e.g. polycrystalline Co). To obtain sufficient stray field to enable reading, a single bit could be composed of an array of of extremely small acicular elements. Results will be described on how the magnetic properties of arrays of elements vary with dimensions and spacing. |
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MI-TuP-26 Fabrication and Transport Measurements of Ni, NiFe, and Fe Sub-micron Wires
Y.D. Park, J.A. Caballero, H.D. Hudspeth, T.J. Schultz, F. Sharifi, J.R. Childress (University of Florida, Gainesville) We report on the fabrication and transport properties of Ni, NiFe, and Fe wires ranging from micrometer to nanometer scale. The patterning process involves the fabrication of a nanodeposition mask realized by e-beam lithography on bilevel resist (PMMA/PMMA-MAA) with feature sizes down to 40 nm. The use of bilevel resist facilitates the lift-off process which results in structures with sharp sidewall profiles. Two approaches are compared. The first is direct nanowire fabrication by deposition of Ni, NiFe, or Fe through the nanomask onto a Si substrate by e-beam evaporation or sputtering. So far, such structures have displayed a large, temperature-independent resistivity, indicating a disordered microstructure. We believe this is due to non-ideal growth conditions below the nanomask. The second is using a Nb nanowire as a mask over previously deposited Ni, NiFe, or Fe thin films. This approach introduces processing steps which alter the structural and magnetic properties of the underlying films. We will compare the structural, magnetic, and magnetotransport properties as a function of temperature and applied field of nanowires prepared by these two methods. This work supported by the Air Force Office of Scientific Research under F49620-96-1-0026 |
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MI-TuP-27 Self Assembled Co/Ag Nanostructures Grown on Mo(110)
E.D. Tober (IBM Almaden Research Center); G. Witte, R. Farrow (Lawrence Berkeley National Laboratory); R.F. Marks (IBM Almaden Research Center); D. Chambliss (IBM T.J. Watson Research Center) Utilizing scanning tunneling microscopy with in situ deposition and low energy electron diffraction,we have examined the compositional ordering in Ag-Co and Ag-Fe thin films grown on Mo(110). When grown on Mo(110) surfaces at 300-473 K, Ag and Co are observed to self-assemble into parallel, adjacent stripes of Ag and Co running along the Mo [001] direction. Such structures are seen to persist over a wide range in stoichiometry from as low as 21% in the overlayer Co content to over 55%. LEED for these structures displays a sharp 1x1 pattern consistent with the Mo(110) surface with additional diffuse satellite spots along the [1-10] direction equivalent to a ~1.8-2.0 nm periodicity. The periodicity of the alternating stripes is measured by STM to be roughly 1.8 nm for equal amounts of Co and Ag deposited. The Co stripe width, however, is seen to vary with the fraction of Co contained in the overall film. At the lowest Co concentrations, stripes exist only in patches within the Ag islands. The compositional ordering is observed to persist beyond the second monolayer in total Ag + Co coverage. These structures can be produced by co-depositing Ag and Co simultaneously or by depositing Co onto a pre-existing Ag film. |
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MI-TuP-28 Probing Activation Volume Distribution of Magnetic Nanostructures via AC Susceptibility
X. Yan (Hong Kong Univ. of Sci. & Tech. and IBM Almaden Research Center); I. Klik, Y.-D. Yao (Academia Sinica, Taiwan); C.R. Chang (National Taiwan University) AC susceptibility was known to be a good technique to probe particle size distribution of a system consists of isolated single-doman magnetic particles embedded in a non magnetic media. We have extended this technique to general magnetic nanostructures in which interactions are present, so that the activation volume distribution can be obtained directly from the measured dc susceptibility χdc(T) and ac susceptibility χ'(T,f), where T and f are temperature and frequency respectively. When there is a broad activation volume distribution, it was shown that the real component of ac susceptibility follows a -log(f) dependence at temperatures near the average blocking temperature of the system. Both the model calculation of the one-dimentional dipole - dipole interaction having either ferromagnetic or antiferromagnetic coupling 1 and the experiments on percolating granular Ni-SiO2 system 2 presents a similar picture with the slope of the -log(f) dependence determined by the interaction strength.
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MI-TuP-29 Generalized Magnetooptical Ellipsometry (GME)
A. Berger, M.R. Pufall (University of California, San Diego) For a complete magnetooptical characterization of any material, it is necessary to determine at least 4 material constants. These 4 constants are normally written as 2 complex numbers; the index of refraction N = n + i*k, and the magnetooptical coupling constant Q = Qr + i*Qi. Typically, these constants are determined by two separate experiments. First, one obtains the optical constants n and k by an ellipsometric measurement. In a second experiment, the magnetically induced rotation φk and ellipticity εk are measured for a defined magnetization orientation. These measurements, in combination with the electromagnetic theory of reflection then allow a determination of the magnetooptical coupling constant Q. We have developed the method of GME, which allows us to achieve such a complete magnetooptical characterization using only one set of measurements. The experimental setup is very simple, containing only two rotatable polarizers as polarization sensitive elements, one for the incoming, one for the reflected beam. With this setup, we measure the fractional change in intensity δI/I on magnetization reversal as a function of both polarizer angles θ1 and θ2. Subsequently, these experimental data are fitted to an analytic function f (θ1, θ2), which describes our set-up and contains the elements of the sample reflection matrix R as fit-parameter. The method was successfully used to measure the optical and magnetooptical constants for permalloy. Furthermore, we were able to utilize this technique to determine the magnetization orientation, because the 3 magnetization components each have a different effect on the reflection coefficients. Thus, GME does not necessarily require a priori knowledge of the magnetization orientation in contrast to all conventional MO-techniques. This work has been supported by the ONR-N000-1495-10541, NSF-DMR-94-00439, and the CMRR at UCSD. |
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MI-TuP-30 UHV Magnetic Force Microscopy on In Situ Grown Iron Thin Films
U. Memmert, P. Leinenbach, J. Lösch, U. Hartmann (Research Center Jülich, Germany) Fe thin films and Fe multilayers on Ag-substrates recently attracted attention during studies of magnetic interlayer coupling effects. UHV magnetic force microscopy (MFM) was used to image the magnetic stray field distribution of in situ grown Fe/Ag thin films and to correlate the results to the surface and interface roughness. The films of 2 - 30 nm thickness were grown on in situ prepared Ag(100)/Fe/GaAs(100) substrates by electron beam evaporation. Before MFM imaging the samples were characterized by LEED/AES and STM. After MFM imaging the samples were ex situ characterized by MOKE. MFM imaging of the as grown films revealed the presence of elongated domains, 1 to 20 µm wide and 10 to 200 µm long, which were always oriented along the same direction. The observation of a domain wall contrast instead of a domain contrast in the images indicates that the direction of the magnetization lies within the surface plane. Within the domains a magnetic ripple structure on a 100 nm scale could be resolved. After touching the sample with the tip, significant modifications of the magnetic stray field distribution were observed. The resulting images indicate that at these positions the magnetization is locally rotated out of the surface plane. |
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MI-TuP-31 Colossal Magnetoresistivity and Hall Effect of Ca-doped LaMnO3 Thin Films
Q.Y. Chen, X.T. Cui, Z.J. Qu, W.N. Kang, W.K. Chu (University of Houston) La1-xCaxMnO3 represents a new class of doped perovskite oxides exhibiting colossal magnetoresistivity (CMR) depending on the doping concentration x. While the CMR properties have been widely studied, the exact mechanism behind these unique characteristics remain unclear. Many believe these are materials of p-doping, others consider spin-orbit coupling as satisfactory explanation for most observed transport phenomena. Hall effect measurements could help shed some light on this issue, but accurate magneto-transport measurements of these materials have been very difficult because of the large contact resistance inherent to their metallic electrical contacts. In this work we have made some successes in that aspect. The Hall effect and magneto-trasport properties of La1-xCaxMnO3 thin films grown on (100) and (110) SrTiO3 substrates have been investigated. The samples were of high quality judged from x-ray diffraction, Rutherford backscattering spectroscopy, and ion channeling analysis. The Curie temperature was about 250K for the (100)-oriented thin films, and was lower for the (110) films, raging from 160K to 200K. We have found that while the Hall resistivity indicated holelike behavior for a wide range of temperature and magnetic field, below 100K there was a trasition from being holelike to being electronlike. Such transition became progressively evident as the field strength increased. A linear scaling ρxy = Aρxxbeta with ß = 1 was found for temperatue above 100K, suggesting a skew scattering mechanism. The relevance of the scaling to the intrinsic material properties will be discussed. |
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MI-TuP-32 Hysteresis Loop Scaling in the Critical Region
I.F. Lyuksyutov, V.L. Pokrovsky (Texas A&M University); J. Erskine (University of Texas, Austin) We study magnetization reversal in an ultrathin magnetic film. This process is modelled by the same process in the two-dimensional Ising model with Glauber dynamics. We consider the scaling behavior of the area Α of the hysteresis loop and coercive field Hc in the critical region of the two-dimensional Ising transition. We distinguish two regimes: almost adiabatic and far from adiabaticity. In the first regime the hysteresis loop scaling is fully determined by static and dynamic exponents at criticality. In the second regime we found that the nucleation time is much smaller in comparison with the domain growth time (until coalescence occur). As a result the magnetization reversal is determined by the domain wall dynamics. We assume that the domain wall velocity v=constant hnu in the limit h≥0. The exponent ν=1 for the pure system, but may deviate from 1 for disordered systems. In the experimental setup the magnetic field oscillates harmonically H=hcos(ωt). We consider sufficiently strong amplitude of the magnetic field, so that the magnetization induced by magnetic field is much larger than the spontaneous magnetization. Assuming that the domain wall motion is the slowest process, we predict that the hysteresis loop area Α scales in the critical region as Α=constω1/(nu+1)h1/(nu+1). For a simplest case ν=1 we have: Α Hc=constω1/2h1/2. Monte-Carlo simulation confirm the ω1/2 scaling behavior over all accessible range of the ω variation (three decades) and of the h variation (one decade). By small decrease of temperature below Tc we observe crossover from the magnetization reversal dominated by domain wall motion to the process dominated by domain nucleation with totally different scaling behavior. With decreasing temperature even the weak quenched disorder changes drastically the domain wall motion. Existing relevant experimental results will be discussed. |
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MI-TuP-33 Structure Dependent Anisotropies in Epitaxial Fe3O4/MgO Superlattice
G. Chern (National Chung-Cheng University, Taiwan); C.L. Chang (Tamkang University, Taiwan); C.K. Lo (Academia Sinica, Taiwan) We have synthesized a series of epitaxial Fe3O4/MgO superlattices by oxygen plasma assisted molecular beam epitaxy. The MgO thickness in these superlattices are held to 20 Å while the Fe3O4 thickness is varied from 10 to 80 Å with 30 to 50 bilayer repeats. Modulation coherency and sharp interface of the superlattices are characterized by in-situ reflection high energy electron diffraction (RHEED) and ex-situ X-ray diffraction (XRD). The electrical and magnetic properties have been studied by current-voltage (I-V) relation and magneto-optical Kerr effect (MOKE) respectively. Transverse I-V measurements, in which the current flows in the direction perpendicular to the planes, show strong non-ohmic effects. Although the MOKE measurements on these oxide superlattices have only 10% reflected intensities relative to those of metal samples, the in-plane anisotropy is clearly shown in the hysteresis loops. Fe3O4 length scale dependence is observed in both the electrical and magnetic measurements. The corelations between reduced dimension of Fe3O4 and the electrical and magnetic anisotropies, and the possible mechanism causing the observed results are discussed. |
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MI-TuP-34 Crystalline Coherence and Electronic Structure in Antiferromagnetically Coupled Fe/Si Multilayers
J.A. Carlisle (Virginia Commonwealth University); A.E. Chaiken, R.P. Michel, L.J. Terminello (Lawrence Livermore National Laboratory); T.A. Callcott (University of Tennessee); D.L. Ederer (Tulane University) Soft x-ray fluorescence spectroscopy has been used to examine the electronic structure of deeply buried silicide thin films which arise in Fe/Si multilayers. These systems exhibit antiferromagnetic (AF) coupling of the Fe layers, despite their lack of a noble metal spacer layer found in most GMR materials. Also, the degree of coupling is very dependent on preparation conditions, especially spacer layer thickness and growth temperature. The valence band spectra are quite different for films with different spacerlayer thicknesses, yet are very similar for films grown at different growth temperatures. This latter result is surprizing since AF-coupling is strongly dependent on growth temperature. Combining near-edge x-ray absorption (NEXAFS) with the fluorescence data demonstrate that the silicide spacer layer in epitaxial films which exhibit AF-coupling are metallic. These results clearly indicate the equal role of crystalline coherence and electronic structure in determining the magnetic properties of these systems. |