AVS2000 Session MI-ThA: Magnetic Devices: GMR & Tunneling

Thursday, October 5, 2000 2:00 PM in Room 206

Thursday Afternoon

Start	Invited?	Item
2:00 PM		MI-ThA-1 The Determination of Magnetostriction for Spin-Valve Devices with 5.0 nm and 10.0 nm Permalloy Layers T.J. Gafron (Boise State University); S.E. Russek (National Institute of Standards and Technology); S.L. Burkett (Boise State University) The objective of this study is to determine the extent of magnetostriction in spin-valves. Spin-Valves were constructed on a silicon substrate using dc magnetron sputter deposition techniques with the following structure: Ta_5.0 /NiFe_{5.0 or 10.0} /Co_1.0 /Cu_3.0 /C0_3.0 /Ru_0.6 /Co_2.0 /FeMn_10.0 /Ta_5.0, where the subscripts denote the layer thickness in nanometers. The films were deposited with a magnetic field applied parallel to the substrate to align the pinned and free layers. Spin-valves were designed in a serpentine shape to maximize magnetostriction effects by increasing the device length. Device widths between 1 and 40 microns and lengths between 100 and 2000 squares were fabricated. Spin-valves tested exhibited a 5-7% change in magnetoresistance and an average ferromagnetic exchange coupling of 0.4 kA/m at 300°K. Devices were subjected to an external magnetic field while a mechanical stress was applied to the backside of the substrate. A four-point probe technique was used to measure device resistance as a function of applied field and mechanical stress. An increase in the anisotropy field, H_k, is observed with increasing mechanical stress. This increase is observed for all devices tested but more distinct for those containing the 5.0 nm Permalloy. Using the curvature of the stressed sample and the thickness of the spin-valve and substrate, magnetostriction is calculated as a function of the applied stress. Results show that maximum magnetostriction occurs abruptly at lower stress values for the 10.0 nm Permalloy while magnetostriction for the 5.0 nm permalloy occurs gradually over a wider range of stress values. Magnetostriction is small (1.50 microns for the 20 micron by 1K square, 5.0 nm NiFe), but the effect is pronounced and impacts device performance as demonstrated by a shift in H_k. Magnetostriction analysis becomes critical as both device complexity and integration levels increase.
2:20 PM		MI-ThA-2 Properties of GMR Multilayers Grown by RF Diode Sputtering W. Zou, X.W. Zhou, R.A. Johnson, H.N.G. Wadley (University of Virginia); D.J. Brownell, D. Wang (Nonvolatile Electronics, Inc.) RF diode sputtering techniques are used for the growth of giant magnetoresistive (GMR) multilayers. The performance of devices synthesized in this manner is a sensitive function of the nanoscale structure and interfacial morphology created during the condensation step of the deposition process. Systematic series of experiments have been conducted to evaluate the dependence of film morphology upon composition of conducting layer. Atomic force microscopy (AFM) shows when CuAgAu is used (instead of pure Cu) the RMS roughness is reduced but occasional hillocks are also formed. By using a newly developed embedded atom method (EAM) alloy potential, a Molecular Dynamics study has been used to investigate the layer by layer growth phenomena and to identify the origin of the relationships between the experimental observations and layer composition. The use of the copper silver gold alloy is found to promote smoother interfaces because silver acts as a surfactant. Surface Auger results reveal the hillocks to be rich in silver a consequence of surface segregation into islands. Novel deposition strategies for morphology control have been proposed.
2:40 PM	Invited	MI-ThA-3 GMR Sensing Elements for the Detection of Magnetic Microbeads in Biosensor Array M. Miller (Naval Research Laboratory) Giant magnetoresistance (GMR) magnetoelectronics has been an area of growing technological and commercial interest. GMR hard disk read heads have enabled a order-of-magnitude increase in storage density. Nonvolatile random access memory (NVRAM) utilizing GMR technology is an area of intense research and prototype devices are being introduced. Still other devices include magnetic field sensors as well as rotation and displacement sensors. A unique application for GMR devices is in the detection of magnetic microbeads commonly used in biological research for cell and biomolecule separation. We are developing a biosensor, the Bead ARray Counter (BARC),¹ that uses these microbeads as labels to detect DNA hybridization at specific areas over an array of GMR sensors micro-fabricated on a chip. Eventually, the BARC chip is envisioned to be akin to GMR NVRAM, with millions of sub-micron GMR elements enabling simultaneous detection of thousands of DNA sequences with high sensitivity and dynamic range. We will discuss the special magnetotransport, micromagnetics, and microfabrication issues required for the development of high-density GMR-based biosensor arrays. This work was done in collaboration with P.E. Sheehan, R. L. Edelstein, C. R. Tamanaha, L. Zhong, S. Bonnak, R. J. Colton, L. J. Whitman, and G.A. Prinz. footnote1Edelstein et al., Biosensors & Bioelectronics 14, 805 (2000).
3:20 PM		MI-ThA-5 Preparation of Magnetic Tunnel Junctions by Ionized Atom Beams S.O. Demokritov, B.F.P. Roos, B. Hillebrands (University Kaiserslautern, Germany) A new oxidation technique for the preparation of thin insulating barriers for magnetic tunnel junctions combined with in-situ resistivity and optical reflectivity measurements is studied. A highly dissociated low energy (30 eV-80 eV) ionized oxygen atom beam from a novel type of an electron cyclotron wave resonance controlled plasma reactor is used to oxidize metallic Al layers and to form an insulating barrier for tunnel junctions. The oxidation process is found to be self limiting. The oxidation depth variate from 1.5 to 2 nm in agreement with performed Monte Carlo simulations.
3:40 PM	Invited	MI-ThA-6 Nonlinear Magneto-Optical Investigations of Magnetic Interfaces Th. Rasing (University of Nijmegen, The Netherlands) Magnetization induced second harmonic generation (MSHG) is a new nonlinear magneto-optical technique that combines interface sensitivity with huge magneto-optical effects. These effects are due to the simultaneous breaking of inversion symmetry (at interfaces) and time-reversal symmetry (by the magnetization). Because most magnetically ordered materials are centrosymmetric in their bulk form, MSHG is a particularly interesting probe to study the magnetization structure of the interfaces in magnetic multilayer systems. Using MSHG, we have found e.g. that the spin orientation at the interface of CoNi/Pt multilayers can be different from the bulk due to specific preparation conditions. Due to both very high magneto-optical contrast and interface sensitivity, fine details of magnetization reversal become visible with MSHG imaging that are not detectable by usual magneto-optics. MSHG also appears to be highly sensitive for the step induced anisotropy in magnetic thin films grown on vicinal surfaces. In addition, effects of interface annealing and oxidation can be observed in situ, which is of great importance for sensor multilayer structures. By using phase sensitive spectroscopic MSHG experiments, the spin-dependent interface density of states can be probed, as was recently demonstrated in a study on a Ni(110) surface. This is of great importance for the understanding of e.g. the spin dependent tunnel current in magnetic tunnel devices. Finally, the use of fs laser excitation allows the probing of ultra fast magnetization dynamics, using pump-probe techniques. Recent results of this will be discussed. Part of this work was supported by FOM, the TMR Network NOMOKE and INTAS 97-0705 (ERBFMRXCT960015) and INTAS 97-0705.
4:20 PM		MI-ThA-8 "Acoustical" and "Optical" Spin Modes of a Fe/Cr/Fe/Cr/Fe Multilayer with Ferro- and Anti-ferromagnetic Couplings F. Nizzoli, L. Giovannini, P. Vavassori, R. Zivieri (University of Ferrara and INFM, Italy) A study of the in-phase ("acoustical") and of the out-of phase ("optical") spin modes in layered ferromagnetic structures with both ferromagnetic and antiferromagnetic coupling is presented, in particular for the system Fe(20Å)/Cr(20Å)/Fe(20Å)/Cr(9Å)/Fe(100Å), for which experimental Brillouin scattering data are available¹ In our model we take magnetocrystalline, uniaxial and shape anisotropies, Zeeman interaction, bilinear and biquadratic exchange interlayer couplings into account. The magnetization equilibrium configuration is calculated as a function of the external field using the steepest descent technique. A calculation of the dynamic magnetization assumed constant in each film gives an explanation of the behavior of the two kinds of modes at different magnetic fields applied along the easy axis. The "acoustical" mode is the lowest frequency one at low applied fields, but frequency exchanges with the "optical" modes occur at higher fields. It is found that the competition between the Zeeman and the biquadratic exchange terms is responsible for the appearance of a soft mode at a critical field marking a second-order phase transition.
4:40 PM		MI-ThA-9 Analysis of Tunneling Magnetoresistance Structures by Low Energy Electron Nanoscale Luminescence Spectroscopy S.H. Goss (The Ohio State University); S.S.P. Parkin (IBM Almaden Research Center); L.J. Brillson (The Ohio State University) The performance of state-of-the-art tunneling magnetoresistive (TMR) heads depends sensitively on the thickness of insulating layers less than a few nanometers thick which separate two magnetic films. We have used low energy electron nanoscale luminescence (LEEN) spectroscopy to observe optical emission from TMR test structures with buried insulating oxides less than a few nm thick. TMR structures grown by DC magnetron sputtering consisted of a 0.8 - 3 nm Al oxide layer on a 2.4 nm CoFe alloy (84:16) sandwiched between a multilayer metal-on-Si substrate and a 4.4 nm CoFe plus Pt overlayer. LEEN excitation energies ranging from 0.5-3 keV enabled us to distinguish between emissions from the buried oxide layers vs. the free surface. We used different compositions, thicknesses, and oxidation exposures to separate Al oxide from transition metal oxide emissions, as well as from the ambient-exposed Pt surface. A broad peak centered at 2.7 eV increased with increasing oxygen plasma exposure at constant Al thickness. It also increased with increasing Al thickness and commensurate oxygen exposure. Emission from oxidized CoFe without Al consists of a featureless emission extending from 1.5 - 3.7eV. Common to all these spectra is emission at 1.8 eV, which energy-dependent LEEN demonstrates is due to the ambient-exposed Pt. Finally, spectral changes of the buried, oxidized Al/CoFe sandwich layers as a function of thickness and oxygen exposure reveal the regime separating oxidation of the Al layer alone from over-oxidation that extends into the CoFe base layer. These results suggest that optical emission from nanometer - thick tunnel layers within TMR structures can be used to assess the extent of oxidation as well as to optimize deposition and process conditions.
5:00 PM		MI-ThA-10 Magneto-optical and Optical Spectroscopies of Fe/Si Multilayered Films Y.P. Lee, T.-U. Nahm, C.O. Kim (Hanyang University, Korea); Y.V. Kudryavtsev (Institute of Metal Physics, Ukraine); K.W. Kim (Sunmoon University, Korea); J.Y. Rhee (Hoseo University, Korea); J. Dubowik (Institute of Molecular Physics, Poland) Fe/Si multilayered films (MLF) exhibiting a strong antiferromagnetic (AF) coupling were studied by optical and magneto-optical spectroscopies. The first set of Fe/Si MLF with a fixed Fe sublayer thickness of 3.0 nm and a variable thickness of Si sublayers (1.0 - 2.2 nm) was prepared by rf-sputtering onto glass substrates at room temperature with the number of repetition of 50. To replicate the spacer silicide layers in the MLF, the second set of Fe/Si MLF with very thin Fe and Si sublayers (0.3 - 0.8 nm and 0.4 - 0.8 nm, respectively) was also deposited. The results were compared with the computer-simulated spectra based on various structural models of the MLF. Neither semiconducting FeSi₂ nor ε-FeSi turned out to be considered as the spacer layer for a strong AF coupling. The optical properties of the spacer extracted from the effective optical response of the MLF strongly support its metallic nature. A reasonable agreement between experimental and simulated equatorial-Kerr-effect spectra was obtained with the fitted optical parameters of the spacer for the FeSi or Fe₅Si₃ stoichiometry. A comparison of the extracted optical properties for the spacer with the calculated ones based on the first principles showed that a B2-phase metallic FeSi compound is spontaneously formed at the interfaces of MLF during deposition. For the Fe/Si system with ultrathin Fe and Si sublayers, our optical data reveal that the overall structure of MLF is close to an amorphous and semiconducting ε-FeSi.