Session Z31 - Spin Transport, Magnetoresistance and Hall Effect.
FOCUS session, Friday morning, March 16
Room 619, Washington State Convention Center

[Z31.001] Interlayer exchange coupling and electric transport in Fe/Cr/Fe multilayers

Based on the fully relativistic spin-polarized Screened Korringa-Kohn-Rostoker method and the Kubo-Greenwood equation as formulated for layered systems the interlayer exchange coupling (IEC) as well as the magnetoresistance for the current in-plane (CIP) geometry is calculated for Fe/Cr/Fe trilayer systems. For the IEC not only short periods of 2ML but also long periods of about 18ML are found. For large Cr thicknesses the magnetic moments oscillate with a period of 8 - 9 ML for even numbers of Cr layers and 18 for odd. The calculated CIP magnetoresistance (MR) and corresponding resistivities are in the range of available experimental data. It is found that the peaks in the CIP-MR have little in common with the oscillation periods characterizing the IEC.

[Z31.002] GMR in Co/Cu and Fe/Cr multilayers - a comparison

Full ab initio calculations for the giant magnetoresistance (GMR) in Co/Cu and Fe/Cr multilayers are presented. The electronic structure of the multilayers is calculated by spin density functional theory using a Screened Korringa-Kohn-Rostoker method. Scattering of impurity atoms in the multilayers is described by means of a Green's-function method. The scattering potentials are calculated self-consistently. The transport properties are treated quasi-classically solving the Boltzmann equation including the electronic structure of the layered system and the anisotropic scattering. The solution of the Boltzmann equation is performed iteratively taking into account both scattering out and scattering in terms (vertex corrections). Furthermore the influence of interface scattering is incorporated by averaging the scattering cross sections of different impurities at various sites in the supercell.

It will be shown that there is an excellent agreement of experimental and numerical results concerning the general trend of GMR in Co/Cu systems depending on the type and the position of impurities. The increase of GMR when impurities are placed within the Co layer and not at interface is directly connected to the quantum confinement in magnetic multilayers. In contrast, the GMR in Fe/Cr multilayers is attributed to spin-dependent interface scattering, especially to the strong spin dependence of electron scattering at chromium impurities in the Fe interface layer. The spin asymmetry of the Fe/Cr interface is crucial and determines the magnitude of GMR.

[Z31.003] Optimal Atomic Configuration for Giant Magnetoresistance Devices

What atomic configuration has the highest giant magnetoresistance(GMR)? This is difficult to answer because the GMR depends on the details of the band-structure, the bulk disorder, and the interface disorder. To search for a near-optimal solution, we have developed a computer code using the simulated annealing algorithm(A. Franceschetti and A. Zunger, Nature, 402, p.60 (1999).) and a 9-band tight-binding GMR method. In each step of the annealing algorithm, a new configuration is generated by a Monte Carlo move that randomly varies either the thickness of a layer or the chemical composition of a monolayer. The GMR of the configuration is then computed, and depending on the GMR, the configuration is either kept or discarded. To handle the amount of computation, we have increased the speed of our GMR code without losing the accuracy. As a result, we can search for the optimized GMR configuration with a unit cell of 50 atoms. We will present results for GMR structures made with Co, Ni, and Cu.

[Z31.004] Perpendicular giant magnetoresistance in spin-valves

We investigate the angular dependence of the conductance and giant magnetoresistance in the current perpendicular-to-the-plane geometry in magnetic spin-valves via a third-nearest-neighbor tight-binding model with s,p and d orbitals. An iterative method is used to calculate the Green's function of the spin-valve. The conductance, due to both minority and majority spin channels, is calculated in for arbitrary angles between the magnetizations of the magnetic layers using the Landauer-Büttiker formula. We find that the leading contribution to the conductance is proportional to cos\left( \phi \right), where \phi is the angle between the magnetizations of the magnetic layers.

[Z31.005] Extraordinary Magnetoresistance At Room Temperature In Non-Magnetic Narrow-Gap Semiconductor/Metal Composites

The magnetoresistance (MR) of a material object contains a physical contribution from the magnetic field dependence of the material parameters such as the mobility or carrier concentration and a geometric contribution from the dependence of the current path and output voltage on the sample shape and electrode configuration. To date, only two classes of magnetic materials, artificially layered metals which exhibit giant MR (GMR) and the manganite perovskites which exhibit colossal MR (CMR) have been considered serious candidates in the effort to improve the room temperature (RT) performance of MR sensors. For both of these classes, the physical MR dominates. In contrast, we have found that non-magnetic narrow-gap semiconductors containing patterned metallic inhomogeneities (shunts), exhibit RT geometric extraordinary MR (EMR) orders of magnitude larger than the physical MR of other materials. EMR in excess of 2000% at 0.05 Tesla and 3,000,000% at 5 T, respectively, has been observed in macroscopic (\sim 1 mm) composite structures of InSb with patterned internal or external shunts.(S.A. Solin et al., Science \bf289), 1530 (2000).^,(T. Zhou, D.R. Hines and S.A. Solin, Appl. Phys. Lett., submitted.) We have been able to quantitatively account for the magnitude of the observed EMR as well as its dependence on the geometry (shape, size and placement of the shunt) using both analytic (Laplace equation with boundary conditions) and computational (Finite Element Analysis)(see the talk by L.R. Ram-Mohan et al., this conference) methods. The effect of scaling EMR structures to mesoscopic dimensions and the possible technological impact of EMR will be discussed.

[Z31.006] Temperature Dependent Uniaxial Tensil Stress Studies Of Epitaxial InSb Films

The carrier mobility is one of the most important parameters in the design of semiconductor magnetoresistance (MR). At low electric and magnetic fields, this mobility is inversely proportional to the effective mass m* of the carriers, while according to band structure \veck\cdot\vecp perturbation theory m* is approximately proportional to the fundamental energy gap E_g. Among III-V compounds, InSb has the smallest E_g, therefore the smallest m* and the highest carrier mobility at room temperature. Since InSb also has the largest lattice constant of the III-V compounds, one would expect an even smaller E_g and higher mobility when InSb is exposed to a tensile stress. We thus designed a tensile stress apparatus that can apply uniaxial tensile stress to InSb films and can also be put into a magnetic cryostat. Using this apparatus, we measured the mobility and carrier concentration of an n-doped InSb film as a function of uniaxial tensile stress along the [100] direction. At room temperature the mobility of the InSb film is found to increase almost linearly with increasing stress, from 40,000 cm^2/Vs at zero stress to 73,000 cm^2/Vs at about 4.5 kbar, while the carrier concentration decreases from 2.1\times10^16cm^-3 to 1.03\times10^16cm^-3. The deformation potential deduced from these measurements is compared with that obtained under uniaxial compression.

[Z31.007] Simulation of Enhanced Geometric Magnetoresistance at Room Temperature by Finite Element Analysis.

The remarkable enhancement in the room temperature magnetoresistance (MR) of van der Pauw disks of narrow-gap semiconductors with concentric metallic inclusions(S.A. Solin et al., Science 289), 1530 (2000). has been modeled using the finite element method (FEM).(O.C. Zienkiewicz and R. L. Taylor, The Finite Element Method), 4th edition (McGraw-Hill, NY, 1988). An action integral is set up for the problem, with derivative boundary conditions at the current leads and floating values at other locations on the disk periphery. The physical region is discretized into triangles of varying size so as to conform to the geometry and provide means of representing the rapidly varying currents at the current leads. The action is evaluated over each triangle using interpolating functions. A variational principle adapted to nodal values is used to determine the potential and the current everywhere using sparse matrix analysis. The evaluated MR is in excellent agreement with experimental results, with no adjustable parameters. The FEM is ideally suited for the computation of this geometry-dependent effect.

[Z31.008] Hot Electron Degradation Of The Extraordinary Magneto-Resistance And Hall Measurements In Inhomogeneous InSb

Pulsed high current resistivity and Hall effect measurements on epitaxial InSb van der Pauw discs of radius r_a with a concentric metallic inclusion of radius r_b are reported. The large magnetoresistance of these samples(S.A. Solin et al., Science \bf289), 1530 (2000). is a result of an increase in the fraction of the total current excluded from the metallic inhomogeneity with increasing applied magnetic field, It is shown that measurement of the Hall voltage yields quantitative information on the extent of the current exclusion from the inclusion, and for r_b/r_a = 12/16, the excluded current varies from 0.4% (at 0 T) to 1% (at 100 mT) of the total current. Pulsed high current resistivity measurements reveal a degradation in the MR of approximately 10% at drive currents of 200mA. An extension of the theory of Wolfe et al. \footnote C.M. Wolfe et al., J. Electrochem. Soc. \bf119, 250 (1972). to include resistance measurements with H\neq0, shows that the reduction in MR results from a decrease in mobility and an increase in carrier concentration observed at high electric fields in InSb Hall bars.

[Z31.009] Anisotropic Magnetoresistance in (III,Mn)V Ferromagnets

We report on a theoretical study of anisotropic magnetoresistance in bulk (III,Mn)V Ferromagnets. Our work is based on a kinetic-exchange model for carrier induced ferromagnetism in these materials, on Boltzmann transport theory, and on the assumption that the main disorder-source is randomness in the spatial arrangements of Mn spins. We find that the resistivity of these materials is very sensitive to external magnetic fields because of the strong coupling between Mn^++ S=5/2 local moments and the valence band quasiparticles that carry the current. Results are presented for the variation of resistivity with external field around hysterisis loops for serveral different relative orientations of the external field and the magnetic easy-axis.

[Z31.010] Hall effect and magnetoresistance of ternary carbide Ti3SiC2.

In this study we report on the transport properties of the new class of Ti-based readily machinable ternary carbides. Ti3SiC2 was recently fabricated and shown to possess a unique combination of physical properties. The Hall effect and longitudinal magnetoresistance have been measured as a function of temperature in the 4K to 300K range and magnetic field up to 5T. The magnetoresistance is dominated by a positive quadratic field dependence at low temperatures. The Hall voltage was a linear function of magnetic field in the wide range of temperatures. The effective carrier density and mobilities were calculated based on sign and value of Hall coefficient. These results will be discussed in terms of a 2-band model.

[Z31.011] Transport Measurements of the Itinerant Ferromagnet CaB[sub 6]

We present a series of magneto-transport and Hall measurements on the novel itinerant ferromagnet CaB[sub 6]. We have studied the transport properties of this compound as a function of doping. Surprisingly, we find that stoichiometric CaB[sub 6] is a semi-metal. As the Ca concentration in the material is reduced, we observe a reduction in the carrier density and semiconducting transport behavior. We will discuss implications of our data with regards to the various excitonic models proposed to describe the observed ferromagnetism in this material.

Part Z of program listing