Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session Y42: Magnetotransport |
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Sponsoring Units: GMAG Chair: Adnan Rebei, Seagate Technology Room: LACC 150B |
Friday, March 25, 2005 11:15AM - 11:27AM |
Y42.00001: Magnetoresistance Measurements in Nanoconstricted Nickel Wires Zachary Keane, Lam Yu, Douglas Natelson Nanoscale constrictions between two ferromagnetic electrodes have been the subject of much recent interest because of reports of ballistic magnetoresistance (BMR). Substantial controversy exists regarding the size and mechanism of this effect. We report preliminary measurements of the electron transport across Ni wires with nanoscale constrictions and tunnel junctions. Test structures are fabricated using a combination of e-beam lithography and electromigration. Sample geometries are chosen to allow independent control of electrode magnetizations. Measurements are performed from room temperature to cryogenic temperatures, at magnetic fields ranging up to 9 T. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y42.00002: Ballistic anisotropic magnetoresistance in nanowires Julian Velev, Renat Sabirianov, Sitaram Jaswal, Evgeny Tsymbal We have performed an \textit{ab-initio} study of the ballistic conductance of very thin ferromagnetic nanowires for magnetization parallel and perpendicular to the axis of the wire. We find that there can be a significant difference in the resistance for the two orientations of the magnetization giving rise to an appreciable ballistic anisotropic magnetoresistance (BAMR).This effect is similar to the AMR observed in the bulk systems. BAMR is due to the change in the number of bands crossing the Fermi energy produced by the spin-orbit interactions. The spin-orbit interactions lift the degeneracy of $d$-type bands for magnetization parallel but not perpendicular to the wire axis. This can cause a change in conductance if the degenerate levels are close to the Fermi energy. We find that BAMR can be either positive or negative. Similar ballistic magnetoresistance effect can be achieved by mechanical deformation caused, for example, by strain or magnetostriction. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y42.00003: Electronic structure and Magnetoresistance of Ni nanocontacts J.J. Palacios, D. Jacob, J. Fernandez-Rossier We address the fundamental question of whether magneto-resistance (MR) of atomic-sized contacts of Nickel is very large because of the formation of a domain wall (DW) at the neck. Using both {\em ab initio} and model Hamiltonian transport calculations we find [1] that, as in the case of non-magnetic electrodes, transport in Ni nanocontacts depends very much on the orbital nature of the electrons. Our results are in agreement with several experiments in both average value of the conductance [2,3] and the fact that MR can be either positive or negative [3]. Contrary to existing claims [4], DW scattering does {\em not} account for large MR in Ni nanocontacts. \newline \newline [1] J.J. Palacios, D. Jacob, J. Fernandez-Rossier, condmat/0406249\newline [2] C. Untiedt {\em et al.}, Phys. Rev. B {\bf 69}, 081401 (R) (2004)\newline [3] M. Viret {\em et al.}, Phys. Rev. B {\bf 66}, 220401 (2002)\newline [4] N. Garc\'ia {\em et al.},Phys. Rev. Lett. {\bf 82}, 2923 1999. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y42.00004: A magnetic-field-effect transistor and spin transport. R.N. Gurzhi, A.N. Kalinenko, A.I. Kopeliovich, A.V. Yanovsky, E.N. Bogachek, Uzi Landman A magnetic-field-effect transistor is proposed that generates a spin-polarized current and exhibits a giant negative magnetoresistance. The device consists of a nonmagnetic conducting channel (wire or strip) wrapped, or sandwiched, by a grounded magnetic shell. The process underlying the operation of the device is the withdrawal of one of the spin components from the channel, and its dissipation through the grounded boundaries of the magnetic shell, resulting in a spin-polarized current in the nonmagnetic channel. The device may generate an almost fully spin-polarized current, and a giant negative magnetoresistance effect is predicted. \footnote{R.N. Gurzhi et al., Appl. Phys. Lett. {\bf 83}, 4577 (2003).} [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y42.00005: Spin Filtering Of Hot Holes in a Metallic Ferromagnet Tamalika Banerjee, Ehtsham Haq, J.C. Lodder, Ronald Jansen Spin-transport of non-equilibrium holes in ferromagnetic metals with energy below the Fermi level has been investigated using Ballistic Hole Magnetic Microscopy. Using a semiconductor/ferromagnet hybrid structure, we show that a thin ferromagnetic film acts as an efficient spin-filter for holes. Unpolarized hot holes injected from an STM tip after transmission through a ferromagnetic stack (NiFe/Au/Co) are collected in the valence band of p-type Si. The hole attenuation length for Co is found to be short and increases from 0.6 to 1.0 nm in the energy range 0.8 to 2 eV below the Fermi level. For a NiFe/Au/Co trilayer, the hole transmission is clearly spin-dependent with a surprisingly large magnetocurrent of 130{\%}. The large spin dependence of the hole transmission as well as the increase of the attenuation length with energy cannot be explained by the phase space available for inelastic decay of hot holes. We discuss other factors to explain such a large spin asymmetry. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y42.00006: Hot-Electron Spin-Transport Through Tantalum Films and Interfaces Huseyin Gokcan, Ronald Jansen, Cock Lodder The spin-valve transistor (SVT) is a hybrid semiconductor/ferromagnet device based on the spin-dependent transmission of hot electrons across a metallic spin-valve sandwiched between a semiconductor emitter and collector. The transistors have the configuration Si / NM / FM / NM / FM / NM / Si and contain ferromagnetic (FM) materials such as Co, NiFe, and various non-magnetic (NM) materials in the spin-valve base. SVT's with 7 nm Au as the NM spacer exhibit a magnetocurrent (MC) of 350{\%}. Interestingly, we find a sharp drop of the MC to only 9{\%} when Au is replaced by 6 nm Ta as the spacer. We also find that Ta has a short hot-electron attenuation length of about 1.5 nm, attributed to the existence of partially empty d-bands. The strong reduction in MC for SVT's with Ta spacer is surprising, since it is known that the large MC in the SVT results from the spin-dependence of the bulk attenuation lengths for majority and minority spins in ferromagnets. We consider spin-dependent scattering at the Ta/FM interfaces and a short spin relaxation time in bulk Ta as possible reasons. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y42.00007: Domain Wall Resistance in Ferromagnetic Wires Renat Sabirianov, John Burton, Sitaram Jaswal, Evgeny Tsymbal We present first-principles studies of the domain wall (DW) resistance in atomic-size cobalt and nickel ferromagnetic wires. Several types of domain walls are considered: Bloch, Neel and linear DW. The DWs are modeled using a constrained geometry with a fixed width of the non-collinear region allowing the electronic degrees of freedom to relax. Electronic structure calculations are performed using a tight-binding LMTO method in real space for monatomic wires and wires of the 4-fold-symmetry. The electronic transport properties are calculated using the Landauer-Buttiker approach. We find that DW resistance decreases very rapidly, on the scale of a few interatomic layers, with increasing DW width for both Bloch and Neel walls. The largest magnetoresistance value of about 250{\%} is predicted for an abrupt DW in a monatomic Co wire. The density of states and the conductance of the Co wires display energy gaps in one spin channel making the magnetoresistance of an abrupt DW for these energies infinitely large. For the abrupt domain wall the large magnetoresistance is observed in some cases even if the ferromagnetic wire has conductance in both spin channels. This work is supported by National Science Foundation and Nebraska Research Initiative [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y42.00008: Effect of Inserted Ni layers on Magnetoresistance of Co/Cu Multilayers Kenji Tanahashi, Takahiro Inomata The effect of the inserted Ni layers on the magnetoresistance(MR) of Co/Cu multilayers has been investigated. It was found that the insertion of Ni layer enhanced the MR ratio of the multilayers. The effect was investigated on sputter deposited Co(1nm)/Cu(2.3nm) multilayers which give one of the maximum MR ratios. The degree of the effect depends on the thickness of Ni layer, and the prominent effect was observed only when the thickness of Ni layer is 0.1-0.2 nm. The symmetric and asymmetric two types of insertion of Ni layer were carried out to see the effect of the interface on the magnetoresistance, since an interface plays a significant role in scattering of conduction electrons. MR ratio is strongly affected by the thickness of Ni layer itself and the total thickness of Co and Ni layers between two Cu layers. The two types of insertion of Ni layer yield little difference in MR ratio, showing that MR effect depends more on the electrical structure which the total thickness of ferromagnetic Co and Ni layers forms than on the morphological change or the interdiffusion at the interfaces due to the insertion of Ni layers. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y42.00009: Anomalous magnetic after-effect near orientation transition in epitaxial [Fe/Cr(100)]10 multilayers Raul Villar, Ruben Guerrero, Rainer Schad, Jose Luis Martinez, Farkhad Aliev Transport and magnetic properties near a field induced orientation transition (OT) from easy to hard axis in magnetization of antiferromagnetically coupled epitaxial [Fe/Cr(100)]$_{10}$ multilayers have been studied down to 1.7K. It was found that in the vicinity of the OT both the amplitude of the magnetic susceptibility anomaly and the relaxation rate of the magnetoresistance change dramatically below a few K. These observations, together with the previously observed strong reduction of zero field magnetic losses at temperatures below 5-7K [1] indicate a qualitative transformation of the magnetic dynamics of antiferromagnetically coupled Fe/Cr multilayers at very low temperatures. [1] F.G. Aliev, et al., Phys. Rev. Lett., v.88, p.187201 (2002). [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y42.00010: Observation of spin-bottleneck due to spin-charge separation in a superconductor Brigitte Leridon, Jerome Lesueur, Marco Aprili Quasiparticles were injected from a ferromagnet ($Ni_{0.8}Fe_{0.2}$) through a tunnel junction into a conventional superconductor (Nb), while the density-of-states of the superconductor was measured through a second tunnel junction with a paramagnet. No significant decrease of the superconductive gap was observed while a noticeable heating of the quasiparticles of the superconductor was measured. A similar experiment performed with current injected from a paramagnet showed no such effect. This can be interpreted in terms of an enhanced recombination time for the spin-polarized quasiparticles. Estimations give for the intrinsic recombination time a value of about $10^{-9}$ s at 2 K, while some measurements of the spin relaxation time in superconducting Nb give about $10^{-8}$ s at the corresponding temperature (Vier, Phys. Lett. A 98, 283, 1983). The existence of intrinsic spin-charge separation in a superconductor (see Kivelson Phys. Rev. B 41, 11693, 1990) is indeed responsible for a decrease of the spin-orbit coupling matrix coefficients leading to infinite spin-relaxation time at zero temperature (as calculated by Yafet, Phys. Lett. A 98, 287, 1983). In our experiment, it ensures a large spin-relaxation time, thus blocking the recombination process and acting as a "spin-bottleneck". Due to the geometry of the experiment, the pure spin excitations are spatially separated from the charge and are able to thermalize above the gap edge. [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y42.00011: Determination of spin diffusion length of gold utilizing lateral spin valves Yi Ji, Axel Hoffmann, Sam Jiang, Sam Bader Spin transport in lateral metallic structures is an intriguing, emerging area, which offers rich opportunities to explore complex spintronic devices. We demonstrate spin injection, diffusion, and detection in a series of lateral spin valves. A 220-nm wide and 20-nm thick gold wire is defined by e-beam lithography on a SiN substrate. Two permalloy (Py) electrodes, an injector and a detector, are subsequently overlaid on the Au wire. The charge current and spin current are separated by connecting current drain and voltage ground to opposite ends of the Au wire. A charge current that passes through the Py injector into the Au induces a spin accumulation, resulting in a split of the chemical potentials for the spin-up and spin-down electrons in the Au. The injected spins diffuse away on both sides of the injector, resulting in a spin current towards the detector even without a charge current. Depending on the relative orientations of the accumulated spin and the detector spin, a voltage contrast can be observed at the detector/Au interface. This separation of charge and spin currents results in a large percentage value of the spin signals, which rules out the possibilities of spurious effects such as AMR. By changing the injector-detector separation, we observe an exponential decay of the spin signals, and thus determine a spin diffusion length of 63 nm in Au at 10 K. [Preview Abstract] |
Friday, March 25, 2005 1:27PM - 1:39PM |
Y42.00012: Topological effects on conduction electrons in ferromagnetic rings Adnan Rebei, Eugen Simanek Using two examples, I will show possible measurable effects of \textit{global }geometry on spin accumulations in ring geometries as a function of the diameter of the nano-ring. Because of the spin character of the conduction electrons and the magnetization in transition metals, we have two possible inequivalent spin structures on a ring which are given by the first Cohomology group of the ring. In the first example, the magnetization is assumed to be everywhere the same with respect to the normal to the ring, i.e., the magnetization has a 360 degrees periodicity. In the second example, the magnetization is assumed to have a 720 degrees periodicity and topologically equivalent to a Mobius band. In both examples the magnetization is locally the same but differ globally. We will argue that as the size of the ring becomes smaller (i.e., around 10 nm or less), the topologically non-trivial spin structure should become more stable. This topological property can manifest in e.g. resistance differential between the two examples. This latter physical effect is expected to be present in addition to the Berry phase. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 1:51PM |
Y42.00013: Induced spin currents in alkali films Funing Song, Gerd Bergmann, Doug Garrett Sandwiches of FeK and FeCs are prepared at helium temperature and under ultra-high vacuum. The mean free path within these sandwiches can exceed the film thickness by a factor of five. This implies almost perfect specular reflection of the electrons at the interfaces. Furthermore, the experiments suggest that the specular reflections for spin-up and spin-down electrons are different at the Fe interface, resulting in a spin current in the alkali films. In order to detect this current, dilute Pb impurities are condensed on top of the free surface of the alkali films. Strong spin-orbit scatterers, such as Pb, introduce an Anomalous Hall Effect in the presence of a spin current, which can be easily detected through straightforward Hall measurements. The results of the AHE experiments clearly indicate the existence of a local spin current. [Preview Abstract] |
Friday, March 25, 2005 1:51PM - 2:03PM |
Y42.00014: Leads as Self-Energies in Nonequilibrium Spin Transport Michael Johnson, Fredrick Michael Open quantum systems consist of semi-infinite leads which transport electronic spin or charge to and from a device. We show here that within the nonequilibrium Green's function technique for continuum systems, the leads can be replaced by simple c-number self-energies. Our approach is a reformulation of a continuum calculation developed by T.E. Feuchtwang and connects this method with calculations on a lattice. While the choice of internal boundary conditions gives a limited variability to the self-energies, a particular choice greatly simplifies calculations in two and three dimensions. We close with an example from spin transport. [Preview Abstract] |
Friday, March 25, 2005 2:03PM - 2:15PM |
Y42.00015: Spin current in an asymmetric quantum well: 2x2 effective Hamiltonian vs multi-band model A. A. Kiselev, K. W. Kim We have conducted an analysis of the problem of electron spin currents in two-dimensional systems with structural asymmetry. It is shown that an attempt to derive this quantity based exclusively on the 2x2 effective Hamiltonian is internally inconsistent and actually misses the dominant contribution to the spin current. In short, this happens because "effective" (i.e., obtained via partial diagonalization of multi-band Hamiltonian and truncation) forms of spin and velocity operators are not sufficient, alone, to define the effective form of their superposition. Proper approach should start with the multi-band Hamiltonian explicitly incorporating spin-orbit-split bands and define an operator of the spin current on a complete system, not on one of its effective subspaces. Understandably, this also resolves ambiguities, associated with the order of spin and velocity operators in the superposition, that are inherently present in the 2x2 approach. [Preview Abstract] |
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Y42.00016: Electrical transport in oxidized iron thin films Jolanta Stankiewicz, Juan Bartolom\'{e} , F\'{e}lix Jimen\'{e}z-Villacorta, Carlos Prieto We report results of electrical resistivity, Hall effect and anisotropic magnetoresistance (AMR) measurements performed on thin films ($\sim 50 $ nm) of oxidized iron in the temperature range from 10 to 300 K, and in magnetic fields up to 0.6 T. The films were sputtered on naturally oxidized Si(100) substrates whose deposition temperature $T_s$ can be varied between 170 and 300 K. Samples were oxidized at room temperature for 30 minutes, and subsequently capped with a SiO$_2$ layer in order to prevent further oxidation. By changing $T_s$ during the growth process we can control the size and shape of crystalline grains in these films. We find that the residual resistivity, carrier concentration, coercive field $H_c$ as well as the magnetoresistance of the films are strongly affected by growth conditions, particularly by the substrate temperature. All measured samples are metallic with a room temperature electron concentration of about $2\times 10^{21}$ cm$^{-3}$. As $T_s$ increases, the resistivity at $T$=10 K decreases from $\approx$ 400 $\mu\Omega$cm (for $T_s$=170 K) to $\approx$ 40 $\mu\Omega$cm (for $T_S$=300 K). The low-temperature coercive field, obtained from AMR data, also decreases from $\approx 500$ to $\approx 300$ Oe as $T_s$ increases. At $T$=300 K, $H_c \approx 100$ Oe for all values of $T_s$. AMR values increase with increasing $T_s$. We find the room-temperature planar Hall effect ratio of about 15\% in the films grown at $T_s\geq$ 250 K. [Preview Abstract] |
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