Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session P22: Focus Session: Spin Transport in Metals |
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Sponsoring Units: GMAG FIAP DMP Chair: Branislav Nikolic, University of Delaware Room: Baltimore Convention Center 319 |
Wednesday, March 15, 2006 11:15AM - 11:27AM |
P22.00001: Disorder, itinerant ferromagnetism and the anomalous Hall effect in two dimensions Partha Mitra, Arthur Hebard This talk will describe research motivated by the lack of consensus on what happens in a band ferromagnet such as iron when the itinerancy of the electrons, which carry spin information, is compromised by disorder. We address this challenging problem by performing \textit{in situ} studies of magnetotransport in a series of films having sheet resistances varying from 50 to 1,000,000 Ohms. In the weakly disordered regime of this two-dimensional system, where the quantum corrections to the conductivity have logarithmic temperature dependence, we find a surprising scaling of the longitudinal and anomalous Hall (transverse) resistances. For higher disorder the scaling breaks down and the anomalous Hall resistance $R_{xy}$ saturates at a constant value near 100 Ohms. These results imply the presence of an \textit{anomalous Hall insulating} state where the longitudinal $L_{xx}$ and transverse $L_{xy}$ conductivities approach zero with a ratio $R_{xy}~=~L_{xy}/L_{xx}^{2}$ that remains constant. [Preview Abstract] |
Wednesday, March 15, 2006 11:27AM - 11:39AM |
P22.00002: Theory of Anomalous Hall Effect in Ferromagnets Shigeki Onoda, Naoyuki Sugimoto, Naoto Nagaosa Mechanism of the anomalous Hall effect (AHE) in ferromagnetic metals has been controversial over many decades. Karplus-Luttinger initiated the discussion by focusing on the intrinsic thermodynamic Hall current produced by the band structure with a spin-orbit interaction. Later, it was argued that instead, scattering by impurity or disorder together with the spin-orbit interaction distorts the electron motion as the skew scattering or the side jump and these extrinsic contributions dominate over the Hall current. Here, we reexamine this issue by fully taking account of both the impurity scattering and the anomalous velocity in terms of the quantum transport theory. We demonstrate that apart from the conventional nonequilibrium transport current, an equilibrium Hall current flows even in the presence of dissipation in metals. This equilibrium Hall current contains the intrinsic one which has a topological non-perturbative nature associated with degeneracy of the band dispersions in the momentum space. We also show that there appears a crossover from the extrinsic regime to the intrinsic as the electron damping rate becomes comparable to or larger than the energy scale of the spin-orbit coupling. This resolves the long standing puzzle on the mechanism and reveals a new small energy scale governing the quantum transport in multi-band systems. [Preview Abstract] |
Wednesday, March 15, 2006 11:39AM - 11:51AM |
P22.00003: Quantum correction to the anomalous Hall conductivity of ferromagnetic metallic films Khandker Muttalib, Peter Woelfle Motivated by new anomalous Hall effect (AHE) data on polycrystalline Fe films we calculate the interaction correction to the AHE for both the skew scattering and the side jump mechanisms. The correction to the longitudinal conductivity is also considered. We use a model of randomly located short range impurity potentials of arbitrary scattering strength, inducing spin-orbit scattering, and an isotropic band ferromagnet. The quantum correction is found to depend sensitively on the strengths of the scattering potential and the spin-orbit interaction. In the limit of weak scattering, known results are recovered. [Preview Abstract] |
Wednesday, March 15, 2006 11:51AM - 12:03PM |
P22.00004: Spin Hall effects in diffusive normal metals. Roman Shchelushkin, Arne Brataas We study transport in normal metals in an external magnetic field. We employ the Keldysh formalism to find transport equations in the presence of the spin-orbit interaction, interaction with magnetic impurities, and nonmagnetic impurity scattering. This system exhibits an interplay between a transverse spin imbalance (spin Hall effect) caused by the spin-orbit interaction, a Hall effect via the Lorentz force, and spin precession due to the Zeeman effect. The spin and charge accumulations are computed numerically in experimentally relevant thin film geometries. The out-of-plane spin Hall potential is suppressed when the Larmor frequency is larger than the spin-flip scattering rate. The in-plane spin Hall potential vanishes at a zero magnetic field and attains its maximum at a finite magnetic field before spin precession starts to dominate. Spin injection via ferromagnetic contacts creates a transverse charge Hall effect that decays in a finite magnetic field due to spin precession. [Preview Abstract] |
Wednesday, March 15, 2006 12:03PM - 12:15PM |
P22.00005: Resistance and Scattering Anisotropy of Al Interfaces with Co, Fe, and Co(91)Fe(9). Nikoleta Theodoropoulou, Thibault Haillard, Amit Sharma, Reza Loloee, William Pratt Jr., Jack Bass The properties of normal/ferromagnetic metal interfaces, described by the interface specific resistance, AR* (A = area, R = resistance) and spin scattering anisotropy, $\gamma $, are of both fundamental interest and practical interest for optimizing current-perpendicular-to-plane (CPP) magnetoresistance (MR) and current-induced magnetization-switching (CIMS) in nanopillars. From measurements of the CPP resistances and MRs of sputtered [Al/F]x$N$ (F= Fe, Co, Co(91)Fe(9)) multilayers with $N$-bilayers, and Al/F-based exchange-biased spin-valves, we are able to estimate 2AR$^{\ast }$ and $\gamma $ for each metal pair at 4.2K. In each case, 2AR* is large and $\gamma $ is small, comparable to values of 2AR* $\sim $ 9 f$\Omega $m$^{2}$ and $\gamma \quad \sim _{ }$0.03 for Permalloy (Py)/Al interfaces [1], and each differing by an order of magnitude from the parameters for well-studied Co/Cu and Py/Cu interfaces (2AR$^{\ast }\sim $ 1 f$\Omega $m$^{2}$, $\gamma \quad \sim $ 0.8). The values of AR* with Al are too large to be explained by the resistivities of alloyed Al/F interfaces. The similarity of results for Py, Fe, Co, and Co(91)Fe(9) strongly suggests that spin dependent scattering at Al/F interfaces is determined mainly by the properties of Al. [1] N. Theodoropoulou et al., J. Appl. Phys. (In Press, 2006). [Preview Abstract] |
Wednesday, March 15, 2006 12:15PM - 12:27PM |
P22.00006: Current-Perpendicular-to-Plane (CPP) Resistance of Cu/Al Interfaces Amit Sharma, Nikoleta Theodoropoulou, Reza Loloee, William Pratt Jr., Jack Bass . The recent discovery [1] that the current-perpendicular-to-plane (CPP) interface specific resistance (area A times resistance R) of Py/Al (Py = Permalloy = Ni(84)Fe(16)) is almost 10 times larger than that of Py/Cu, led us to examine also the resistances and magnetoresistances of Py/Cu/Al/Cu/Py exchange-biased spin-valves (EBSV) and [Cu/Al]x$N$ multilayers with $N$ repeats. Using two different techniques, we estimate the interface specific resistance of sputtered Al/Cu interfaces as 2AR(Al/Cu) $\sim$ 2 f-ohm-m$^2$ at 4.2K. However, some of the data from these two techniques, as well as from the Cu/Al/Cu EBSV studies, show unusual behaviors that suggest that the Cu and Al atoms might not always stay where they are deposited. We will describe our CPP-resistance results, and plan to supplement them with x-ray and cross-sectional TEM studies. [1] N. Theodoropoulou et al., J. Appl. Phys. (In Press, 2006). [Preview Abstract] |
Wednesday, March 15, 2006 12:27PM - 1:03PM |
P22.00007: Spin transport and spin-flip scattering in magnetic multilayer structures Invited Speaker: The existence of spin-flip scattering at the interface between ferromagnetic (F) and nonmagnetic (N) layers of magnetoresistive F/N/F structures can significantly reduce the size of the magnetoresistance, limiting the sensitivity and increasing the power consumption of F/N/F devices such as GMR magnetic field sensors, magnetic read heads, and MRAM's~[1]. Detecting and measuring the degree of spin flip scattering in F/N/F structures can allow further optimization in such devices as well as increase the understanding of interfacial spin transport. Our nonlocal spin injection and detection experiments on mesoscopic Co-Al$_2$O$_3$-Cu-Al$_2$O$_3 $-Co spin valves provide evidence for the existence of interfacial spin-flip scattering in magnetoresistive devices~[2]. By extending the conventional picture of spin-dependent interfacial resistances (R$_{\uparrow}$, R$_{\downarrow}$) to include two additional spin-flip scattering channels (R$_{\uparrow\downarrow}$,R$_{\downarrow\uparrow}$)~[3] we have shown that the nonlocal resistance contains information about both the degree of spin polarization and the degree of spin-flip scattering at the F/N interface. The magnitudes of R$_{\uparrow\downarrow}$ and R$_{\downarrow\uparrow}$ depend on the relative orientation of the detector magnetization and the nonequilibrium magnetization in the normal metal. We have observed that the difference in spin-flip scattering between up and down channels vanishes at low temperatures, but for T$>$100K it increases nonlinearly with temperature. Further evidence for the presence of interfacial spin-flip scattering can be obtained from noise measurements, which are extremely sensitive to the microscopic transport details. \noindent [1] \textit{Spin Dependent Transport in Magnetic Nanostructures}, edited by S. Maekawa and T. Shinjo (Taylor \& Francis, New York, 2002). \noindent [2] S. Garzon, I. \v{Z}uti\'{c}, and R. A. Webb, Phys. Rev. Lett. \textbf{94}, 176601 (2005). \noindent [3] E. I. Rashba, Eur. Phys. J. B \textbf{29}, 513 (2002). [Preview Abstract] |
Wednesday, March 15, 2006 1:03PM - 1:15PM |
P22.00008: Measurement of spin-diffusion length in sputtered Ni films Charles Moreau, William Pratt, Norman Birge The spin-diffusion length of the electron represents a fundamental transport parameter and plays a central role in the development of spintronic devices. While several measurements of the spin-diffusion length in normal metals and in ferromagnetic alloys exist, measurements in elemental ferromagnets (Ni, Fe, Co) have been scarce. We present a novel sample geometry using giant magnetoresistance (GMR) for the measurement of the spin-diffusion length in elemental ferromagnets with weak scattering asymmetry. We report the first measurement of the spin-diffusion length of Ni using an exchanged-biased Permalloy-based spin-valve, Py/Ni/Cu/Py, where the Ni layer acts as a “GMR spoiler” layer when its thickness becomes greater than the spin-diffusion length in Ni. [Preview Abstract] |
Wednesday, March 15, 2006 1:15PM - 1:27PM |
P22.00009: Magneto-transport in Fe$_{3}$O$_{4}$/Nb:SrTiO$_{3}$ schottky junction diode Darshan Kundaliya, S.B. Ogale, J. Higgins, T. Venkatesan, L.F. Fu, N.D. Browning Among the half metallic ferromagnets, Fe$_{3}$O$_{4}$ is of particular interest because of its half metallicity, high curie temperature and a charge ordering transition at 120K (popularly known as Verwey transition (T$_{V}))$. These materials are also expected to show 100{\%} spin polarization. In view of these fascinating properties, we studied temperature dependent transport, magnetic, structural and interface characteristics of epitaxial schottky junctions between Fe$_{3}$O$_{4}$ and Nb:SrTiO$_{3}$ (with different Nb concentrations). Epitaxial thin films of Fe$_{3}$O$_{4}$ were grown on Nb:SrTiO$_{3}$ substrates by PLD technique. The films show epitaxial growth along (100)-axis direction. We also performed HR-TEM and EELS study to ensure a better quality interface. In the temperature range above T$_{V}$, 300K-130K, the I-V characteristic shifts towards higher forward bias voltage upon lowering temperature. On further decreasing temperature (below T$_{V})$, the trend is reversed. Junction parameters such as the Schottky barrier height (\textit{$\phi $}$_{B})$ and ideality factor (\textit{$\eta $}) are extracted using thermionic emission theory at all temperatures. These parameters show interesting and systematic trends above and below T$_{V}$. From the magnetic field dependence of non linear I-V characteristics data, a spin polarization of $\sim $80{\%} is estimated for the magnetite electrode at T$_{V}$. [Preview Abstract] |
Wednesday, March 15, 2006 1:27PM - 1:39PM |
P22.00010: Spin filtering of hot electrons in ferromagnetic layers Driss Lamine, Nicolas Rougemaille, Georges Lampel, Yves Lassailly, Jacques Peretti We present a spin dependent transport experiment where spin polarized electrons, injected from vacuum, are spin-filtered when entering into a thin ferromagnetic layer. The role of the interface between this spin-filter layer and the ``current collector'' is analysed. In a first geometry, the ``current collector'' is a semiconductor and the transmitted current is measured through a Fe/Oxide/GaAs diode. In that case, the measured electrons are selected at the interface, at an energy higher than the Schottky barrier. In a second geometry, the spin filter is a self standing thin layer Au/Co/Au and the ``current collector'' is a faraday cup. In that case, only electrons that overcome the vacuum energy level of the gold are measured. In both cases, the spin dependent part of the collected current is measured when the ferromagnetic layer is reversed. We have found an energy domain where both the collected current and its spin dependence increase over orders of magnitude. The role of the interface in the spin-dependent transport is discussed and a spin dependent transport model is presented. [Preview Abstract] |
Wednesday, March 15, 2006 1:39PM - 1:51PM |
P22.00011: Mesoscopic Conductance Fluctuations in Cobalt Nanoparticles Yaguang Wei, Xiya Liu, Liyuan Zhang, Dragomir Davidovic We~present measurements of mesoscopic conductance fluctuations in Cobalt particles of diameter 200nm. Samples are made by e-beam lithography and shadow metal deposition. Co particles are not single domain; domain walls are nucleated at the contacts between Co and Cu-reservoirs. We obtain the dependence of~peaks in differential resistance with the applied voltage and the magnetic field during the magnetization reversal process at 0.03K temperature. The conductance fluctuations with the magnetic field are caused by a mechanism different from the usual Aharonov-ohm effect. In particular, domain walls are found to generate significant mesoscopic fluctuations. We obtain that electron transfer across the domain wall is associated with a phase change of about 5$\pi $. We explain how this phase-shift arises from a not perfectly parallel spin-transport across domain walls. The dephasing time is very short, $\tau _\phi \sim ps$. Fast dephasing is correlated with the strong magnetocrystalline anisotropy in Co. This work was performed in part at the Georgia-Tech electron microscopy facility. We thank P. Brouwer for valuable discussions. This research is supported by the David and Lucile Packard Foundation grant 2000-13874 and Nanoscience/Nanoengineering Research Program at Georgia-Tech. [Preview Abstract] |
Wednesday, March 15, 2006 1:51PM - 2:03PM |
P22.00012: Spin transport through atomic scale chromium Coulomb islands Marc van Veenhuizen, Jagadeesh Moodera Electrical current through metallic islands coupled via tunnel barriers to external leads is governed by the Coulomb repulsion and can be brought down to single electron transport. The spin-degeneracy of the electrons can be lifted by choosing both the leads and the islands to be magnetic. The combination of spin-splitting and Coulomb blockade creates a device geometry capable of resonant tunneling of a single spin-direction only. Maximum effect can be obtained by minimization of the size of the Coulomb islands in order to suppress spin-relaxation. We report on our efforts to make a spin-resonant tunneling device using atomic size clusters of chromium atoms, submerged in an alumina-barrier in a conventional magnetic tunnel junction set-up. The $300x300\;\mu\rm{m}^2$ size magnetic tunnel junction consists of a cobalt bottom electrode, an aluminum-oxide tunnel barrier, a delta-doping layer of chromium in the range of $1-6\;\AA$, an alumina tunnel barrier, and a permalloy top-electrode. Transport measurements reveal Coulomb blockade behavior. [Preview Abstract] |
Wednesday, March 15, 2006 2:03PM - 2:15PM |
P22.00013: Orbital nature of spin magnetic moment Chih-Piao Chuu, Ming-Che Chang, Qian Niu In view of the application of spintronics, it is important to understand the physical observable of the spin, i.e. its magnetic moment. We show by constructing a wave-packet in the upper bands of the Dirac equation that the spin magnetic moment (Bohr magneton) is a direct result of the self-rotation in the wave-packet. In this sense, an non-relativistic electron is really a rotating charged object, confirming the original speculation on the physical nature of the electron spin. In a Bloch band of a crystal, a wave-packet can acquire additional self-rotating orbital angular momentum, giving rise to a change of the spin magnetic moment and causing a modification of the g-factor from 2. [Preview Abstract] |
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