Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J29: Focus Session: Spin Currents in Metals - New and Miscellaneous Topics |
Hide Abstracts |
Sponsoring Units: GMAG DMP FIAP Chair: Samir Garzon, University of South Carolina Room: 333 |
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J29.00001: Electronic transport in ferromagnetic conductors Christian Wickles, Wolfgang Belzig We theoretically study ferromagnetic conductors using the Stoner model to describe the interaction between electron spin and magnetization. The latter can, in general, depend on time and space. We include impurity scattering processes for the electrons that lead to momentum relaxation, spin-flip and spin-dephasing. Utilizing Keldysh theory, we derive transport equations which allow to access interesting quantities such as domain wall resistance, electronic contribution to the magnetization damping coefficients and forces induced by non-equilibrium electron distributions in the presence of current flow. On the other side, a magnetization that shows temporal and spatial variation can induce current flow in the electron system. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J29.00002: Bulk Spin Pumping and Bulk Spin Transfer Torque in Two-band Magnetic Conductors Wayne Saslow For nonmagnetic materials, irreversible thermodynamics shows that thermal conduction and electrical conduction have a new and independent cross-coupling that yields a thermoelectric and an electrothermal effect. All of these terms are dissipative. However, for nonuniform two-band conducting magnets (e.g., within domain walls), electrical conduction and magnetization dynamics are cross-coupled by the up-band and down-band conductivities, without a need for a new cross-coupling. This yields both a bulk spin pumping term driving the current and a bulk spin transfer torque term driving the magnetization. Adiabatic in space, these terms are dissipative. In addition to these spin transfer and spin pumping terms corresponding to existing transport coefficients, for each spin component there are two types of additional transport coefficients. One type modifies the dissipative conductivity-driven terms in spin pumping and spin transfer torque, and itself is dissipative. The other type, non-adiabatic in space, is non-dissipative. We consider the situation where there is a spin current but no net current. Thermal effects are also considered, with temperature gradients having the same symmetry as gradients in the up and down spin electrochemical potentials. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J29.00003: Influence of a Transport Current on Magnetic Anisotropy Ion Garate, Allan MacDonald The microscopic understanding of the spin transfer torque (STT) is an essential ingredient in the quest to develop optimized spintronic devices. It is well-known that STT occurs whenever electric currents travel through non-collinear magnetic systems. In contrast, it is often overlooked that current-induced torques may also arise in uniformly magnetized systems due to the intrinsic spin-orbit coupling in the band structure of the ferromagnet. We relate this effect to the change in magnetic anisotropy in presence of a current, and use simple models to estimate the possible role of transport currents to modify the direction of the ferromagnetic easy axis and assist magnetization reversal. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J29.00004: Quantifying Spin Hall Effects in Gold Hall Bars Goran Mihajlovic, John E. Pearson, Samuel D. Bader, Axel Hoffmann, Miguel Angel Garcia Spin Hall effects manifest the fundamental interdependence between charge and spin transport. We studied these effects experimentally by measuring the non-local resistance,$ R_{nl}$, in mesoscopic Au Hall bars where spin current generation and detection are spatially separated in two side arms, while a bridging arm acts as the spin conduit. The measured $R_{nl}$ decreases monotonically with decreasing temperature, changing sign from positive to negative. This can be understood by modeling $R_{nl}$ as a sum of two components; a positive, ohmic component, arising from the charge current, and a negative component, due to spin Hall effects and spin diffusion. By varying the spacing between the side arms, the components can be separated. We determined the spin diffusion length, the$_{ }$spin Hall angle and the spin Hall conductivity. We found spin Hall angles of order 0.1, with a temperature dependence proportional to the resistivity, while the spin Hall conductivity was almost temperature independent. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J29.00005: Investigation of Spin-Torque Effects on the Exchange Bias of Ferromagnet/Antiferromagnet Bilayers Kiran V. Thadani, R.A. Buhrman, D.C. Ralph Spin-polarized current, generated by one ferromagnetic layer in a magnetic multilayer structure, can deposit spin angular momentum into a second ferromagnetic layer, causing it to either reversibly switch its orientation or oscillate in steady state at microwave frequencies. Recent calculations and experiments have investigated the possibility that spin torque might also alter the structure of an antiferromagnet [1], thereby affecting the exchange-bias field produced by the antiferromagnet on an adjacent ferromagnetic layer [2, 3]. Here we report studies made using nanopillar samples in which the free magnetic layer is exchange-biased to an antiferromagnet, which allow a direct measurement of the magnitude of the exchange bias and its current dependence. We will also investigate the degree to which the exchange bias alters the damping of the free-layer magnet and the extent to which the effective damping can be controlled with current. [1] A. S. Nunez et al., Phys. Rev. B 73, 214426 (2006). [2] Z. Wei et al., Phys. Rev. Lett. 98, 116603 (2007). [3] S. Urazhdin et al., Phys. Rev. Lett. 99, 046602 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:51PM |
J29.00006: Tunnel barrier enhanced voltage signals generated by magnetization precession of a single ferromagnetic layer Invited Speaker: A variety of experimentally observed phenomena involving nonlocal magnetization dynamics in magnetic multilayers are due to two complementary effects: (i) the transfer of spin angular momentum accompanying charge currents driven by the applied bias voltage between ferromagnetic layers results in torques that (for sufficiently high current densities) generate spontaneous magnetization precession and switching; and (ii) the precessing magnetization of a ferromagnet (FM) pumps spins into adjacent normal metal layers (NM) with no applied bias. In particular, the spin pumping effect is a promising candidate for realizing a spin battery device [1] as a source of elusive pure spin currents (not accompanied by any net charge transport) emitted at the FM/NM interface, where steady magnetization precession of the FM layer is sustained by the absorption of external rf radiation under the FMR conditions. We report the electrical detection of magnetization dynamics in an Al/AlO$_{x}$/Ni$_{80}$Fe$_{20}/$Cu tunnel junction, where a Ni$_{80}$Fe$_{20}$ ferromagnetic layer is brought into precession under the ferromagnetic resonance (FMR) conditions. The dc voltage generated across the junction by the precessing ferromagnet is enhanced about an order of magnitude compared to the voltage signal observed in Cu/FeNi/Pt structures [2]. A structure of Cu (100nm)/Al (10nm)/AlO$_{x}$ (2.3nm)/Ni$_{80}$Fe$_{20}$ (20nm)/Cu (70nm)/Au (25nm) was fabricated on a Si substrate with a 1$\mu$m thick thermal oxide layer. The bottom-most 100 nm Cu layer was patterned into a coplanar waveguide (CPW) and the rest of the structure was patterned into a pillar structure on the signal line of the CPW. Dc voltages $\sim\mu$V were observed in the Al/AlO$_{x}$/Ni$_{80}$Fe$_{20}$/Cu tunnel junction when the Ni$_{80}$Fe$_{20}$ is in the ferromagnetic resonance. The dc voltages increase as the precession cone angle and frequency increase. We discuss the relation of this phenomenon to magnetic spin pumping and speculate on other possible underlying mechanisms responsible for the enhanced electrical signal. \\[4pt] [1] A. Brataas et al. PRB 66, 060404 (2002)\\[0pt] [2] M. V. Costache et al. PRL 97, 216603 (2006) [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J29.00007: FMR and voltage induced transport in normal metal-ferromagnet-superconductor trilayers Hans Joakim Skadsem, Arne Brataas, Jan Martinek In recent years, hybrid nanoscale circuits containing normal conductors, ferromagnets, and superconductors have been realized. These structures allow observation and understanding of the competition between ferromagnetism and superconductivity. In this talk, we consider charge and spin transport in normal metal-ferromagnet-superconductor trilayers induced by bias voltage and/or magnetization precession. Transport properties are discussed in terms of time-dependent scattering theory. We compute the charge and spin current response to bilinear order in precession frequency and bias voltage and express the results in terms of spin-dependent conductances. Simplified conductance expressions are obtained when the ferromagnet is longer than the transverse spin coherence length. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J29.00008: Spin torque from tunneling through impurities in a magnetic tunnel junction Turan Birol, Piet Brouwer We study impurity-mediated transport in a magnetic tunnel junction (MTJ) in the sequential tunneling regime. We address the conductance of the MTJ as well as the spin transfer torque. We show that the torque from impurity-mediated tunneling can be distinguished from that from direct tunneling through its dependence on the barrier thickness and the angle between the ferromagnetic moments. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J29.00009: Thermoelectric transport and thermal spin currents in ferromagnetic films and nanostructures Azure Avery, Rubina Sultan, Barry Zink For fundamental physics, understanding the mechanisms behind giant magnetoresitance (GMR) and its related properties, magnetoresistance (MR) and magnetothermopower (MTEP), is crucial, especially for nanoscaled structures. Though progress has been made in understanding electron transport through magnetic thin films and multilayers, far less is understood about the mechanisms behind thermal transport in these systems. This is due, in part, to the difficulty of measuring thermal properties of these low-dimensional systems. We present a robust technique for accurately measuring thermal conductivity ($ k_{\parallel} $), thermopower ($ \alpha $), and MTEP in nanoscale magnetic materials using micromachined silicon nitride thermal isolation structures. We outline the fabrication of the structures and present our measurement results for ferromagnetic thin films and nanowires. Finally, we present how this technique is applied to testing the validity of current models explaining the mechanisms of thermal transport, such as thermal spin currents, in ferromagnetic films and nanostructures. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J29.00010: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J29.00011: Magnetoresistive junctions based on epitaxial graphene and h-BN Oleg Yazyev, Alfredo Pasquarello Using a first-principles approach, we investigate the structural, magnetic and transport properties of interfaces based on epitaxially grown monolayer graphene and hexagonal boron nitride ($h$-BN) in combination with ferromagnetic transition metals (Fe, Co and Ni). Such structurally well defined interfaces based on (111) fcc or (0001) hcp transition metals can be produced using simple manufacturing processes. Our calculations predict magnetoresistance ratios over 100\% for certain junction compositions. In addition, such systems feature strong antiparallel (Fe and Co) and parallel (Ni) exchange coupling across the interface combined with low junction resistance. The predicted properties position such magnetoresistive junctions as an interesting alternative to the currently used giant and tunneling magnetoresistance systems and make them suitable for practical applications. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J29.00012: Spin injection and transport in single layer graphene spin valves Wei Han, Wei-Hua Wang, Keyu Pi, Kathy McCreary, Wenzhong Bao, Yan Li, Chun Ning Lau, Roland Kawakami Graphene is an attractive material for spintronics due to its tunable carrier concentration and polarity, weak spin-orbit coupling, and the prediction of novel spin-dependent behavior. We investigate the spin injection and transport in single layer graphene (SLG) spin valves at room temperature. Raman spectroscopy is used to identify SLG. SLG spin valve devices are fabricated by growing cobalt electrodes, defined by electron beam lithography, on top of SLG. Nonlocal resistance measurements are performed on these SLG spin valve devices in order to study the spin injection and transport properties. Our results show that the nonlocal magnetoresistance (MR) is dependent on the gate voltage. Also, the nonlocal MR shows some variation as a function of DC bias current. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J29.00013: Falling Magnets and Electromagnetic Braking Christopher Culbreath, Peter Palffy-Muhoray The slow fall of a rare earth magnet through a copper pipe is a striking example of electromagnetic braking; this remarkable phenomenon has been the subject of a number of scientific paper s [1, 2]. In a pipe having radius R and wall thickness D, the terminal velocity of the falling magnet is proportional to (R\^{}4)/D. It is interesting to ask what happens in the limit as D becomes very large. We report our experimental observations and theoretical predictions of the dependence of the terminal velocity on pipe radius R for large D. [1] Y. Levin, F.L. da Silveira, and F.B. Rizzato, ``Electromagnetic braking: A simple quantitative model''. \textit{American Journal of Physics}, \textbf{74}(9): p. 815-817 (2006). [2] J.A. Pelesko, M. Cesky, and S. Huertas, Lenz's law and dimensional analysis. \textit{American Journal of Physics}, \textbf{3}(1): p. 37-39. 2005. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700