Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D8: Unconventional Spin Torques |
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Sponsoring Units: GMAG Chair: Shufeng Zhang, University of Missouri Room: 414/415 |
Monday, March 16, 2009 2:30PM - 3:06PM |
D8.00001: Thermal Spin Transfer Torques Invited Speaker: The coupling between spin and charge in electronic transport is studied in the field of spintronics. Heat currents are coupled to both charge and spin currents as well [1]. This extension of spintronics to what may be called ``spin caloritronics'' recently enjoys renewed attention [2]. The spin-transfer torque associated with electric currents can excite magnetizations in nanostructures, switching magnetic configuration in spin valves and move domain walls in magnetic wires when exceeding critical values of the order of $10^7$Acm$^{-2}$ [3]. Also heat currents transfer spin angular momentum [4], either intrinsically or via the thermoelectric generation of particle spin currents. We predict that temperature differences of the order of 100 K over typical metallic nanostructures cause effects equivalent to the critical charge current densities. In this talk I will give a brief review of various aspects of spin caloritronics with emphasis on thermal spin transfer torques. This work has been carried out in collaboration with Moosa Hatami, Qinfang Zhang, Paul Kelly, Hans Joakim Skadsem, Arne Brataas and Sadamichi Maekawa. \\[4pt] [1] M. Johnson and R.H. Silsbee, Phys. Rev. B 35, 4959 (1987).\\[0pt] [2] International Workshop on Spin Caloritronics, Lorentz Center of Leiden University, 9-13 February 2009, http://www.lorentzcenter.nl/lc/web/2009/323/info.php3?wsid=323\\[0pt] [3] D. C. Ralph and M. D. Stiles, J. Magn. Magn. Mater. 320, 1190 (2008).\\[0pt] [4] M. Hatami, G.E.W. Bauer, Q. Zhang, and P.J. Kelly, Phys. Rev. Lett. 99, 066603 (2007). [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D8.00002: Nonequilibrium intrinsic spin torque in a single nanomagnet Invited Speaker: The spin transfer torque usually observed in metallic and tunneling spin-valves, as well as magnetic domain walls, comes from the transfer of the transverse spin-current of conduction electrons to the magnetization [1]. Therefore, it requires both a non collinear configuration of the magnetic structure (or inhomogeneous magnetic texture in the case of domain walls) and magneto-resistive effects. However, a number of magnetic systems show magneto-resistive effects in a single magnetic layer, such as anisotropic magnetoresistance (AMR) [2]. In the presence of spin-orbit interaction (SOI) the electron scattering depends on the magnetization direction. Recent theoretical studies suggest that in such systems, a transverse component of the spin density builds up, due to the spin-dependent scattering introduced by the spin-orbit coupling. Consequently, a transverse spin density arises from intrinsic properties of the band structure without the need of non-collinear magnetization texture. In the case of a single ferromagnet with spin-orbit interaction, the exchange interaction between the accumulated spin and the magnetization gives rise a spin torque on the magnetization. We show that this torque can be used to control the magnetization direction injecting current densities as low as 10\^{}5-10\^{}6 A/cm\^{}2, comparable or lower than the spin transfer effect. We first study the general case of a single ferromagnetic layer with spin-orbit interaction and then focus on the cases of effective Hamiltonians, such as Rashba and Dresselhaus SOI, as well as Luttinger hole systems. We discuss the relation between the spin torque and the spatial inversion symmetry of various forms of spin-orbit couplings and compare this spin torque with the conventional spin transfer torque. We finally discuss several magnetic systems for possible experimental realization. This work was done in collaboration with Shufeng Zhang. [1] ] J.C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, Phys. Rev. B 54, 9353 (1996). [2] T.R. McGuire and R.I. Potter, IEEE Trans. Mag. 11, 1018 (1975). [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D8.00003: Spin transfer torques in antiferromagnets and magnetic semiconductors Invited Speaker: Spin transfer torques (STT) in ferromagnetic metals can be understood in terms of conservation of total spin, allowing a simple evaluation and interpretation of these torques in terms of spin currents. STTs also occur in antiferromagnets, which have no net spin and different symmetries than ferromagnets, resulting in qualitatively different torques. We consider a structure with a compensated antiferromagnetic layer and a ferromagnetic layer. We find a STT on both layers, which vanishes when the layers' order parameters are either collinear or perpendicular. This torque can drive the magnetization of a thin film ferromagnet to be perpendicular to the easy plane. In dilute magnetic semiconductors, strong spin orbit coupling in the semiconductor host implies that spin is not even approximately conserved, requiring modifications of the microscopic calculation of the STT. We describe these modifications and present results from first principles calculations of STT in GaMnAs. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:54PM |
D8.00004: Spin-Transfer-Torques at a Ferromagnet/Antiferromagnet Interface Invited Speaker: Spintronics in ferromagnetic systems is built on a complementary set of phenomena in which the magnetic configuration of the system influences its transport properties and vice versa. Giant magnetoresistance (GMR) [1] and spin- transfer-torque (STT) [2] phenomena are typical examples of such interconnections. Recently, MacDonald and co-workers [3] predicted that corresponding effects ought to occur in systems where ferromagnetic (F) components are replaced by antiferromagnets (AFM). I will present our experimental search for these new AFM effects which may potentially lead to a new all-antiferromagnetic spintronics where antiferromagnets are used in place of ferromagnets. In particular I will focus on our experiments with exchange-biased spin valves [4] where extreme current densities were found to affect the exchange bias at F/AFM interface [5-7]. As exchange bias is known to be associated with interfacial AFM magnetic moments, our observation can be taken as the first evidence of STT effect in AFM materials. \\[4pt] [1] M. N. Baibich et al., Phys. Rev. Lett. 61, 2472 (1988); G. Binasch et al., Phys. Rev. B 39, 4828 (1989). \\[0pt] [2] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, J. Appl. Phys. 81, 4880 (1997); M. Tsoi et al., Phys. Rev. Lett. 80, 4281 (1998). \\[0pt] [3] A. S. N\'u\~nez et al., Phys. Rev. B 73, 214426 (2006); \\[0pt] [4] Z. Wei et al., Phys. Rev. Lett. 98, 116603 (2007). \\[0pt] [5] S. Urazhdin and N. Anthony, Phys. Rev. Lett. 99, 046602 (2007). \\[0pt] [6] X-L.Tang et al., Appl. Phys. Lett. 91, 122504 (2007). \\[0pt] [7] N. V. Dai et al., Phys. Rev. B77, 132406 (2008). [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:30PM |
D8.00005: Effects of polarizer dynamics on current-induced behaviors in magnetic multilayer nanopillars Invited Speaker: Magnetic nanodevices usually include two magnetic layers - a polarizing ``fixed'' layer, and a ``free'' layer, whose roles are determined by their relative thicknesses. I will describe our measurements of spin transfer in nanopillars with similar thicknesses of the ``polarizer'' and the ``free'' layer. In the first sample type, both layers were patterned into similar lateral dimensions. Spectroscopic measurements of current-induced dynamics showed incoherent bipolar excitations. Thermally-activated reversal statistics exhibited dependencies on magnetic field and applied current dramatically different from the ``standard'' samples with a thick polarizing layer. I will also discuss our results for samples in which only the free layer was patterned into a nanopillar, while the polarizing layer was left extended with dimensions of several micrometers. These samples exhibit coherent precession of only the extended layer, only the polarizer, or both, depending on the relative thicknesses of the two layers. The transition between the ``free''-like and ``fixed''-like behaviors of each layer occured over a small range of thickness. I will show that current-induced behaviors of our samples can be understood in terms of the dynamical coupling between ferromagnets induced by spin transfer. This coupling can result suppression of the current-induced precession, incoherent dynamics, or for certain geometries in enhancement of current-induced dynamics in magnetic bilayers. [Preview Abstract] |
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