Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F10: Invited Session: Spin-Orbit Transfer Torques in Magnetic Bilayers |
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Sponsoring Units: GMAG Chair: Kyung-Jin Lee, Korea University Room: 309 |
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F10.00001: The spin Hall effect in transition metal-ferromagnetic material bilayer devices Invited Speaker: Chi-Feng Pai The strong spin-orbit interaction from certain heavy metal/ferromagnetic material bilayer systems has been shown to be intense enough to drive the magnetization into steady dynamics and/or magnetic switching via spin transfer torque mechanism. The spin Hall effect, which describes the generation of a transverse spin current from a longitudinal charge current, plays an important role in these bilayer devices that typically contain a heavy transition metal underlayer. Here we demonstrate that the spin Hall effect induced spin transfer torque (SHE-STT) from Ta and W based systems can be utilized to control the magnetization direction in magnetic tunnel junctions through a three-terminal device architecture. We also demonstrate DC current induced dynamics in the magnetic layer due to the SHE-STT in these three-terminal devices. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F10.00002: Interfacial current induced torques in Pt|Co|GdOx Invited Speaker: Geoffrey S. D. Beach |
Tuesday, March 19, 2013 9:12AM - 9:48AM |
F10.00003: Rashba spin-orbit coupling and orbital chirality in magnetic bilayers Invited Speaker: Hyun-Woo Lee The phenomenon of the Rashba spin-orbit coupling is examined theoretically for an ultrathin magnetic layer in contact with a non-magnetic heavy metal layer. From first-principles calculation, large Rashba parameter of order 1 eV$\cdot$\AA\ is obtained, which is strong enough to generate large spin transfer torque of spin-orbit coupling origin. Large Rashba parameter is attributed to the orbital mixing of 3$d$ magnetic atoms and non-magnetic heavy elements with significant atomic spin-orbit coupling. Interestingly the magnitude and sign of the parameter vary from energy bands to bands, which we attribute to band-specific chiral ordering of orbital angular momentum. Through a simple tight-binding model analysis, we demonstrate that $d$-orbital hybridization allowed by the breaking of structural inversion symmetry generates band-specific chiral ordering of orbital angular momentum, which combines with atomic spin-orbit coupling to give rise to band-specific Rashba parameter. The band-dependence of the Rashba parameter is discussed in connection with recent experiments and we argue that the dependence may be utilized to enhance device application potentials. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:24AM |
F10.00004: Vector measurements of the current induced effective fields in Ta/CoFeB/MgO heterostructures Invited Speaker: Masamitsu Hayashi Ultrathin magnetic heterostructures exhibit a variety of rich physics owing to the strong effects from the interfaces. Power efficient current induced magnetization switching and domain nucleation, fast current driven domain wall motion have been observed in ultrathin Co or CoFeB layer sandwiched between a heavy metal (Pt, Ta) and an oxide. Most of the current (or voltage) induced effects in these systems can be represented by the ``effective magnetic fields'', which illustrate the strength and direction of the torque exerted on the magnetic moments. A comprehensive understanding of the effective fields is key to the development of magnetic nano-devices aimed for memory and logic applications. We have studied the current induced effective field vector in Ta\textbar CoFeB\textbar MgO heterostructure to reveal the underlying physics of the interaction between the magnetic moments and current in such structure. A low current lock-in detection scheme is used to evaluate the effective field vector. The CoFeB layer is perpendicularly magnetized owing to the interface magnetic anisotropy of CoFeB\textbar MgO. We find that the effective field is very sensitive to the thickness of the Ta and CoFeB layers. The effective field even changes its direction when the Ta layer thickness is varied, indicating that there are competing effects that contribute to the effective field generation. We discuss our results in light of the spin Hall effect and an effect due to Rashba-like Hamiltonian. (Acknowledgment: FIRST program) [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 11:00AM |
F10.00005: Giant spin Hall effect in CuBi alloys Invited Speaker: Yoshichika Otani Spintronic devices manipulating pure spin currents, flows of spin angular momentum without net charge current, should play an important role in low energy consumption electronics for next generation. This explains the current interest for the spin Hall effect (SHE) which provides a purely electrical way to create spin currents without ferromagnets and magnetic fields. In this work, we have studied extrinsic SHEs in Cu-based alloys [1]. Copper itself does not show any SHE, but by adding impurities with strong spin-orbit interactions such as Ir and Bi, the extrinsic SHEs can be generated and one can tune the SH angle which represents the maximum yield of conversion of charge to spin current density. The SH resistance was measured by means of spin absorption method using a lateral spin valve structure with an inserted wire of SHE material [1]. We found that Cu$_{99.5}$Bi$_{0.5}$ exhibited a very large negative SH resistance whereas Pt and a Cu$_{99}$Ir$_{1}$ alloy had positive SH resistances. From nonlocal spin valve measurements with the SHE materials, we can obtain the spin absorption rates as well as the spin diffusion lengths of the SHE materials. The spin Hall angle was determined by fitting experimental data to two theoretical models, i.e., a purely 1D model [2] and a 3D spin transport model based on an extension to 3D of the Valet-Fert formalism [3]. For Pt and CuIr alloys, the spin diffusion lengths are smaller than their thickness (20 nm), and the SH angles obtained from the 1D and 3D analyses are similar to each other (about 2{\%} for both Pt and CuIr). For CuBi alloys, however, the analysis in the 3D model gave much larger SH angle of about - 24{\%} than the 1D of about -12{\%}. More interestingly the fact that Bi impurities generated much larger SH angle than Pt and Ir, was consistent with a recent ab-initio theoretical calculation [4].\\[4pt] [1] Y. Niimi et al., Phys. Rev. Lett. 106 (2011) 126601; Y. Niimi et al., Phys. Rev. Lett. 109 (2012) 156602.\\[0pt] [2] S. Takahashi and S. Maekawa, Phys. Rev. B 67 (2003) 052409.\\[0pt] [3] T. Valet and A. Fert, Phys. Rev. B 48 (1993) 7099.\\[0pt] [4] M. Gradhand et al., Phys. Rev. B 81 (2010) 245109. [Preview Abstract] |
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