Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars
Katine, J.A., F. J. Albert, F.J., Buhrman, R.A., Myers, E.B., & Ralph, D.C., 2000. Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars. Phys. Rev. Lett. 84, 3149
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Given the potential application for spin torques it took about three years to confirm the spin-transfer torque prediction. The primary obstacle was the requirement that the cross-section of the pillar for the current is on the nanometer scale, e.g., 100x100 nanometers. The reason was the torque created by the spin current is in competition with the effect, from Ampère’s law, that as a magnetic field surrounds a current, its magnitude increases with the cross-sectional area and is proportional to the charge current density. Spin torque is only dependent on spin current density and is uniform across the cross section of a pillar. To avoid the competition between the two effects, nanolithographic techniques had to be perfected so that one could produce these nano-pillars with a minimum of roughness on the surface of the pillar so as to minimize scattering of the current there. The pillar had to comprise of a minimum of two magnetic layers separated by a nonmagnetic layer, and current conducting leads that could be larger in cross section than the nano-pillar, provided that a good contact existed. Also, one had to assure that roughness at interfaces was reasonable so as to not have excessive scattering.
At first current-induced magnetization switching was observed in metallic pillars, pretty much at the predicted threshold current density. While the Current Perpendicular to the Plane Magnetoresitance (CPP-MR) of these pillars is quite small, on the order of 2-3%, the torque is not determined by the GMR, i.e., the change in charge current with alignment of magnetic layers. But, rather, by the factors governing the spin current. Then, after a few more years, this current-induced switching was observed in MTJs for threshold current-densities about, or lower than, those for metallic spin valves. The lower threshold comes from using magnetic electrodes with smaller magnetization densities than those for metallic spin valves. There are at least two advantages of working with tunnel junctions: first, one can readily drive their response into the nonlinear regime, so that both the TMR and spin torque dependence on bias can be determined, and, second, the analysis can be based on the equilibrium band structure, inasmuch as the current-induced accumulations in the electrodes do not invalidate ones conclusions based on a ballistic view of the transport. The low current densities and the fact that the overwhelming drop in resistance occurs in the barrier region of a tunnel junction [the changes due to accumulation are not important] conspire to make calculations based on equilibrium band structure reliable. As the drop in voltage is confined to the barrier region in a tunnel junction “hot electrons” are created in the presence of finite voltage drops. These electrons leave the Fermi level of one metallic electrode and can arrive at the second electrode-barrier interface at an energy eV [ e is the charge of an electron and V the drop in potential] above the Fermi level in the second electrode. Eventually the excited electrons lose this energy and drop to the lower Fermi level; this allows one to determine the effects of inelastic processes, such as magnon creation at the interfaces between the electrodes and the barrier, on changes in resistance, TMR, and spin torque .
 Peter M Levy and Albert Fert, Phys. Rev. Lett. 97, 097205 (2006); Phys. Rev. B 74, 224446 (2006).
F. J. Albert, J. A. Katine, R. A. Buhrman, and D. C. Ralph, Appl. Phys.Lett. 77, 3809 (2000)
J. Grollier, V. Cros, A. Hamzic, J. M. George, H. Jaffrès, A. Fert, G. Faini, J. Ben Youssef, and H. Legall, Appl. Phys. Lett. 78, 3663 (2001).
Describe the differences between the torque on the magnetism in a ferromagnetic metal derived from spin currents as compared to the charge current?
The above article is reprinted with permission from the author(s) of Katine, J.A., F. J. Albert, F.J., Buhrman, R.A., Myers, E.B., & Ralph, D.C., 2000. Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars. Phys. Rev. Lett. 84, 3149. Copyright (2000) by the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society .
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