Influence of top electrode on the current-induced magnetic switching in magnetic nanopillars

T. Yang, J. Hamrle, T. Kimura, Y. Otani
2005 Applied Physics Letters  
Magnetic nanopillars with variable top electrodes were fabricated to clarify the roles of the spin current and the spin accumulation in the current-induced magnetic switching. The critical switching current is significantly increased when the size of the top electrode is comparable to that of the nanopillar. This result implies that the dominant contribution in the current-induced magnetic switching is not the spin accumulation, but the spin current. The magnetic nanopillar with a
more » ... ith a ferromagnet/nonmagnet/ ferromagnet layered structure is attractive for the application to the magnetic random access memory, which has parallel ͑P͒ and antiparallel ͑AP͒ magnetic configurations, switchable by either an external field or a dc current. 1 Since the first dc current induced magnetic switching was experimentally demonstrated using such nanopillars by Katine et al., 2 there have been many experimental 3-6 and theoretical 7-15 studies on this type of systems. Nevertheless, the roles of the spin current and the spin accumulation have not been clarified yet. Usually, when a charge current j e given by a sum of the spin-up j ↑ and the spin-down j ↓ currents travels through alternating magnetic and nonmagnetic layers, the spin current j s ͑=j ↑ − j ↓ ͒ is evolved as a consequence of the spindependent scattering. Accumulated spins at the interfaces accordingly build the split in the electrochemical potential ⌬ ͑= ↑ − ↓ ͒. As the simplest case, the spin-up and down currents are correlated with the spin accumulation as in following one-dimensional diffusion equations 16 where , , , and e are, respectively, the spin-diffusion length, the electrochemical potential, the electrical conductivity, and the electronic charge. Despite the earlier set of diffusion equations, the roles of the spin accumulation and the spin current in the current-induced magnetic switching are still a point of controversy. Theories based on the spin transfer 1, [8] [9] [10] [11] [12] [13] [14] [15] 17 suggest that the spin current transfers the transverse component of the spin angular momentum to the local magnetic moment at the interface upon flowing into the adjacent magnetic layer, and thereby a torque is exerted on the local moment, resulting in spin-wave excitation or magnetic switching. Notably the theory based on the spin accumulation 7 claims that the nonequilibrium magnetization of the accumulated spins applies an effective exchange field on the local magnetization, described as the nonequilibrium exchange interaction for the case of two magnetic layers separated by a nonmagnetic one. The effective field thus forces the local magnetization to take the orientation of the accumulated spins. Remarkable is that even among the spintransfer theories, divergence appears in terms of the origin of the spin current. Recently, it has been reported 11-13 that the spin accumulation theoretically gives rise to a transverse spin current at the interface much larger than the spin current carried by the polarized charge current. Therefore, we have three possible contributions to the current-induced magnetic switching, such as the spin current carried by the polarized charge current ͑hereafter the spin current͒, the transverse spin current induced by the spin accumulation, and the effective field due to the spin accumulation. It is important to clarify the roles of these three possible contributions for further theoretical and practical understandings. In this letter our main purpose is to clarify the roles of the spin current and the spin accumulation. If the spin current gives dominant contribution, the polarization of the charge current should be increased to reduce the critical switching current. If the spin accumulation gives dominant contribution, no matter whether it induces the effective field or the transverse spin current, the spin accumulation should be enhanced. The studied nanopillars comprise an electron-beam evaporated layered structure of Cu ͑60 nm͒ /Co ͑40 nm͒ /Cu ͑6 nm͒ /Co ͑2.5 nm͒ /Au ͑15 nm͒ /Cu ͑50 nm͒. The layers of Cu ͑60 nm͒ and Cu ͑50 nm͒ serve as the bottom and top electrodes, respectively. The extended bottom Co ͑40 nm͒ layer has a fixed magnetization during the currentinduced magnetic switching, while the top Co ͑2.5 nm͒ layer has freely switchable magnetization. For this study, the effect of the spin current needs to be separated from the effect of the spin accumulation. This can be realized by adjusting the size of the top electrode. a͒ Electronic mail: APPLIED PHYSICS LETTERS 87, 162502 ͑2005͒
doi:10.1063/1.2093921 fatcat:bcvdqcjrbvgzhlxnnzgptumcmy