Giant spin-accumulation signal and pure spin-current-induced reversible magnetization switching

Abstract

A number of proposed next-generation electronic devices, including novel memory elements 1 and versatile transistor circuits 2 , rely on spin currents, that is, the flow of electron angular momentum. A spin current may interact with a magnetic nanostructure and give rise to spin-dependent transport phenomena, or excite magnetization dynamics 1-11 . In contrast to a spin-polarized charge current, a pure spin current does not produce any charge-related spurious effects 12,13 . One way to produce a pure spin current is non-local electrical-spin injection 12-18 , but this approach has suffered so far from low injection efficiency. Here, we demonstrate a significant enhancement of the non-local injection efficiency in a lateral spin valve prepared with an entirely in situ fabrication process. Improvements to the interface quality and the device structure lead to an increase of the spin-signal amplitude by an order of magnitude. The generated pure spin current enables the magnetization reversal of a nanomagnet with the same efficiency as in the case of using charge currents. These results are important for further theoretical developments in multiterminal structures 2 , but also with a view towards realizing novel devices driven by pure spin currents.In a vertical spin-valve nanopillar consisting of a ferromagnet/non-magnet/ferromagnet trilayer, the magnetic state can be switched between the antiparallel and the parallel configurations by applying a charge current [1][2][3][4][5][6][7][8][9][10][11] . This charge-current-induced magnetization switching (CIMS) is the result of a direct transfer of spin angular momentum from the spin current carried along the charge current to the localized magnetic moment in the ferromagnet. Separation of the charge and spin components raises the possibility of chargeless pure spin-current-induced magnetization switching (pure spin CIMS).The pure spin current transfers only spin angular momentum, and thus provides an attractive means to manipulate the magnetic state in magnetic nanostructures as well as a quiet electrical background for experimental studies. The pure spin current I S can be generated by the diffusion of the accumulated spins 12-20 in a metallic lateral spin-valve (LSV) structure with non-local electrical spin injection, as shown in Fig. 1a. When the spin accumulation at the interface between the permalloy (Py) and Cu wires on the detector side is non-collinear to the Py magnetization, the transverse component of the pure spin CuAu Py

Kondo physics in non-local metallic spin transport devices

“…This non-local geometry has enabled significant advances in the understanding of spin transport in metals [4][5][6][7][8][9][10][11][12][13][14][15][16]18,22,[24][25][26][27][28][29][30][31][32][33][34][35][36][37] , but has also highlighted serious discrepancies with theory. The primary example of the latter is the surprising non-monotonicity in the temperature (T) dependence of DR NL , and thus the deduced l N , in the transparent interface limit.…”

“…Surface spin relaxation 10 , reduced dimensions 9 and surface 18 / bulk 8 (magnetic) impurities have all been advanced, but no consensus has emerged. One issue contributing to this is the focus on only a small set of materials, with Ni 80 Fe 20 /Cu and Ni 80 Fe 20 / Ag accounting for a major fraction of prior work 10,18,28,[30][31][32][33][34][35]39 .…”

Nanopillar junctions

“…For STT devices, lateral and non-local devices have been recently demonstrated [66]. These devices enable pure spin currents to be detected using a non-local spin valve structure.…”

“…To fabricate these devices, a multi-step process is required, which involves interface cleaning stages. For optimal device operation, fabrication methods in which no interface is exposed during the processing are desirable, such as the angled deposition method of Otani et al [66].…”

“…Rather than flowing through the second FM, the charge current flows out of the spacer layer and the spin current into the top FM layer. A pure spin current can also deliver spin torque [66] and hence can drive the magnetization of the top layer without any charge current flowing through it. No vertical non-local spin valve device has yet been demonstrated but such a device could be ideal for studying the role of interface in spin torque being transferred through pure spin current.…”