Spin-transfer switching in an epitaxial spin-valve nanopillar with a full-Heusler Co2FeAl0.5Si0.5 alloy

Abstract

We report magnetization switching by spin-transfer torque in an epitaxial spin-valve nanopillar made with a half-metallic full-Heusler Co2FeAl0.5Si0.5 (CFAS) alloy. The CFAS/Ag/CFAS spin valves showed a magnetoresistance ratio of 7%–9%, and spin-transfer switching was clearly observed in the nanopillar by applying a relatively small dc current (∼106 A/cm2 in current density). Statistical analysis based on a thermal activation model revealed an averaged critical current density (Jc0) of 9.3×106 A/cm2 with a thermal stability factor (KV/kBT) of ∼40.

Micromagnetic simulation of spin-transfer switching in a full-Heusler Co2FeAl0.5Si0.5 alloy spin-valve nanopillar

“…Furubayashi et al 15 found a large magnetoresistance ratio of 6.9% at room temperature (RT) for Co 2 FeAl 0.5 Si 0.5 (CFAS)/ Ag/CFAS spin-valves. Sukegawa et al 16 showed that the resistance-current curves of Co 2 FeAl 0.5 Si 0.5 spin-valves exhibited a two-step switching process, which is similar to the behavior of the Fe/Ag/Fe nanopillar 17 and originated from the interplay between the magnetocrystalline anisotropy of CFAS layers and STT. Unlike most cases, for both Co 2 FeAl 0.5 Si 0.5 and Fe/Ag/Fe spin-valves it is believed that there exists an intermediate (I) state, besides the parallel (P) and antiparallel (AP) spin configurations, with the direction of the spins perpendicular to their original and final states when an appropriate negative current is applied.…”

“…1, we studied a spin-valve device with geometry similar to the structure of the spin-valve in Ref. 16 [CFAS (20 nm)/Ag (4 nm)/CFAS (2.5 nm) of elliptical cross-section area (250 Â 190 nm 2 )]. We employ a Cartesian coordinate system where the x-axis is the long axis of the ellipse along the CFAS [110] direction (easy axis) and the y-axis is along the short axis ([1 dynamics are described using a generalized Landau-LifshitzGilbert-Slonczewski (LLGS) equation, 1,2 which can be written as…”

“…The magnetic parameters are adopted 16 as follows: saturation magnetization M s ¼ 9.0 Â 10 5 A/m, exchange constant A ¼ 2.0 Â 10 À11 J/m, Gilbert damping parameter a ¼ 0.01, spin polarization factor g ¼ 0.76, and magnetocrystalline anisotropy constant K 1 ¼ À1.0 Â 10 4 J/m 3 . The initial magnetizations of free and pinned layers are along the Àx axis and þx axis, respectively.…”

“…It was hypothesized that those two switching processes were caused by different switching mechanisms. 16 The detailed features and full understanding of the switching mechanism of Heusler alloy spin-valves call for further investigations.…”

“…The critical current density J c is 9.0 Â 10 6 A/cm 2 , consistent with the experimental value of 9.3 Â 10 6 A/cm 2 . 16 Furthermore, the switching time was discussed and the mechanism of spin-transfer switching was elucidated based on the magnetization trajectory and magnetization distribution.…”