Enhancing the spin transfer torque in magnetic tunnel junctions by ac modulation

Abstract

The phenomenon of spin transfer torque (STT) has attracted a great deal of interests due to its promising prospects in practical spintronic devices. In this paper, we report a theoretical investigation of STT in a noncollinear magnetic tunnel junction under ac modulation based on the nonequilibrium Green's function formalism, and derive a closed-formulation for predicting the time-averaged STT. Using this formulation, the ac STT of a carbon-nanotube-based magnetic tunnel junction is analyzed. Under ac modulation, the low-bias linear (quadratic) dependence of the in-plane (out-of-plane) torque on bias still holds, and the sin θ dependence on the noncollinear angle is maintained. By photon-assisted tunneling, the bias-induced components of the in-plane and out-of-plane torques can be enhanced significantly, about 12 and 75 times, respectively. Our analysis reveals the condition for achieving optimized STT enhancement and suggests that ac modulation is a very effective way for electrical manipulation of STT.

Transient spin current under a thermal switch

“…Finally, we shall have a short discussion about the dependence of transient spin current on the noncollinear angle θ. In our model, the magnetization direction of lead R is along z, thus the x(y) component of the spin current actually corresponds to the in-plane (out-of-plane) spin transfer torque (STT), respectively [9]. For simplicity, we choose identical leads, i.e., the same coupling parameters of leads L and R, γ Lσ =γ Rσ , and focus on the transient spin current in lead R. Results are demonstrated in Fig.…”

“…Due to the retarded nature of Green's functions, all time-related quantities in A(ε, t) and B l (ε, t) vanish when t → ∞, which means that only the dc component persists to infinite time. Assuming that the initial population on the central QD is zero, we can prove that [9]…”

“…The general form of a spin current flowing from the central region to lead l is ( = 1) [9] J spin l (t) = − ss s kα∈l,n∈C Assuming that the hopping between lead l and the central region does not cause spin-flipping, i.e., t is ,js = t i,j δ ss is spin-independent, we have…”

“…Due to the simultaneous variation of the temperature, it is not clear about whether there is an intrinsic enhancement of spin currents in the transient regime or not. Although electrically-induced transient phenomena are well investigated for charge currents [13][14][15][16][17] and spin currents [9,18,19], little is known about thermally-induced transient spin currents.…”

“…This issue can be solved by using spintronic devices, which have the potential advantages of faster data processing speed, lower power consumption, and higher integration densities [1,2]. In realizing spintronic devices, a major challenge is to control and manipulate spin currents, which are useful for spin injection into semiconductors [3][4][5] and magnetic reorientation of ferromagnets [6][7][8][9].…”