Bias-voltage dependence of perpendicular spin-transfer torque in asymmetric MgO-based magnetic tunnel junctions

Abstract

1,2(STT) allows the electrical control of magnetic states in nanostructures [3][4][5] . The STT in magnetic tunnel junctions (MTJs) is of particular importance owing to its potential for device applications 6,7 . It has been demonstrated [8][9][10][11] that the MTJ has a sizable perpendicular STT (τ ⊥ , field-like torque), which substantially affects STT-driven magnetization dynamics. In contrast to symmetric MTJs where the bias dependence of τ ⊥ is quadratic [8][9][10]12,13 , it is theoretically predicted that the symmetry breaking of the system causes an extra linear bias dependence 11. Here, we report experimental results that are consistent with the predicted linear bias dependence in asymmetric MTJs. The linear contribution is quite significant and its sign changes from positive to negative as the asymmetry is modified. This result opens a way to design the bias dependence of the field-like term, which is useful for device applications by allowing, in particular, the suppression of the abnormal switching-back phenomena.The STT is composed of two vector components (Fig. 1a): the in-plane torque (τ ) and the perpendicular torque (τ ⊥ , also called 'field-like torque') normal to τ . Whereas τ ⊥ in fully metallic nanopillars is negligible and has been ignored in the analysis of experimental data 14 , τ ⊥ in MgO-based MTJs can be 10 ∼ 30% of τ (refs 8-10). Previous theoretical 12,13 and experimental 8-10 studies indicate that τ ⊥ is a symmetric function of the voltage V at low voltages;where the bias-independent contribution C 0 is also known as the interlayer exchange coupling 15,16 . A more recent theoretical study 11 indicates however that the symmetric bias dependence is expected only in symmetric MTJs and that extra antisymmetric components may appear ( (MTJ2) is 175%/117% (195%/123%) at 4.2 K/300 K (Fig. 1b). Owing to the asymmetry in the MTJs, the current-voltage characteristics are not symmetric even in the parallel magnetic configuration (P state) (Fig. 1c). Note that (d 2 I /dV 2 ) P at V = 0 has opposite sign for the MTJ1 and MTJ2, reflecting the different asymmetries in the two MTJs.To determine the bias dependence of τ ⊥ , we used the thermal activation model 17,18 ,where the upper (lower) signs apply to the antiparallel-to-parallel (parallel-to-antiparallel) switching, t ± is the relaxation time, f 0 is the attempt frequency (= 10 9 s −1 ), k B is the Boltzmann constant, T * is the junction temperature taking into account the bias-induced heating and V ± C is the critical voltage for magnetization switching at T * = 0 K. Here, the factor (1 ∓ V /|V ± C |) arises from the voltage dependence of the energy barrier due to the in-plane torque τ . The junction temperature T * is obtained from commonly used estimation methods (see the Methods section and Supplementary Note S1). Here, E ± B readswhere E 0 B is the intrinsic energy barrier, H ext is the external magnetic field applied along the easy axis, H sh is the shift field owing to the orange-peel coupling 19 as well as the interlayer exchange couplin...

Anomalous temperature dependence of current-induced torques inCoFeB/MgOheterostructures with Ta-based underlayers

“…Recently, it has been shown [11][12][13][14] that the spin orbit torque also possesses a damping-like torque 6,7 and a fieldlike torque 15 , analogous to the spin transfer torque in magnetic tunnel junctions [16][17][18][19][20] . In many systems, however, the field-like torque is much larger than the damping-like torque 11,14 , and its direction is pointing opposite to the incoming spin direction of the electrons (assuming the spin source is the spin Hall effect) 11,14,21 .…”

Estimation of thermal stability factor and intrinsic switching current from switching distributions in spin-transfer-torque devices with out-of-plane magnetic anisotropy

“…9,10 For devices with in-plane magnetization, the exponent n is set to 1. In the limit t p τ 1, equations (1) and (2) can be combined to express the switching probability as:…”

Spin-transfer-torque efficiency enhanced by edge-damage of perpendicular magnetic random access memories

“…The time step for integration is 0.05 ps and all calculations are performed at T=0K. 4 We use following parameters for an undamaged cell: the perpendicular magnetic anisotropy energy density K u is 10 7 erg/cm 3 , M S is 1000 emu/cm 3 , and the exchange stiffness constant A ex is 10 -6 erg/cm. We model the edge-damaged region around the rim with the width of 5 nm (see Fig.…”

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