Spin-transfer dynamics in magnetic tunnel junctions with an out-of-plane magnetized free layer and in-plane polarizer

Spin-torque nano-oscillators may be exploited for wireless communication applications [1-3]. It has recently been demonstrated that devices with an in-plane (IP) magnetized polarizer and out-of-plane (OOP) free layer allow for maximizing the output power, as the full parallel(P)-to-antiparallel (AP) resistance variation can be exploited in the limit of 90° precession angle [1]. However, in this geometry stable precession can only be sustained if the spin-transfer torque (STT) shows an asymmetric dependence on the angle between the free and the polarizing layer, like in metallic devices [1]. Nevertheless, recent experimental reports showed that spin-transfer driven dynamics can also be sustained in similarly designed MgO-based magnetic tunnel junctions (MTJs), with high output powers, but lacking an intrinsic STT angular asymmetry [4-5].
Here, we explore potential mechanisms for sustaining steady-state precession in MgO-MTJs with IP polarizer and OOP free layer. To this end, we analytically solve the Landau-Lifshitz-Gilbert-Slonczewski equation for a typical device with circular cross-section, under perpendicular applied fields and currents (Fig. 1). We assume that the precession trajectory is approximately circular around the direction perpendicular to the plane, as set by the effective field [1]. Since the magnitude of the STT is determined by the voltage across the barrier, at each point along the trajectory, we convert the current into an equivalent voltage value, taking into account the bias dependence of the resistance for the instant angle between the magnetic moments of the two layers. We assume the bias dependence of the AP state resistance to be linear, the P state resistance to be constant and a simple cosine angular dependence of the resistance with bias. We find that for constant current, the bias dependence of the resistance inherently induces an STT angular dependence asymmetry that it is sufficient to sustain precession and high output powers for relatively low values of applied current and field.
References:
[1] W. H. Rippard, A. M. Deac, M. R. Pufall, et al., Physical Review B 81, 014426 (2010).
[2] A. M. Deac, A. Fukushima, H. Kubota, et al., Nature Physics 4, 308 (2008).
[3] S. I. Kiselev, J. C. Sankey, I. N. Krivorotov, et al., Nature 425, 380 (2003).
[4] H. Kubota, K. Yakushiji, A. Fukushima, et al., Applied Physics Express 6, 103003 (2013).
[5] T. Taniguchi, H. Arai, S. Tsunegi, et al., Applied Physics Express 6, 123003 (2013).