Switching voltages and back-hopping in magnetic tunnel junctions with different geometries

A spin-polarized current flowing through a ferromagnet can exert a torque on the local magnetization [1,2], which induce switching or steady state precession. Spin-transfer switching can be used as writing scheme in magnetic random access memory (MRAM), while spin-torque-driven precession can be exploited to design RF oscillators for telecommunication devices. Presently, the majority of spin-torque devices are based on a magnetic tunnel junction (MTJ) with an MgO barrier. A key step towards the practical implementation as MRAM elements is the reduction of the critical voltages [3].
Several groups have reported that MTJs exhibit the so-called ‘back-hopping’, whereby reliable switching is achieved with voltages of the order of the switching voltage, while a larger applied bias induces a telegraph-noise behaviour [4,5]. Back-hopping is characteristic for MTJs, since it has not been observed in metallic multilayers, and raises concerns for designing industrially-competitive MRAM devices. Here, we demonstrate that a potential cause for this phenomenon is the field-like (out-of-plane) spin-torque, which has been found to be much larger in MgO-MTJs than in metallic spin-valves, where it can be neglected [6]. In MgO-MTJs, however, the field-like torque can be of the order of 30% of the in-plane torque [7], and needs to be taken into account. We evaluate the switching phase diagram by analytically and numerically solving the modified Landau-Lifshitz-Gilbert equation which includes both (in-plane) (Slonczewski-like) and field-like torque terms for different geometries. The quadratic dependence of the field-like torque on the applied voltage [8] translates into a more complex correlation between the critical bias and the external field, altering the shape of the phase diagram as demonstrated experimentally [9]. It also explains back-hopping at a large bias for specific geometries, in agreement with experimental results.

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[3] Z. Diao et al., J. Phys.: Cond. Mat. 19 (2007) 165209.
[4] J. Z. Sun et al., J. Appl. Phys. 105 (2009) 07D109.
[5] T. Min et al., J. Appl. Phys. 105 (2009) 07D126.
[6] M. A. Zimmler et al., Phys. Rev. B 70, 184438 (2004).
[7] J. C. Sankey et al., Nat. Phys. 4, 67 (2008).
[8] C. Heiliger and M. Stiles, Phys. Rev. Lett. 100 (2008) 186805.
[9] S.-C. Oh et al., Nat. Phys. 5, 898 (2009).