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]. This phenomenon is currently intensively investigated due to its potential application in magnetic random access memory (MRAM) or in telecommunication devices. Presently, the structure of choice for spin-torque devices includes a magnetic tunnel junction (MTJ) with an MgO barrier, due to their large magnetoresistance signals. However, a key step towards the practical implementation as MRAM elements is the reduction of the critical voltages, in order to keep the size of the selection transistor down and compete with existing technologies [3]. In addition, magnetic tunnel junctions also exhibit a somewhat obscure behaviour referred to as ‘back-hopping’, whereby reliable switching to the desired state is achieved for applied voltages of the order of the critical voltage, but a larger applied bias induces a telegraph-noise behaviour [4, 5]. Back-hopping is characteristic for MTJs, as it has not been observed in metallic multilayers, and poses concerns for designing industrially-competitive MRAM devices.

We evaluate the switching voltages and their temperature dependence by analytically and numerically solving the modified Landau-Lifshitz-Gilbert equation which includes both Slonczewski-like (in-plane) and field-like (out-of-plane) torque terms for different geometries. Its quadratic dependence on the applied voltage [6] translates into a more complex correlation between the critical bias and the external field, altering the shape of the phase diagram. It also explains back-hopping at a large bias for specific geometries, in agreement with experimental results.

References:

[1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996)
[2] L. Berger, Phys. Rev. B 54, 9359 (1996)
[3] Z. Diao et al., J. Phys. D: Cond. Mat. 19, 165209 (2007)
[4] J. Z. Sun, J. Appl. Phys. 105, 07D109 (2009)
[5] T. Min et al., J. Appl. Phys. 105, 07D126 (2009)
[6] C. Heiliger and M. Stiles, Phys. Rev. Lett. 100, 186805 (2008)