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

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

Kowalska, E.; Sluka, V.; Fowley, C.; Aleksandrov, Y.; Bernert, K.; Lindner, J.; Fassbender, J.; Deac, A. M.

Spin-torque nano-oscillators (STNOs) are novel spintronics devices which may be exploited in order to design energy-efficient, frequency-tunable receivers and transmitters for wireless technology purposes [1-3]. Indeed, such devices are considerably smaller than conventional oscillators, having lateral dimensions in the range of a few tens of nanometers. Moreover, it has been demonstrated that STNOs can generate signals with high quality factors (in metallic point-contacts) and output powers in order of μW (in MgO-based magnetic tunnel junctions (MTJs) nanopillars). These levels are compatible with applications, thus justifying the interest which they have attracted as potential mobile Information-Telecommunication Technology devices.
To date, most studies focusing on spin-transfer driven dynamics have been carried out on devices with both the free and the reference layers magnetized in-plane. In this configuration, under application-desirable conditions (i.e., close to zero applied field), steady-state precession mainly occurs on clam-shell trajectories centered on the direction defined by the in-plane shape anisotropy. Consequently, only a fraction of the full magnetoresistance amplitude translates into the radio-frequency output power. However, it has been demonstrated that devices utilizing an in-plane (IP) magnetized polarizer (also acting as read-out layer) and out-of-plane (OOP) magnetized free layer allow for the full parallel-to-antiparallel resistance variation to be exploited in the limit of 90° precession angle [1]. In this particular geometry, it has been shown that steady-state precession can only be sustained if the spin-transfer torque exhibits an asymmetric dependence on the angle between the free and the polarizing layer [1].
Nevertheless, it has been very recently demonstrated experimentally that spin-transfer driven dynamics can also be sustained in similarly designed MgO-based MTJs, in spite of the fact that such devices do not exhibit any asymmetry in the spin-torque angular dependence [4-5].
Here, we explore potential mechanisms for sustaining steady-state precession in MgO-Based MTJs with IP polarizing and OOP free layer. To this end, we analytically solve the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation for a nano-pillar MTJ with circular cross-section, under a constant applied current (Fig. 1). We take into account both the in-plane and the field-like spin-torque terms, whose magnitude is determined not by the current, but by the corresponding voltage across the barrier. Steady-state precession can be sustained if the in-plane spin-torque term and the damping torque compensate over a full precession period.
Our results show that the stable dynamics occur only for negative current, for electrons flowing from the free to the reference layer (Fig. 2). According to our calculations, at small finite fields, the precession angle increases gradually from around zero to 90° with increasing current. High output powers (in the limit of 90° precession) can be obtained for relatively low values of applied current and field, which is beneficial from the point of view of STNOs application.


[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] T. Taniguchi, H. Arai, S. Tsunegi, et al., Applied Physics Express 6, 123003 (2013).
[5] H. Kubota, K. Yakushiji, A. Fukushima, et al., Applied Physics Express 6, 103003 (2013).

Keywords: spin-torque nano-oscillator (STNO); MgO-based magnetic tunnel junctions; tunnel magnetoresistance (TMR); spin dynamics

  • Poster
    INTERMAG 2014 - IEEE International Magnetics Conference, 04.-08.05.2014, Dresden, Germany

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Publ.-Id: 21402