Spin-transfer effects in MgO-based tunnel junctions with an out-of-plane free layer and in-plane polarizer


Spin-transfer effects in MgO-based tunnel junctions with an out-of-plane free layer and in-plane polarizer

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

Spin-torque nano-oscillators (STNOs) are novel devices which may be exploited for wireless communication applications [1-3]. In particular, it has recently been demonstrated that STNOs 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 (P)-to-antiparallel (AP) resistance variation to be exploited in the limit of 90° precession angle, thereby maximizing the output power [1]. However, for this specific geometry, 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, such as in the case of fully metallic devices [1]. Nevertheless, it has recently been reported that dynamics have been experimentally observed in similarly designed MgO-based MTJs under constant applied electrical current, in spite of the fact that such devices do not exhibit any asymmetry in the spin-torque angular dependence [4,5]. These results have so far been interpreted based on the formalism for metallic devices, including the spin-torque angular dependence.
Here, we explore potential mechanisms for sustaining steady-state precession in MgO-based MTJs with an IP polarizer and an OOP free layer. To this end, we analytically and numerically solve the Landau-Lifshitz-Gilbert-Slonczewski equation for a nano-pillar MTJ with circular cross-section, under a constant perpendicular applied current and field. To sustain steady-state precession, the energy supplied by the in-plane spin-torque term and energy dissipated through damping must compensate over a full precession period.
In an MgO-MTJ, the magnitude of the STT is determined by the voltage across the barrier [6]. As the magnetization of the free layer precesses around the put-of-plane direction, the angle between the magnetic moments of the two layers changes and through the magnetoresistance effect the voltage changes if the experiment is conducted at constant applied current [7,8]. This cosine-like angular dependence of the MTJ resistance effectively introduces a spin-torque angle dependence asymmetry. In addition, for a given angle, the resistance exhibits a specific bias dependence, with the resistance of the AP state decreasing approximately linearly with increasing bias, while remaining mostly constant in the P configuration. In this work, we demonstrate that the spin-torque angular asymmetry exhibited in such systems is sufficient to sustain STT-driven dynamics.
Fig. 1 shows dynamic and static phase diagrams of the STNO obtained when neglecting (Fig. 1(a) and 1(c)) and taking into account (Fig. 1(b) and 1(d)) the bias dependence of the AP state resistance. In both cases, stable dynamics occur only for positive currents (colored area), defined as electrons flowing from the free to the reference layer. In MTJs exhibiting no bias dependence of the resistance (dRAP/dV = 0 Ω/V), the onset current for steady-state dynamics (solid lines) scales linearly with the applied current. High output powers can be obtained for relatively low values of applied currents and fields for realistic MTJ parameters, which is beneficial from the point of view of applications. Introducing an experimentally realistic value of dRAP/dV affects mostly the steady-state dynamics, while most of the trends observed for static states are maintained (Fig. 1(b) and 1(d)). Indeed, in this case current-driven precession is only excited for fields lower than the effective anisotropy of the free layer (but still only for positive currents). Moreover, while the symmetry versus field sign is conserved, the onset current no longer increases linearly with the field, but rather exhibits a parabolic-like dependence.

Keywords: spin-torque nano-oscillators (STNOs); magnetic tunnel junctions (MTJs)

  • Poster
    International Colloquium on Magnetic Films and Surfaces (ICMFS 2015), 12.-17.07.2015, Cracow, Poland

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Publ.-Id: 22252