Microwave oscillations in magnetic tunnel junction of hybrid geometry
Spin-torque nano oscillators (STNOs) [1-7] for the microwave technology are one of the key applications of spin-transfer torque (STT) related phenomena. It has been demonstrated that STNO with hybrid geometry [4,6-8], i. e. utilizing an in-plane magnetized fixed layer and an out-of-plane free layer, allow for maximizing the output power, as the full parallel-to-antiparallel resistance variation can be exploited in the limit of 90° precession angle . In addition, this confguration maximises the output power as it favours large angle precession [6,7], helps to reduce the critical current , and can provide functionality regardless of applied magnetic field or current history [4,7]. State-of-the-art devices, exploiting MgO-based STNOs [10,11], can exhibit output power orders of magnitude higher (as high as µW [3,12]) over their fully metallic GMR predecessors (limited to a few nW), all the while being about 50 times smaller than present devices used in mobile telecommunication . TMR devices further benefit from lower operational current density of order 1 MA/cm² [6,13,14] (one order of magnitude lower as compared to the case of metallic spin valves [1,2]).
Within this project, we experimentally and theoretically investigate STT-driven dynamics in MgO-based MTJs of hybrid geometry (Fig. 1), taking into account the voltage bias dependencies of the magnetoresistance (Fig. 2 b) and the spin-transfer torque. In our theoretical studies, we assume that the spin-transfer torque asymmetry results from the cosine-type angular dependence of the tunnel magnetoresistance ratio, and try to confirm experimentally that it is in fact the responsible mechanism for sustaining the precession. Moreover, we demonstrate that the reduction in the TMR due to its bias dependence suppresses the STT angular dependence asymmetry, and thus the dynamics itself.
Fig. 1. Principle of the microwaves generation in a spin-torque nano oscillator based on the hybrid geometry magnetic tunnel junction. The free layer (M_free) has an out-of-plane easy axis, and the fixed layer (M_fixed) is magnetized in the sample plane. Via the spin-transfer torque, an applied d.c. current induces precession of the free layer magnetization resulting in an alternating tunnel magnetoresistance (TMR), and thus in the generation of radio-frequency (RF) voltage.
Fig. 2. Electrical characteristics of the hybrid geometry STNO. (a), Magnetoresistance curves measured at an in-plane and an out-of-plane magnetic field. The magnetization directions of the “free” and the “fixed” layers are depicted by the upper and lower arrows, respectively. (b), Resistance versus bias voltage showing the linear decrease in the resistance for the antiparallel state and approximately constant resistance for the parallel state. Sample supplied by collaborators from the AIST, Tsukuba, Japan.
Fig. 3. Frequency spectra of STNO sample versus d.c. current showing a decrease of the frequency, as well as an increase of the signal magnitude, with increasing current. Measurement was conducted at constant magnetic field of 30 mT. Sample supplied by collaborators from the AIST, Tsukuba, Japan.
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