Abstract
We theoretically investigate the influence of the field-like spin torque term on the oscillation characteristics of spin transfer oscillators, which are based on MgO magnetic tunnel junctions (MTJs) consisting of a perpendicular magnetized free layer and an in-plane magnetized pinned layer. It is demonstrated that the field-like torque has a strong impact on the steady-state precession current region and the oscillation frequency. In particular, the steady-state precession can occur at zero applied magnetic field when the ratio between the field-like torque and the spin transfer torque takes up a negative value. In addition, the dependence of the oscillation properties on the junction sizes has also been analyzed. The results indicate that this compact structure of spin transfer oscillator without the applied magnetic field is practicable under certain conditions, and it may be a promising configuration for the new generation of on-chip oscillators.
Highlights
We theoretically investigate the influence of the field-like spin torque term on the oscillation characteristics of spin transfer oscillators, which are based on MgO magnetic tunnel junctions (MTJs) consisting of a perpendicular magnetized free layer and an in-plane magnetized pinned layer
The results indicate that this compact structure of spin transfer oscillator without the applied magnetic field is practicable under certain conditions, and it may be a promising configuration for the new generation of on-chip oscillators
For the finite-sized free layer considered here, the components of N are calculated in the following simulations as Nx = Ny = 0.0256 and Nz = 0.9488, and this leads to Hd = 16.8 kOe, Hk = 0 kOe
Summary
Since the initial theoretical predictions[1,2] and subsequent experimental observations[3,4,5,6] of the magnetization dynamics driven by spin transfer torque (STT), there has aroused tremendous and continuous research interest.[7,8,9,10,11,12] The phenomenon of the spin transfer can be used to switch the magnetization from one static configuration to another one[4,5] or produce a steady precession of magnetization.[6,7,8] In particular, this steady-state precession can generate an AC voltage oscillation with the same frequency, that known as spin-torque oscillators (STOs). The STOs have high frequency microwave, large tuning range via applied field or current,[13] high modulation rates,[14] small footprints, and the same compatibility with the standard complementary metal-oxide-semiconductor process. This structure was able to exhibit a reduced threshold current, a high power and a high Q factor.[28,29] It has been theoretically demonstrated that this configuration is capable of generating microwave signal in zero applied magnetic field by taking into account the field-like torque.[30] the dependence of oscillation behavior on the field-like torque still remains unclear. The dependence of the oscillation behavior on the junction sizes has been analyzed; it was found that the critical current Jc and the corresponding oscillation frequency decrease with increasing junction surface area
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