Spin-transfer-torque-induced rf oscillations in CoFeB/MgO/CoFeB magnetic tunnel junctions under a perpendicular magnetic field

We investigated spin-transfer-torque (STT)-induced rf oscillation with a magnetic field applied perpendicular to the plane of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs). For the rf measurement with the magnetic field normal to the MTJ plane, the magnetic field dependence of the peak frequency (f ₀), the linewidth (Δf), and the power spectral density (PSD) were well interpreted as behavior in a linear regime, which showed the minima of f₀ and Δf and the maximum of PSD at the demagnetizing field of the free layer. However, no clear onset of STT-induced rf oscillation was observed even with a large bias current applied. Under the magnetic field slightly tilted from the normal direction, on the other hand, the spectral shape strongly depended on the bias current. With the current in the direction to stimulate STT-induced rf oscillation, a rapid blueshift of f₀ and an abrupt increase in PSD were observed together with a remarkable broadening of Δf, indicating that STT-induced rf oscillation was achieved. The critical current for STT-induced rf oscillation estimated from the variation in Δf was also in good agreement with the bias current at which the peak of the second harmonics of f₀ appeared in the rf spectrum.