Anisotropic frequency response of spin-torque oscillators with applied field polarity and direction

We have measured spin-torque-driven oscillations of Ni80Fe20 free-layer nanocontacts as a function of field direction. For a given field axis angle (10 degrees from the surface normal) and magnitude, simply changing the field polarity can significantly alter the device output. The critical current I-c, the frequency of oscillation f, the amplitude, and the f vs current f (I) all change with field polarity and change if the applied field axis is rotated about the surface normal. Spin-torque ferromagnetic resonance measurements show that the linear resonant frequency f and the slope of f (I) at currents much less than I-c also vary by tens of MHz and tens of MHz/mA, respectively, with field polarity. These results are consistent with the idea that the mode excited by spin torque interacts strongly with a different subregion of the magnetization in the vicinity of the contact for each field direction, resulting in potential variations in the anisotropy field, Oersted field, and spin-torque magnitude as a function of field direction.