Linear frequency modulation by weak bipolar magnetic fields for a vortex-mode oscillation in a nanocontact magnetoresistive spin-torque-oscillator

Spin torque oscillations near zero magnetic fields were investigated in a nanocontact magnetoresistive device. A typical auto-oscillation mode was observed with high amplitude and narrow linewidth in a vortex domain state of the free layer around the zero field. The number of branches was reduced to a single one with an increase of current over the threshold value. As a result, we observed a linear frequency change between a wider field range across zero fields with a high amplitude and a narrow linewidth for the oscillation. The frequency response to small bipolar applied fields is applicable to a weak-field sensor.