Spin Torque Effect in Magnetic Low-Frequency Noise Due to Asymmetric Oscillation

In this letter, the influence of spin-transfer torque (STT) on magnetic read sensor noise in the test band of 1-200 MHz has been investigated experimentally using the macrospin model with STT term. The results reveal a dependence of the integrated noise power on the direction of bias current when an external field is applied. In a positive field direction, positive bias current produces stronger noise than negative current. In the presence of negative field direction, the relationship is reversed. The hypothesis of asymmetric oscillation noise modification by STT is confirmed by corresponding changes of a correlation between the signal in the test band, and amplitude fluctuations of thermally driven ferromagnetic resonance in the sensor free layer, both derived from high-bandwidth waveforms. The effect is a consequence of the spin torque impact on the effective damping constant, which adjusts the shape of the asymmetric oscillation spectrum in a test band.