Theoretical Study of Spin-torque Oscillator Coupled with Nano-magnet by Dipole-dipole Interaction

The dynamics of a spin-torque-oscillator (STO) coupled with a nano-magnet through dipole-dipole interaction was studied numerically by using the macrospin model for the application of the STO as a read head sensor of hard disk drives. We found that the current, which is required to induce the oscillation of the free-layer (FL) of the STO, depends strongly on the distance between the FL and the nano-magnet as well as on the relative orientation of the magnetizations between them. To determine the dynamics of the STO it is indispensable to consider the dynamics of the dipole-coupled nano-magnet. We showed that we could detect the orientation of the magnetization of a nano-magnet, or a recording-bit, by the modulation of the oscillation frequency of the STO.