  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.
