  We investigated the dynamics of a novel design of spin torque oscillator
(STO) for microwave assisted magnetic recording. Using Ni$_{80}$Fe$_{20}$
(NiFe) as the polarizer and Fe$_{67}$Co$_{33}$ (FeCo) as the field generating
layer, we experimentally observed the magnetization reversal of NiFe, followed
by multiple signals in the power spectra as the bias voltage increased. The
signals reflected the out-of-plane precession (OPP) mode oscillation of both
FeCo and NiFe, as well as the magnetoresistance effect of the STO device, which
had the frequency equal to the difference between the oscillation frequency of
NiFe and FeCo. Such dynamics were reproduced by micromagnetic simulation. In
addition to the merit of realizing the OPP mode oscillation with a simple and
thin structure suitable for a narrow gap recording head, the experimental
results using this design suggested that a large cone angle of $\sim$
70$^{\circ}$ for the OPP mode oscillation of FeCo was achieved, which was
estimated based on the macrospin model.
