  The self-oscillation of the magnetization in a spin torque oscillator (STO)
with a perpendicularly magnetized free layer and an in-plane magnetized pinned
layer in the absence of an applied magnetic field was studied by numerically
solving the Landau-Lifshitz-Gilbert equation. It was pointed out that
field-like torque was necessary to realize stable self-oscillation in this type
of STO at zero field. The numerical simulation at finite temperature showed
that the presence of the field-like torque led to a high power with a
relatively high oscillation frequency.
