  We investigate the origin of the experimentally observed varying
current-frequency nonlinearity of the propagating spin wave mode in
nano-contact spin torque oscillators. Nominally identical devices with 100 nm
diameter are characterized by electrical microwave measurements and show large
variation in the generated frequency as a function of drive current. This
quantitative and qualitative device-to-device variation is described in terms
of continuous and discontinuous nonlinear transitions between linear current
intervals. The thin film grain microstructure in our samples is determined
using atomic force and scanning electron microscopy to be on the scale of 30
nm. Micromagnetic simulations show that the reflection of spin waves against
the grain boundaries results in standing wave resonance configurations. For a
simulated device with a single artificial grain, the frequency increases
linearly with the drive current until the decreased wavelength eventually
forces another spin wave anti-node to be formed. This transition results in a
discontinuous step in the frequency versus current relation. Simulations of
complete, randomly generated grain microstructures additionally shows
continuous nonlinearity and a resulting device-to-device variation in frequency
that is similar to the experimental levels. The impact of temperature from 4 K
to 300 K on the resonance mode-transition nonlinearity and frequency noise is
investigated using simulations and it is found that the peak levels of the
spectral linewidth as a function of drive current agrees quantitatively with
typical levels found in experiments at room temperature.
