  Using a spin-polarized muon beam we were able to capture magnetic dynamics in
an amorphous niobium pentoxide thin film. Muons are used to probe internal
magnetic fields produced by defects. Magnetic fluctuations could be described
by the dynamical Kubo-Toyabe model considering a time-dependent local magnetic
field. We state that observed fluctuations result from the correlated motion of
electron spins. We expect that oxygen vacancies play a significant role in
these films and lead to a complex magnetic field distribution which is
non-stationary. The characteristic average rate of magnetic field change is on
the order of 100~MHz. The observed dynamics may provide insight into potential
noise sources in Nb-based superconducting devices, while also highlighting the
limitations imposed by amorphous oxides.
