  We study the static and dynamic magnetic properties of epitaxially strained
$\gamma$-$\mathrm{Fe_2O_3}$ (maghemite) thin films grown via pulsed-laser
deposition on MgO substrates by SQUID magnetometry and cryogenic broadband
ferromagnetic resonance experiments. SQUID magnetometry measurements reveal
hysteretic magnetization curves for magnetic fields applied both in- and out of
the sample plane. From the magnetization dynamics of our thin films, we find a
small negative effective magnetization in agreement with a strain induced
perpendicular magnetic anisotropy. Moreover, we observe a non-linear evolution
of the ferromagnetic resonance-linewidth as function of the microwave frequency
and explain this finding with a model based on slowly relaxing impurities, the
so-called slow relaxor model. By investigating the magnetization dynamics in
our maghemite thin films as a function of frequency and temperature, we can
isolate the temperature dependent contribution of the slowly relaxing
impurities to the resonance linewidth and, in particular, observe a sign change
in the effective magnetization. This finding provides evidence for a transition
of the magnetic anisotropy from a perpendicular easy axis to an easy in-plane
anisotropy for reduced temperatures.
