Magnetic resonance at the Verwey transition in epitaxial $Fe_3O_4$*

$Fe_3O_4$ is of interest due to its potential applications in the field of spintronics. Previous studies on magnetite films and $Fe_3O_4$/Cr/NiFe spin valves indicate the presence of a uniaxial anisotropy when the magnetite is grown in a magnetic field. Two 170 nm thick films of $Fe_3O_4$ were reactively sputtered simultaneously onto (100) MgO substrates without and with an applied field (100 Oe). Epitaxy was confirmed by in-plane x-ray diffraction and optimal oxygen stoichiometry is confirmed by the 119 K Verwey transition temperature ($T_V$). Ferromagnetic resonance (FMR) measurements at 35 GHz in the plane of the film revealed four-fold anisotropy confirming high quality (100) epitaxy, with an additional uniaxial contribution present in the field-grown sample. Temperature dependent FMR on the sample grown without field clearly reflects $T_V$ in the linewidth and in-plane and out-of-plane anisotropies. The in-plane uniaxial anisotropy for the sample grown with field exhibits an even stronger temperature response at $T_V$. Detailed discussions of thermal variations of magnetization, anisotropy and relaxation processes will be presented.