Resonant damping in $Fe_{3} O_{4} $/Ag below the Verwey Transition

The temperature dependence of damping in epitaxial $Fe_{3} O_{4} $/Ag bilayers grown on [001] MgO substrates was investigated as a function of Ag thickness. The $Fe_{3} O_{4} $ layers were fixed at 350 nm thick, while the Ag thicknesses ranged from 0 to 500 nm. The epitaxial nature of the films was verified by in plane XRD of the $Fe_{3} O_{4} $ (311) and (220) directions. Ferromagnetic resonance (FMR) measurements at 9.2 GHz were carried out with the sample film normal to the applied magnetic field and at temperatures ranging from 30 to 295 K. All samples exhibited easy-plane anisotropy consistent with thin-film shape effects and a sample magnetization of approximately 400 $emu/cm^{3}$. Room temperature resonance line-widths were largely independent of Ag layer thickness and remained fairly constant with decreasing temperature until the Verwey transition ($T_{V} \simeq 110 K$), below which damping increases dramatically for all samples. Of particular note is the influence of the Ag layer thickness on the low temperature damping, wherein a peak in magnitude is observed at approximately 50 nm of Ag. This unexpected damping resonance will be discussed in terms of spin pumping into the Ag in conjunction with the changing $Fe_{3} O_{4} $magnetodynamics associated with the Verwey transition.