Low Damping Spinel Ferrites for Spin Pumping

Spin pumping phenomena at ferromagnet/metal interfaces have extensive applications ranging from electronic control of magnetic orientation to generation and manipulation of pure spin currents. Few spin pumping experiments have utilized spinel ferrites for the magnetic layer due to their typically large damping. Here we report on the spin pumping in low damping spinel ferrite based bilayers. (Ni,Zn)AlFeO$_{\mathrm{4}}$ thin films exhibit a damping factor as low as 3x10$^{\mathrm{-3}}$. These films also exhibit large negative perpendicular magnetic anisotropy of \textgreater 1 T, leading to low resonance fields three times smaller at 10 GHz than the prototypical spin pumping ferrimagnet Y$_{\mathrm{3}}$Fe$_{\mathrm{5}}$O$_{\mathrm{12}}$ (YIG). Upon addition of only 1.5 nm of a Pt film, Gilbert damping more than doubles, and we observe significant in-plane DC voltage during ferromagnetic resonance. These two effects indicate a large amount of spin pumping from the ferrite to Pt with an estimated effective spin mixing conductance of 3x10$^{\mathrm{18\thinspace }}$m$^{\mathrm{-2}}$, comparable to that reported for YIG/Pt. This system demonstrates the promise of spin pumping phenomena in the spinel ferrite family of materials.