Low Damping in Spinel Ferrite Thin Films Enabled by Chemical Substitution*

Spinel ferrites are versatile magnetic insulators whose properties can be tuned by chemical substitution of their constituent elements. However, it has been a challenge to realize spinel ferrite thin films with sufficiently low damping for emerging spintronic applications. We achieve low damping in spinel ferrite films by substituting a large fraction of Fe with Al. Films of thickness $\approx $25 nm and nominal compositions Ni$_{\mathrm{0.65}}$Zn$_{\mathrm{0.35}}$Al$_{\mathrm{x}}$Fe$_{\mathrm{2-x}}$O$_{\mathrm{4}}$ are grown on single-crystal MgAl$_{\mathrm{2}}$O$_{\mathrm{4}}$(001) substrates by pulsed laser deposition. Fully coherent growth of Ni-Zn ferrite (x$=$0) on MgAl$_{\mathrm{2}}$O$_{\mathrm{4}}$ is elusive due to the large substrate-film lattice mismatch of \textgreater 3{\%}. High concentrations of Al (x$\ge $0.5) decrease the lattice constant of the ferrite such that the films are fully strained to the substrate and highly crystalline. As the Al concentration is increased from x$=$0.5 to 1.0, the Gilbert damping parameter $\alpha $ is reduced from $\approx $0.02 to $\approx $0.005, lower than $\alpha $ of permalloy. These low-damping spinel ferrite thin films facilitate the development of new spintronic devices based on insulating oxides.