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.