The Role of Iron in Magnetic Damping of Mg(Al,Fe)2O4 Spinel Ferrite Thin Films

We have investigated magnesium aluminum ferrite thin films with a range of iron concentrations and identified the optimal iron content to obtain high crystalline quality thin films with the low magnetic damping required for spin current-based applications. Epitaxial MgAl2−x FexO4 films with 0.8 < x < 2.0 were grown by pulsed laser deposition on single crystal MgAl2O4 substrates and were characterized structurally and magnetically. We find that the x = 1.5 composition minimizes the room-temperature magnetic damping with a typical Gilbert damping parameter of αeff=1.8×10−3. This minimized damping is governed by a competition between the more robust magnetic ordering with increased iron content, x, and the more defective structure due to larger film-substrate lattice mismatch with increased iron content. The temperature-dependent magnetization curves indicate that Tc is suppressed below room temperature for iron content x≤1.2 and eventually suppressed entirely for x = 0.8. X-ray magnetic circular dichroism results indicate that for all x the magnetic moment is dominated by Fe3+ cations distributed in a 60:40 ratio on the octahedral and tetrahedral sites, with minimal contribution from Fe2+ cations. Films with x=1.4−1.6 exhibit very strong ferromagnetic resonance and low Gilbert damping with αeff=(1.8−6)×10−3, making them ideal candidates for microwave and spintronic applications.