Evidence of anomalous switching of the in-plane magnetic easy axis with temperature in Fe ₃ O ₄ film on SrTiO ₃ :Nb by v-MOKE and ferromagnetic resonance

The evolution of the magnetic anisotropy directions has been studied in a magnetite (Fe₃O₄) thin film grown by infrared pulsed-laser deposition on SrTiO₃(100):Nb substrate. The magnetic easy axes at room temperature are found along the in-plane 〈100〉 film directions, which means a rotation of the easy axis by 45° with respect to the directions typically reported for bulk magnetite and films grown on single-crystal substrates. Moreover, when undergoing the Verwey transition temperature, T_V, the easy axis orientation evolves to the 〈110〉 film directions. This anomalous behavior has been demonstrated by measuring first the angular dependence of coercivity and remanence well above and below T_V by high-resolution vectorial magneto-optical Kerr effect (v-MOKE). Ferromagnetic resonance (FMR) measurements have additionally proven a well-defined fourfold magnetic anisotropy induced during growth with confirmed easy axis directions along 〈100〉 for T > T_V and 〈110〉 for T < T_V. These results provide a clear proof of the possibility of tuning magnetic anisotropy in Fe₃O₄ thin films by proper control on the growth parameters and substrate choice.