Extremely High Ferromagnetic Resonance Frequency Induced by Triclinic Lattice Distortion in Epitaxial FeCo/MgAl₂O₄ (001) Films

Theoretically, tetragonal lattice distortion of FeCo epitaxial films can result in a very large in-plane magnetic anisotropy field, leading to an extremely high ferromagnetic resonance (FMR) frequency. Herein, Fe 75 Co 25 $\left(\text{Fe}\right)_{75} \left(\text{Co}\right)_{25}$ thin films are epitaxially grown on (001) MgAl2O4 single-crystal substrates. A triclinic lattice distortion with a & NOTEQUAL;b & NOTEQUAL;c $a \neq b \neq c$ , instead of a tetragonal one, is found in the FeCo films. The cubic symmetry breaking leads to a deviation of easy axes from the 100 $100$ directions, forming a distribution of magnetic moments with a strong perpendicular magnetic anisotropy (PMA) along the out-of-plane [001] directions and a deviation of the in-plane components from the ([10 100]) directions. The effective field of the former is as high as 1.5-2.5 T, enough to overcome the thin film shape anisotropy, while that of the latter stays at a low value of around 0.05 T. The strain-induced PMA gradually relaxes to in-plane for thicker films with a strained sublayer remaining. As a result, an extremely high out-of-plane FMR frequency over 40 GHz is achieved, accompanied by a lower in-plane FMR frequency around 8 GHz. This study provides a possible approach to prepare self-biased soft magnetic films with extremely high-resonance frequency for applications in microwave-integrated circuits.