Compositional Study of Low Damping Epitaxial Lithium Aluminum Ferrite Films

The development of ferromagnetic insulators with ultra-low damping and perpendicular magnetic anisotropy (PMA) is of great interest for the study of spin wave-based phenomena, as well as for application in spin wave spintronics. Recently, we have developed epitaxial thin films of the ferrimagnetic spinel structure Li_0.5(Al,Fe)_2.5O₄ (LAFO) with ultra-low magnetic damping. In this talk, we present the study of the effect of composition on the magnetic properties of LAFO by varying the Fe concentration, examining Li_0.5Al_(0.5)Fe_(2.0)O₄ (LAFO2.0) and Li_0.5Al_(1.0)Fe_(1.5)O₄ (LAFO1.5) films. Both compositions exhibit good epitaxy and excellent crystallinity on single crystal (001) oriented MgAl₂O₄ (MAO) substrates. X-ray diffraction indicates that both compositions are compressively strained in-plane, although to a lesser degree for LAFO1.5 as it has a smaller lattice parameter from higher Al doping. Static magnetometry reveals LAFO2.0 has disproportionately high saturation magnetisation of 200kA/m, more than twice that of LAFO1.5, suggesting a more detailed investigation of the Fe site distribution is in order. Both compositions show a low Gilbert damping parameter on the order of ~ 7x10^-4 for films 10-15nm thick in broadband ferromagnetic resonance (FMR). Stabilizing epitaxial, low damping LAFO films with variable composition, strains and magnetic properties make them promising for future spin wave spintronics applications.