Aluminum substitution in low damping epitaxial lithium ferrite films

The development of ferromagnetic insulators with ultra-low damping and tunable magnetic properties 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 two compositions of LAFO with different Al substitution concentrations. We examine Li0.5Al_(0.5)Fe_(2.0)O₄ (LAFO x=0.5) and Li_0.5Al_(1.0)Fe_(1.5)O₄ (LAFO x=1.0) films and find that the degree of Al substitution significantly tunes the saturation magnetization and in-plane magnetic anisotropy while maintaining low magnetic damping with Gilbert damping parameter on the order of ~ 7x10^-4 for films 10-15 nm thick measured via broadband ferromagnetic resonance (FMR). Element-specific X-ray magnetic circular dichroism measurements confirm that the magnetism originates from the majority of Fe³⁺ cations occupying the octahedral sites, which are displaced by increased Al3+ substitution. High-resolution X-ray diffraction and reciprocal space maps show that both compositions are under compressive strain and exhibit excellent epitaxy and crystallinity on single crystal (001) oriented MgAl₂O₄ (MAO) substrates in films up to ~35 nm. Stabilizing epitaxial, low damping Li_0.5(Al,Fe)_2.5O₄ films with variable composition, strains and magnetic properties make them very promising for future spin-wave spintronics applications.