Tailoring the Lithium Concentration in Thin Lithium Ferrite Films Obtained by Dual Ion Beam Sputtering

Thin films of lithium spinel ferrite, LiFe₅O₈, have attracted much scientific attention because of their potential for efficient excitation, the manipulation and propagation of spin currents due to their insulating character, high-saturation magnetization, and Curie temperature, as well as their ultra-low damping value. In addition, LiFe₅O₈ is currently one of the most interesting materials in terms of developing spintronic devices based on the ionic control of magnetism, for which it is crucial to control the lithium's atomic content. In this work, we demonstrate that dual ion beam sputtering is a suitable technique to tailor the lithium content of thin films of lithium ferrite (LFO) by using the different energies of the assisting ion beam formed by Ar⁺ and O₂⁺ ions during the growth process. Without assistance, a disordered rock-salt LFO phase (i.e., LiFeO₂) can be identified as the principal phase. Under beam assistance, highly out-of-plane-oriented (111) thin LFO films have been obtained on (0001) Al₂O₃ substrates with a disordered spinel structure as the main phase and with lithium concentrations higher and lower than the stoichiometric spinel phase, i.e., LiFe₅O₈. After post-annealing of the films at 1025 K, a highly ordered ferromagnetic spinel LFO phase was found when the lithium concentration was higher than the stoichiometric value. With lower lithium contents, the antiferromagnetic hematite (α-Fe₂O₃) phase emerged and coexisted in films with the ferromagnetic Li_xFe_6-xO₈. These results open up the possibility of controlling the properties of thin lithium ferrite-based films to enable their use in advanced spintronic devices.