The Influence of Substrate Treatment on the Growth Morphology and Magnetic Anisotropy of Epitaxial CrO₂ Films

Summary

Epitaxial CrO₂ thin films deposited on HF-cleaned TiO₂ (100) substrates exhibit very strong strain anisotropy, while those grown without the HF treatment step are essentially strain-free and display bulk-like magnetic properties. The HF treatment enhances the surface smoothness of the TiO₂ (100) substrates thus leading to the growth of epitaxially strained CrO₂ films. The magnetic easy axis of these films changes orientation with thickness, switching from the in-plane c-axis direction for thick films to the b-axis direction for thinner films (< 50 nm). Similarly, over a thickness range, a change of the easy axis direction is also observed with lowering temperature. Ion-beam irradiation of the substrate surface prior to growth also results in the growth of strained CrO₂ films, although the amount of strain is less than that observed for HF-treated substrates. The magnetic properties as a function of thickness have also been studied for as-deposited “thick” CrO₂ films that are slowly chemically etched down in thickness. Unlike as-grown thin films below 50 nm thickness that have the easy axis along the b direction, the chemically etched down CrO₂ thin films of equivalent thickness retain their easy axis alignment along the c-direction, but display a significantly enhanced coercivity. The observed differences in the switching behavior between the as-deposited films and those that are chemically etched can be qualitatively attributed to changes in the strain relaxation mechanism.