Influence of substrate treatment on the growth morphology and magnetic anisotropy of epitaxial CrO2 films

Epitaxial CrO 2 thin films deposited on HF-cleaned TiO 2 (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 TiO 2 (100) substrate thus leading to the growth of epitaxially strained CrO 2 films. The magnetic easy axis of these films changes orientation with thickness, switching from the in-plane c-axis direction for relatively 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 CrO 2 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’ CrO 2 films that are slowly chemically etched down in thickness. Unlike asgrown thin films below 50 nm thickness that have the easy axis along the b-direction, the chemically etched down CrO 2 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.