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

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) substrate 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 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 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. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.