Magnetic properties of thin CrO2 layers supported on polycrystalline TiO2

Thin layers of CrO2 have been prepared on polycrystalline rutile TiO2 supports by thermal decomposition of wet-impregnated Cr(NO3)3 in flowing oxygen under ambient pressure conditions. The nanocomposite material has been characterised by a range of techniques including X-ray powder diffraction and X-ray photoelectron spectroscopy; and magnetic properties have been investigated by SQUID magnetometry. The loading of the nitrate on the support is shown to provide a simple way of controlling the CrO2 thickness. The saturation magnetisation and the coercivity, and their variation with temperature, show a strong dependence on the layer thickness. The thinnest (1.4 nm) layers investigated display essentially superparamagnetic behaviour with a small magnetic hysteresis even at 5 K and a magnetisation that scales with H/T. By contrast, thicker (20 nm) layers show magnetic behaviour similar to that of bulk CrO2. There is a critical upper CrO2 thickness beyond which support assisted promotion of the formation of the thermodynamically unstable CrO2 breaks down.