Synthesis and morphological stability in CrO2 single crystals of a half-metallic ferromagnetic compound

Abstract

Single crystals of a half-metallic ferromagnetic compound CrO2 were synthesized using controlled thermal evaporation of CrO3 in an autoclave at 1.5 bar pressure at moderate temperature 673 K. These CrO2 crystals lay in a Rutile tetragonal crystal structure of D144h:P42/mnm space group, with lattice parameters a = 0.4423 nm and c = 0.2918 nm. The lattice volume 0.0571 nm3 determines 4.88 g/cm3 density, assuming a lattice consisting of two CrO2 formula units. Scanning electron microscopic images reveal rectangular bars of single CrO2 crystals of 6.0-8.0 μm lengths and 1.0-1.5 μm widths. When heating in ambient air pressure the crystals relieve part of the oxygen according to the CrO2 → Cr2O3 phase transformation with a sharp peak in the first derivative of the thermogravimetric curve at temperature as early as 570 K, with a partial mass loss of 1.2%. A complete transformation appears around 775 K, with 3.0% mass loss. The CrO2 → Cr2O3 conversion reflects in a sharp endothermic peak in DTA at 576 K. The CrO2 crystals have the saturation magnetization of 68 emu/g, coercivity ~340 Oe, and remanent magnetization ~ 61 emu/g at room temperature.