High oxygen pressure synthesis, magnetic properties, and magnetic resonance of pure CrO2 powders

The purity of ferromagnetic chromium dioxide (CrO2) is critical for its applications and fundamental research. However, the presence of Cr2O3 contamination is frequently observed in CrO2 obtained commercially or prepared under ambient or low oxygen pressures. High oxygen pressure is crucial to keep CrO2 thermodynamically stable during preparation. In this study, the synthesis conditions for pure CrO2 were optimized by controlling oxygen pressure, temperature, and reaction time. Rod-like CrO2 particles with straight edges and an average length of 5 μm were successfully synthesized under an oxygen pressure of 13 MPa. Different from commercial CrO2 particles, no Cr2O3 layer was observed over the surface of the prepared CrO2 particles. The CrO2 powders showed a much lower coercivity and a much higher saturation magnetization than that of commercial CrO2, indicating high purity of the samples. X-ray photoelectron spectroscopy analysis further confirmed a +4-oxidation state of Cr on the surface of the CrO2 products. A symmetric derivative line with a relatively broad linewidth of 30.1 mT and a shifted g-factor of 1.96 was observed via electron paramagnetic resonance in CrO2 at 405 K, indicating strong electron correlations within the system. This study optimized the preparation process for large scale synthesis of pure CrO2.