Structure and Magnetic Properties of Cr2O3/CrO2 Nanoparticles Prepared by Reactive Laser Ablation and Oxidation under High Pressure of Oxygen

Cr2O3 nanoparticles were prepared via one-step reactive laser ablation of Cr in oxygen. The metastable CrO2 phase was obtained through the subsequent oxidation of Cr2O3 nanoparticles under O-2 with gas pressures of up to 40 MPa. The as-prepared Cr2O3 nanoparticles are spherical or rectangular in shape with sizes ranging from 20 nm to 50 nm. High oxygen pressure annealing is effective in producing meta-stable CrO2 from as-dried Cr2O3 nanoparticles, and the Cr2O3 nanoparticles exhibit a weak ferromagnetic behavior with an exchange bias of up to 11 mT that can be ascribed to the interfacial exchange coupling between uncompensated surface spins and the antiferromagnetic core. The Cr2O3/CrO2 nanoparticles exhibit an enhanced saturation magnetization and a reduced exchange bias with an increasing faction of CrO2 due to the elimination of uncompensated surface spins over the Cr2O3 nanoparticles when exposed to a high pressure of O-2 and/or possible phase segregation that results in a smaller grain size for both Cr2O3 and CrO2.