Cr 2 O 3 nanoparticles were prepared via one-step reactive laser ablation of Cr in oxygen. The metastable CrO 2 phase was obtained through the subsequent oxidation of Cr 2 O 3 nanoparticles under O 2 with gas pressures of up to 40 MPa. The as-prepared Cr 2 O 3 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 CrO 2 from as-dried Cr 2 O 3 nanoparticles, and the Cr 2 O 3 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 Cr 2 O 3 /CrO 2 nanoparticles exhibit an enhanced saturation magnetization and a reduced exchange bias with an increasing faction of CrO 2 due to the elimination of uncompensated surface spins over the Cr 2 O 3 nanoparticles when exposed to a high pressure of O 2 and/or possible phase segregation that results in a smaller grain size for both Cr 2 O 3 and CrO 2 .

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