Structure and Magnetic Properties of Cr₂O₃/CrO₂Nanoparticles Prepared by Reactive Laser Ablation and Oxidation under High Pressure of Oxygen

Abstract

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 .

Magnetic Li–M (M = Ni, Ni0.8Cu0.2, Cr) layered oxides nanoparticles for Li-ion batteries electrodes

“…The ZFC M(T) measurement (figure 6(a)) shows the presence of a magnetic transition at about 365 K (inset of figure 6(a)) and a second transition slightly above 60 K. The transition at 365 K could be related to CrO 2 , which is a known ferromagnet, although the Curie temperature of such phase is rather higher: T C = 396 K [37]. However, it has to be considered that a system of Cr 2 O 3 /CrO 2 nanoparticles is reported in literature to show a Curie temperature T C = 386 K [38], thus meaning that a size effect could reduce the transition temperature. The kink at about 300 K could represent instead a signal of the possible presence of Cr 2 O 3 , which is a canted (or frustrated) antiferromagnet with a Néel temperature T N = 306 K, although its contribution is expected to be hindered by the overwhelming role of the ferromagnetic phase.…”