Cr 2 O 3 surface layer and exchange bias in an acicular CrO2 particle

It has been found by electron diffraction and high-resolution transmission electron microscopy that each commercial CrO2 particle is enclosed by an epitaxial Cr2O3 layer ∼2.5 nm thick. The epitaxial relationships of Cr2O3 layers on CrO2 particles have been identified. Magnetic hysteresis loops measured at different temperatures (5–300 K), after the sample was cooled from 350 K with a 50 kOe field, were found to be perfectly symmetrical, i.e., no exchange bias was observed, which is unexpected for ferromagnetic/antiferromagnetic core/shell structured particles. To gain a deeper understanding of the phenomenon, the “as-received” commercial CrO2 particles were annealed at 600 °C in air for 1 h to thicken the Cr2O3 shells. Exchange bias was clearly observed in the annealed particles with thicker Cr2O3 shells. At 5 K, the exchange bias field reached 220 Oe. These experimental observations can be explained well within the framework of the random field model.