Abstract
Exchange bias and inverted hysteresis loops were observed in the Cr2O3 coated CrO2 particles with a wide distribution of the blocking temperature. The ferromagnetic coupling between the CrO2 cores and Cr2O3 shell is responsible for exchange bias observed below 292 K. In the temperature range of 261 K-similar to340 K, the particles show inverted hysteresis loops, i.e., the negative remanence and coercivity. The most interesting observation is that the particles show an exchange-biased (or shifted) inverted hysteresis loops in between 261 K and 292 K; and that the maximum negative remanence appears at 292 K where exchange bias disappears. An antiferromagnetic type of interaction between the blocked particles and the superparamagnetic particles is believed to be responsible to the inverted hysteresis loops. (C) 2004 American Institute of Physics.