Inverted hysteresis in exchange biased Cr₂O₃ coated CrO₂ particles

Exchange bias and inverted hysteresis loops were observed in the Cr ₂O₃ coated CrO₂ particles with a wide distribution of the blocking temperature. The ferromagnetic coupling between the CrO₂ cores and Cr₂O₂ shell is responsible for exchange bias observed below 292 K. In the temperature range of 261 K- ∼ 340 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.