Origin of unusual thermomagnetic behaviors in maghemite

•The increase of saturation magnetization with temperature was observed in maghemite.•The FC magnetization was observed to be less than ZFC magnetization in maghemite.•The origin of these anomalies was due to the competition of magnetic interactions.

Usually, in any magnetic system, the saturation magnetization increases as decreasing ambient temperature and the field-cooled magnetization is no less than that in zero field-cooled procedure. However, in this work, we observed diametrically opposite experimental phenomena in γ-Fe2O3 microspheres. And we found that the presence of Fe3+ spin chaos provoked by the competition of strong Fe3+(A)O2−Fe3+(A) and Fe3+(B)O2−Fe3+(B) interactions was the root cause for all of these thermomagnetic anomalies. Our findings were supported by exploring the physical significance of Boltzmann H-theorem through quantum mechanics analyses and estimating entropy changes on the basis of Clausius-Clapeyron type equation. It is anticipated that this article will shed new light on the understanding of thermomagnetic behaviors in ferrimagnetic materials.