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.