Interesting magnetic response of the nuclear fuel material UO2

We report results of dc magnetization measurements on UO2. Our study reveals a deviation from standard Curie-Weiss (CW) paramagnetic behavior below 280 K, and that is followed by a transition to antiferromagnetic state with marked thermomagnetic irreversibility below T-N = 30.6 K. The zero field cooled (ZFC) magnetization exhibits distinct structures not usually observed in the antiferromagnets, whereas field cooled (FC) magnetization looks more like a ferromagnet. The thermomagnetic irreversibility continues to exist in a subtle way even in the paramagnetic regime well above T-N. This behavior along with the deviation from CW law highlight non-standard nature of the paramagnetic state. Magnetic response below T-N changes significantly with the increase in the applied magnetic field. In the isothermal magnetic field variation of magnetization, a subtle signature of a magnetic field-induced phase transition is observed. All these experimental results highlight the non-trivial nature of the magnetic state in UO2.