  Magnetic response of uranium dioxide (UO2) has been investigated through
temperature and magnetic field dependent dc magnetization measurements. UO2 is
a paramagnet at room temperature. The magnetic susceptibility, however,
deviates from Curie-Weiss (CW) like paramagnetic behavior below T = 280 K.
Further down the temperature UO2 undergoes phase transition to an
antiferromagnetic state below TN = 30.6 K. The zero field cooled (ZFC) and
field cooled (FC) magnetizations exhibit some distinct thermomagnetic
irreversibility below TN. The temperature dependence of the FC magnetization is
more like a ferromagnet, whereas ZFC magnetization exhibits distinct structures
not usually observed in the antiferromagnets. In low applied magnetic field
this thermomagnetic irreversibility in magnetization exists in a subtle way
even in the paramagnetic regime above TN up to a fairly high temperature, but
vanishes in high applied magnetic fields. Deviation from CW law and
irreversibility between ZFC and FC magnetization indicate that the paramagnetic
state above TN is not a trivial one. Magnetic response below TN changes
significantly with the increase in the applied magnetic field. Thermomagnetic
irreversibility in magnetization initially increases with the increase in the
strength of applied magnetic field, but then gets reduced in the high applied
fields. A subtle signature of a magnetic field induced phase transition is also
observed in the isothermal magnetic field vartaion of magnetization. All these
experimetal results highlight the non-trivial nature of the antiferromagnetic
state in UO2
