Metamagnetic Phase Transition and Anomalous Hysteresis in FeCl₂·2H₂O

Magnetization in metamagnetic FeCl 2 ·2H 2 O has been studied in detail under both a pulsed and a static magnetic fields at temperatures between 1.5 K∼14 K. At high temperatures, a transition from the antiferromagnetic (A.F.) to the ferromagnetic ( M s / 3 ) states at H c 1 and that from the M s / 3 to the ferrimagnetic ( M s ) states at H c 2 are observed. A large hysteresis has been observed at H c 1 even under a slowly varying field. As the temperature is lowered, a transition from the A.F. to a M s / 2 states followed by that from the M s / 2 to the M s / 3 states accompanying with that from the M s / 3 to the M s states takes place in an increasing field. The M s / 2 state which is a high energy state in this salt becomes stable with decreasing temperature. Below 2.24 K, a large hysteresis at H c 2 is observed even under a slowly varying field. It is shown that the main origin of the anomalously large hysteresis comes from the energetic barrier due to a strong single ion anisotropy energy. The appearance of a new phase ( M s / 2 state) arises from the fact that a M s / 2 state is constructed by a simple reversal of down spins in the A.F. state, while the M s / 3 state by a complicated rearrangement of up and down spins.