Anomalous heat capacity of antiferromagnet FeBr2 in a magnetic field

Heat capacity measurements in external magnetic fields (H) have been performed on the Ising antiferromagnet FeBr2. The temperature dependence of the magnetic heat capacity shows two anomalies for 1.4 T less than or equal to H less than or equal to 2.9 T. The anomaly at a higher temperature indicates the phase transition from the paramagnetic to antiferromagnetic phase. The anomaly at a lower temperature is formed by a sharp peak superposed on a broad shoulder. The existence of two peaks shows the occurrence of a new phase transition in addition to the antiferromagnetic transition in magnetic fields. A Monte Carlo simulation for the heat capacity reproduces the peak indicating the antiferromagnetic transition and the bread shoulder. The nonuniform spin configurations in the layers with negative moment, which is caused by the competition between the nearest-neighbor ferromagnetic and the next-nearest-neighbor antiferromagnetic interactions in the triangular Fe-planes, is shown to produce the broad shoulder. (C) 1997 American Institute of Physics.