ANOMALOUS HEAT-CAPACITY OF ANTIFERROMAGNET FEBR2 IN A MAGNETIC-FIELD

Heat capacity measurements in external magnetic fields (H) have been p erformed on the Ising antiferromagnet FeBr2. The temperature dependenc e of the magnetic heat capacity shows two anomalies for 1.4 T less tha n or equal to H less than or equal to 2.9 T. The anomaly at a higher t emperature indicates the phase transition from the paramagnetic to ant iferromagnetic phase. The anomaly at a lower temperature is formed by a sharp peak superposed on a broad shoulder. The existence of two peak s shows the occurrence of a new phase transition in addition to the an tiferromagnetic transition in magnetic fields. A Monte Carlo simulatio n for the heat capacity reproduces the peak indicating the antiferroma gnetic transition and the bread shoulder. The nonuniform spin configur ations in the layers with negative moment, which is caused by the comp etition between the nearest-neighbor ferromagnetic and the next-neares t-neighbor antiferromagnetic interactions in the triangular Fe-planes, is shown to produce the broad shoulder. (C) 1997 American Institute o f Physics.