Specific-heat anomaly in the Ising antiferromagnet FeBr₂ in external magnetic fields

Specific-heat measurements have been performed on the Ising antiferromagnet FeBr₂ in external magnetic fields. The temperature dependence of the magnetic specific heat shows two anomalies in external fields below the metamagnetic transition field. The anomaly at a higher temperature indicates the phase transition from the paramagnetic to antiferromagnetic phase. The lower-temperature anomaly shows up in the form of a peak superposed on a broad shoulder. The peak becomes sharp with the increase of magnetic field up to 2.9 T, which shows the existence of a new phase under a magnetic field. A theoretical analysis based on the pair approximation and Monte Carlo simulations reproduces the peak at the higher temperature and the broad shoulder. The broad shoulder appears as a result of competition between the nearest-neighbor ferromagnetic and the next-nearest-neighbor antiferromagnetic interactions in the triangular Fe lattice planes.