Neutron-scattering studies of a phase transition in the metamagnet FeBr₂ under external magnetic fields

Neutron-scattering experiments have been performed on the metamagnet FeBr₂ under external magnetic fields. We find an anomaly in the temperature (T) and magnetic-field (H) dependences of the intensity of the (2,0,1/2) antiferromagnetic Bragg scattering at the temperature T₁(H) at which an anomaly in specific heat has been observed in addition to the anomaly at the Néel temperature T_N(H). We argue that our results are consistent with the theoretical H-T phase diagram of a metamagnet in which the tricritical point decomposes into a critical end point and a bicritical end point. We locate the latter point at T_cr≅10.8 K and H_cr≅1.4 T in this compound.