Inverse Magnetocaloric Effects in Van der Walls Multiferroic NiBr₂

Recently, there has been a lot of interest in studying two-dimensional (2D) van der Waals nickel dihalides (NiX₂, where X= Br or I) compounds because of their multiferroic properties. Single-crystal NiBr₂ samples are synthesized by chemical vapor transport in a two-zone furnace using powder Ni samples with Br as a transport agent. X-ray diffraction studies of single crystal samples show excellent quality, and its pattern matches well with the reported trigonal phase. Temperature-dependent magnetization indicates two magnetic phase transitions: the first at 44 K and another at approximately 23 K. Both in-plane and out-of-plane isothermal magnetizations are performed for fields up to 9 T between 10 and 100 K with 2 K increments. The change in magnetic entropy, estimated from this data, shows interesting features. Two inverse magnetocaloric effects are observed: one at the first transition, which is more prominent along the c-axis than the ab-plane, and another at around 30 K. Another sharp peak in the magnetic entropy change observed at the second transition likely signifies a transformation from the antiferromagnetic to the helical magnetic phase, which is identified as a first-order phase transition based on the Arrott plot analysis. Beyond this, magnetic entropy decreases with further lowering of temperature. Our studies show a complex magnetic phase diagram for these materials.