Specific heat and magnetocaloric effect study of multiple field-induced phase transitions in HoGa2

Both temperature and field dependences of specific heat C and magnetocaloric effect (MCE) were measured on a HoGa2 single crystal in fields up to 8 T at low temperatures down to 0.1 K. The magnetic fields were applied in the [100] direction, for which intermediate magnetic phases appear. Significantly Ho nuclear contribution to C is found to dominate below 1 K and its field-independent character reveals that the value of Ho magnetic moments does not change visibly in the measured field range irrespective of the magnetic phases. From anomalies observed in the C and MCE data, the reported held-vs-temperature phase diagram is corroborated thermodynamically. One of the phase transitions occurring at T-t = 6.6 K in zero field is found to be of first-order transition. Magnetic entropy determined by the MCE measurements shows an enhancement in the intermediate phases and small humps at the phase boundaries. Based on a crystalline-electric-field model, the nuclear specific heat, magnetic entropy of the 4f electrons, and anisotropy in the magnetization;can be explained consistently. Possible origins of the reported giant magnetoresistance caused by the appearance of the intermediate phases will be discussed.