Specific heat study of the magnetic superconductor HoNi₂B₂C

The complex magnetic transitions and superconductivity of HoNi₂B₂C were studied via the dependence of the heat capacity on temperature and in-plane field angle. We provide an extended, comprehensive magnetic phase diagram for B‖[100] and B‖[110] based on the thermodynamic measurements. Three magnetic transitions and the superconducting transition were clearly observed. The 5.2 K transition (T_N) shows a hysteresis with temperature, indicating the first-order nature of the transition at B=0 T. The 6 K transition T_M, namely the onset of the long-range ordering, displays a dramatic in-plane anisotropy: T_M increases with increasing magnetic field for B‖[100] while it decreases with increasing field for B‖[110]. The anomalous anisotropy in T_M indicates that the transition is related to the a-axis spiral structure. The 5.5 K transition T^* shows similar behavior to the 5.2 K transition, i.e., a small in-plane anisotropy and scaling with Ising model. This last transition is ascribed to the change from a^* dominant phase to c^* dominant phase.