Specific heat study of the magnetic superconductorHoNi2B2C

The complex magnetic transitions and superconductivity of HoNi 2 B 2 C 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.