Calorimetric investigations of HoNi₂B₂C

The rare-earth nickel borocarbides show an intriguing competition between magnetism and superconductivity. For HoNi₂B₂C, this leads to a rich phase diagram with superconducting and magnetic phase transitions. Besides these competing electronic interactions, also the nuclear magnetic moment of Holmium may influence the superconducting state due to an expected strong hyperfine-enhanced nuclear polarization. In order to study this in more detail, we performed high-resolution specific-heat measurements by use of a continuous relaxation-time method. While the superconducting transition at about 8 K results in a very small, but resolvable specific-heat jump, at lower temperatures there are at least three independent magnetic-ordering transitions. One of these shows a lambda-like anomaly with a small hysteresis indicating a first-order phase transition. Additionally, the increasing specific heat below 1 K can be taken as evidence for a pronounced hyperfine contribution.