Specific Heat Study of the Magnetic Superconductor HoNi2B2C

TL;DR

This study investigates the magnetic and superconducting phase transitions of HoNi2B2C using heat capacity measurements, revealing complex magnetic behavior, anisotropic effects, and detailed phase diagrams.

Contribution

It provides a comprehensive magnetic phase diagram and identifies the nature and anisotropy of multiple magnetic transitions in HoNi2B2C.

Findings

Three magnetic transitions and one superconducting transition observed.

The 5.2 K transition is first order with hysteresis.

The 6 K transition shows strong in-plane anisotropy related to spiral structure.

Abstract

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.