Observation of a first-order phase transition in single-crystal ErRh4B4 at Hc2

Detailed magnetization curves show a first-order phase transition in single-crystal ErRh4B4 at Hc2 for the temperature range 3.3T1.4 K when the field is parallel to the a axis, along which the crystal shows strong paramagnetism. Below 3.3 K, the discontinuity in magnetization at Hc2 grows with decreasing temperature and moves to lower fields, ultimately developing into the spontaneous moment in the ferromagnetic state. In the c direction, where the crystal exhibits weak paramagnetism, the superconducting magnetization curves show ordinary type-II behavior.