Observation of a first-order phase transition in single-crystal Er${\mathrm{Rh}}_4$${\mathrm{B}}_4$ at $H_{c2\mathrm{}}$

Detailed magnetization curves show a first-order phase transition in single-crystal Er${\mathrm{Rh}}_4$ ${\mathrm{B}}_4$ at $H_{c2\mathrm{}}$ for the temperature range $3.3\gtrsim T\gtrsim 1.4$ 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 $H_{c2\mathrm{}}$ 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.