Anisotropy and pressure dependence of the upper critical field of the ferromagnetic superconductorUGe2

The temperature and pressure dependence of the upper critical field, $H_{c2\mathrm{}},$ of the ferromagnetic superconductor ${\mathrm{UGe}}_2$ is reported for fields applied along all three crystallographic axes. For fields parallel to the easy magnetic a axis, the relationship between an unusual reentrant behavior of $H_{c2\mathrm{}}$ and a field-induced transition associated with a change in the electronic density of states is reviewed. For transverse field directions a significant evolution in the behavior of $H_{c2\mathrm{}}$ with pressure is found. As the pressure is decreased the dependence of $H_{c2\mathrm{}}$ on temperature for fields along the crystal’s c axis acquires a positive curvature that extends from the critical temperature, $T_c,$ down to almost the lowest temperature measured ${(T}_c/10)$ where $H_{c2\mathrm{}}$ exceeds the usual weak coupling paramagnetic and orbital limits.