Extremely large and anisotropic upper critical field and the ferromagnetic instability in UCoGe

Aoki, Dai*; Matsuda, Tatsuma; Taufour, V.*; Hassinger, E.*; Knebel, G.*; Flouquet, J.*

Magnetoresistivity measurements with fine tuning of the field direction on high quality single crystals of the ferromagnetic superconductor UCoGe show anomalous anisotropy of the upper critical field . for -axis () in the orthorhombic crystal structure is strongly enhanced with decreasing temperature with an S-shape and reaches nearly 20 T at 0 K. The temperature dependence of shows upward curvature with a low temperature value exceeding 30 T, while at 0K is very small (0.6 T).Contrary to conventional ferromagnets, the decrease of the Curie temperature with increasing field for -axis marked by an enhancement of the effective mass of the conduction electrons appears to be the origin of the S-shaped curve. These results indicate that the field-induced ferromagnetic instability or magnetic quantum criticality reinforces superconductivity.