Extremely Large and Anisotropic Upper Critical Field and the Ferromagnetic Instability in UCoGe

Abstract

Magnetoresistivity measurements with fine tuning of the field direction on\nhigh quality single crystals of the ferromagnetic superconductor UCoGe show\nanomalous anisotropy of the upper critical field H_c2. H_c2 for H // b-axis\n(H_c2^b) in the orthorhombic crystal structure is strongly enhanced with\ndecreasing temperature with an S-shape and reaches nearly 20 T at 0 K. The\ntemperature dependence of H_c2^a shows upward curvature with a low temperature\nvalue exceeding 30 T, while H_c2^c at 0 K is very small (~ 0.6 T). Contrary to\nconventional ferromagnets, the decrease of the Curie temperature with\nincreasing field for H // b-axis marked by an enhancement of the effective mass\nof the conduction electrons appears to be the origin of the S-shaped H_c2^b\ncurve. These results indicate that the field-induced ferromagnetic instability\nor magnetic quantum criticality reinforces superconductivity.\n