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 H_c2. H_c2 for H // b-axis (H_c2^b) 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 H_c2^a shows upward curvature with a low temperature value exceeding 30 T, while H_c2^c at 0 K is very small (~ 0.6 T). Contrary to conventional ferromagnets, the decrease of the Curie temperature with increasing field for H // b-axis marked by an enhancement of the effective mass of the conduction electrons appears to be the origin of the S-shaped H_c2^b curve. These results indicate that the field-induced ferromagnetic instability or magnetic quantum criticality reinforces superconductivity.
@article{arxiv.0910.1157,
title = {Extremely Large and Anisotropic Upper Critical Field and the Ferromagnetic Instability in UCoGe},
author = {Dai Aoki and Tatsuma D. Matsuda and Valentin Taufour and Elena Hassinger and Georg Knebel and Jacques Flouquet},
journal= {arXiv preprint arXiv:0910.1157},
year = {2011}
}
Comments
5 pages, 4 figures, accepted for publication in J. Phys. Soc. Jpn