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

## TL;DR

This study reveals highly anisotropic and extremely large upper critical fields in UCoGe, linked to field-induced ferromagnetic instability and quantum criticality, highlighting complex interplay between magnetism and superconductivity.

## Contribution

It provides detailed measurements of H_c2 anisotropy in UCoGe and connects the enhancement of superconductivity to ferromagnetic instability under magnetic fields.

## Key findings

- H_c2^b reaches nearly 20 T at 0 K with S-shape temperature dependence.
- H_c2^a exceeds 30 T at low temperatures.
- H_c2^c remains very small (~0.6 T) at 0 K.

## Abstract

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.