Assessment of the U and Co magnetic moments in UCoGe by X-ray magnetic circular dichroism

TL;DR

This study uses X-ray magnetic circular dichroism to analyze the magnetic moments of uranium and cobalt in UCoGe, revealing details about electron localization, magnetic coupling, and the origin of magnetism related to superconductivity.

Contribution

First direct XMCD measurement of U and Co magnetic moments in UCoGe, clarifying their magnitudes, coupling, and electron localization, advancing understanding of its magnetic and superconducting properties.

Findings

U-5f electrons are close to 3 in count.

U moments are significant and delocalized.

Cobalt moments are induced and small.

Abstract

The ferromagnetic superconductor UCoGe has been investigated by high field X-ray magnetic circular dichroism (XMCD) at the U-M$_{4,5}$ and Co/Ge-K edges. The analysis of the branching ratio and XMCD at the U-M$_{4,5}$ edges reveals that the U-5$f$ electrons count is close to 3. The orbital ($\sim0.70\,\mu_B$) and spin ($\sim-0.30\,\mu_B$) moments of U at 2.1K and 17T (H//c) have been determined. Their ratio ($\sim-2.3$) suggests a significant delocalization of the 5$f$ electron states. The similar field dependences of the local U/Co and the macroscopic magnetization indicate that the Co moment is induced by the U moment. The XMCD at the Co/Ge-K edges reveal the presence of small Co-4$p$ and Ge-4$p$ orbital moments parallel to the macroscopic magnetization. In addition, the Co-3$d$ moment is estimated to be at most of the order of 0.1$\mu_B$ at 17T. Our results rule out the possibility of an unusual polarisability of the U and Co moments as well as their antiparallel coupling. We conclude that the magnetism which mediates the superconductivity in UCoGe is driven by U.