Microscopic magnetic properties of the ferromagnetic superconductor UCoGe reviewed by x-ray magnetic circular dichroism

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 $5f$ electrons count is close to 3. The orbital $(\sim 0.70{\mu }_B)$ and spin $(\sim -0.30{\mu }_B)$ moments of U at 2.1 K and 17 T (H$\parallel$c) have been determined. Their ratio $(\sim -2.3)$ suggests a significant delocalization of the $5f$ 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 $4p$ and Ge $4p$ orbital moments parallel to the macroscopic magnetization. In addition, the Co $3d$ moment is estimated to be at most of the order of $0.1{\mu }_B$ at 17 T. Our results rule out the possibility of an unusual polarizability 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.

Figure 1

XANES (black curves, left axis) and XMCD (red curves, right axis) spectra measured at the U $M_5$ and $M_4$ edges for a UCoGe single crystal in a magnetic field of 17  T applied along the c axis at 2.1 K as a function of the incident photon energy. The integrated areas under the white lines are obtained after subtraction of the continuum modeled by an arctan function (broken lines). The XMCD spectrum has been corrected for self-absorption and for incomplete polarization rate.

Figure 2

Uranium magnetization (left axis, red symbols) recorded at the maximum XMCD signal in UCoGe at the U $M_4$ edge at 2.1 K with H$\parallel$c (open circles) and H$\parallel$b (full squares), together with the macroscopic magnetization curves (right axis, black straight line for H$\parallel$c and dashed line for H$\parallel$b), from [7].

Figure 3

XANES (left axis, black curves) and XMCD (right axis, red curves) spectra in UCoGe recorded at the $K$ edges of Co (dotted lines) and Ge (straight lines) at 2.1 K and 17 T with H$\parallel$c.

Figure 4

Element specific magnetization recorded at the maximum XMCD signal in UCoGe at the Co $K$ absorption edge (right axis, black squares). Comparison with the U magnetization measured at the maximum XMCD signal at the U $M_4$ edge at 2.1 K with H$\parallel$c (left axis, red open circles).

Figure 5

XANES (left axis, in black) and XMCD (right axis, in red) spectra of an UCoGe single crystal (H$\parallel$c, dotted lines) and of an YCoGe polycrystal (straight lines) recorded at the $K$ edge of Co at 2.1 K and 17 T.