Muon Spin Rotation and Relaxation in the Superconducting Ferromagnet UCoGe

We report zero-field muon-spin rotation and relaxation measurements on the superconducting ferromagnet UCoGe. Weak itinerant ferromagnetic order is detected by a spontaneous muon-spin precession frequency below the Curie temperature $T_C=3\mathrm{K}$. The ${\mu }^{+}$ precession frequency persists below the bulk superconducting transition temperature $T_{\mathrm{sc}}=0.5\mathrm{K}$, where it measures a local magnetic field $B_{\mathrm{loc}}=0.015\mathrm{T}$. The amplitude of the $\mu \mathrm{SR}$ signal provides unambiguous proof for ferromagnetism present in the whole sample volume. We conclude ferromagnetism coexists with superconductivity on the microscopic scale.

Figure 1

Typical zero-field $\mu \mathrm{SR}$ spectra in polycrystalline UCoGe: (a) in the paramagnetic phase at $T=5.5\mathrm{K}$ [solid line fit to Eq. (1)], and (b) in the ferromagnetic phase at $T=0.02\mathrm{K}$, well below the superconducting transition temperature [solid line fit to Eq. (2)].

Figure 2

Temperature variation of the normalized asymmetries of the ferromagnetic and paramagnetic phase in UCoGe. Below $\sim 1.5\mathrm{K}$ magnetism is observed in the whole sample volume. The solid lines are to guide the eye.

Figure 3

(a) Resistivity versus temperature of polycrystalline UCoGe. (b) Spontaneous muon precession frequency $\nu (T)$ of UCoGe and (lower inset) ${\mathrm{URh}}_{0.4}{\mathrm{Co}}_{0.6}\mathrm{Ge}$. The solid lines represent fits to a phenomenological order-parameter function (see text). Notice the 2% decrease of $\nu (T)$ below the bulk $T_{\mathrm{sc}}$ of 0.5 K [1]. Upper inset: ${\lambda }_2(T)$ of UCoGe.