Anisotropic Critical Magnetic Fluctuations in the Ferromagnetic Superconductor UCoGe

We report neutron scattering measurements of critical magnetic excitations in the weakly ferromagnetic superconductor UCoGe. The strong non-Landau damping of the excitations we observe, although unusual, has been found in another related ferromagnet, ${\mathrm{UGe}}_2$ at zero pressure. However, we also find that there is a significant anisotropy of the magnetic correlation length in UCoGe that contrasts with an almost isotropic length for ${\mathrm{UGe}}_2$. The values of the magnetic correlation length and damping are found to be compatible with superconductivity on small Fermi-surface pockets. The anisotropy may be important to explain why UCoGe is a superconductor at zero pressure while ${\mathrm{UGe}}_2$ is not.

Figure 1

Constant $\mathbf{Q}=(200)$ scans at several temperatures for FLEX (7 g with $E_f=5.0\mathrm{meV}$) and 4F1 (5 g with $E_f=4.5\mathrm{meV}$). The background scattering determined from rotating the sample away from $Q=(200)$ has been subtracted. The inset shows the elastic peak divided by 150. The solid curves in the main panel are fits to the equations described in the text at 20, 2, and 100 K (in the order of decreasing scattering intensity at small $E$).

Figure 2

Neutron scattering normalized counts for different temperatures and energy transfers at $\mathbf{q}=(200)$ measured on 4F1. The solid lines are for a simultaneous fit to all the data with a single free parameter ${\gamma }_0=0.41(3)\mu \mathrm{eV}$ (plus a temperature independent background term) as described in the text (counts decrease with increased energy transfer).

Figure 3

Constant energy scans taken on (a) 4F1 and (b) MACS. The data on MACS have been integrated over $L=[-1.15,-0.85]$ and $K=[-1.1,-0.9]$ and taken with $E_f=5.0\mathrm{meV}$ with the samples ($7\mathrm{g}+14\mathrm{g}$) aligned in the ($0,K,L$) scattering plane. The data taken on 4F1 are for 18 K and with $E_f=4.5\mathrm{meV}$ with the sample (5 g) in the ($H,K,0$) scattering plane. The horizontal scales equate reciprocal wavelengths between the different axes.

Figure 4

Constant $E=0.75\mathrm{meV}$ slices measured with MACS at (a) $T=20\mathrm{K}$ and (b) $T=40\mathrm{K}$. A background at $T=1.5\mathrm{K}$ has been subtracted from both data sets. The two samples ($7\mathrm{g}+14\mathrm{g}$) were aligned in the ($0,K,L$) scattering plane.