Existence of Anisotropic Spin Fluctuations at Low Temperature in the Normal Phase of the Superconducting Ferromagnet UCoGe

Thermal conductivity measurements have been performed on the superconducting ferromagnet UCoGe down to very low temperature and under magnetic field. In addition to the electronic quasiparticle thermal conductivity, additional contributions to the thermal transport are detected: they are sensitive to the amplitude and direction of the magnetic field, and at low temperature, they display a strong anisotropy with the heat current direction. We identify these contributions as arising from magnetic fluctuations. Detection of such fluctuations on the thermal transport in 3D weak ferromagnets is very rare if not unique, and pledges for a strongly itinerant character of the magnetism of UCoGe.