Strong Correlation between Ferromagnetic Superconductivity and Pressure-enhanced Ferromagnetic Fluctuations in UGe2

We have measured magnetization at high pressure in the uranium ferromagnetic superconductor UGe2 and analyzed the magnetic data using Takahashi’s spin fluctuation theory. There is a peak in the pressure dependence of the width of the spin fluctuation spectrum in the energy space T0 at Px, the phase boundary of FM1 and FM2 where the superconducting transition temperature Tsc is highest. This suggests a clear correlation between the superconductivity and pressure-enhanced magnetic fluctuations developed at Px. The pressure effect on TCurie/T0, where TCurie is the Curie temperature, suggests that the less itinerant ferromagnetic state FM2 is changed to a more itinerant one FM1 across Px. Peculiar features in relations between T0 and Tsc in uranium ferromagnetic superconductors UGe2, URhGe, and UCoGe are discussed in comparison with those in high-Tc cuprate and heavy fermion superconductors.