# Strong correlation between ferromagnetic superconductivity and pressure-enhanced ferromagnetic fluctuations in UGe$_2$

## TL;DR

This study reveals a strong link between ferromagnetic superconductivity and pressure-induced magnetic fluctuations in UGe$_2$, showing that magnetic fluctuation peaks correlate with the highest superconducting transition temperatures.

## Contribution

It demonstrates the correlation between magnetic fluctuations and superconductivity in UGe$_2$, highlighting pressure-dependent changes in ferromagnetic states and their impact on superconductivity.

## Key findings

- Peak in spin fluctuation spectrum at $P_x$ coincides with maximum $T_{sc}$.
- Transition from FM2 to FM1 ferromagnetic state across $P_x$.
- Comparison of $T_0$ and $T_{sc}$ relations with other uranium superconductors.

## Abstract

We have measured magnetization at high pressure in the uranium ferromagnetic superconductor UGe$_2$ 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 $T_0$ at $P_x$, the phase boundary of FM1 and FM2 where the superconducting transition temperature $T_{sc}$ is highest. This suggests a clear correlation between the superconductivity and pressure-enhanced magnetic fluctuations developed at $P_x$. The pressure effect on $T_{Curie}/T_0$, where $T_{Curie}$ is the Curie temperature, suggests that the less itinerant ferromagnetic state FM2 is changed to a more itinerant one FM1 across $P_x$. Peculiar features in relations between $T_0$ and $T_{sc}$ in uranium ferromagnetic superconductors UGe$_2$, URhGe and UCoGe are discussed in comparison with those in high-$T_c$ cuprate and heavy fermion superconductors.