Infuence of pressure on magnetic phase transitions in the ferromagnetic superconductor UGe$_2$ -- phenomenological approach

TL;DR

This paper develops a thermodynamic model to describe pressure effects on magnetic phase transitions in the ferromagnetic superconductor UGe$_2$, highlighting how pressure influences the order of these transitions.

Contribution

It introduces a phenomenological model incorporating pressure dependence of Curie temperature to explain magnetic phase transitions in UGe$_2$, including the change from crossover to first-order transitions.

Findings

Transition remains crossover below a critical pressure.

Above critical pressure, transition becomes first-order.

Model successfully describes phases with same structure but different magnetization.

Abstract

We propose a thermodynamic model of free energy expansion up to eighth order of magnetisation to describe the complex magnetic phase transitions in ferromagnetic superconductor UGe$_2$. The model successfully describes transitions between ordered phases which take place without changing of magnetic structure but only of magnetisation which is the case of UGe$_2$ where two magnetic phases with same structure but with different magnitude occur with decreasing of temperature. We consider the infuence of pressure on magnetic transitions in the simplest form of including it only in pressure dependence of Curie temperature for the transition between the paramagnetic and themagnetic phase with the lower magnetic moment FM1. Our results show that for pressures lower than some limiting pressure the transition betweenlow-and high-magnetisation phase remains of crossover type. Above this limiting pressure this transition changes to real thermodynamic transition of first order.