Some remarks on the magnetic phase diagram of ferromagnetic superconductor UGe2

TL;DR

This paper investigates the pressure-induced change in the order of the ferromagnetic phase transition in UGe2, using Landau theory and magneto-elastic interactions to explain the transition from first to second order.

Contribution

It introduces a Landau free energy model including magneto-elastic effects to explain the change in phase transition order in UGe2 under pressure.

Findings

Transition from first to second order at 1.42 GPa

Calculated entropy jump at the first order transition

Proposed pressure dependence of Curie temperature

Abstract

The phase transition to ferromagnetic order in itinerant ferromagnetic superconductor UGe$_2$ as function of pressure shows change of phase transition order. At low temperature and high pressure the transition is of first order and at pressure of about 1.42 GPa the order changes to second. On the basis of Landau expansion of free energy up to the sixth order in magnetization we calculate the phase diagram taking into account the magneto-elastic interaction as the mechanism responsible for this change. We propose a simple Stoner-like dependence of the Curie temperature on the pressure and present the results for measurable thermodynamic quantities as the entropy jump at the first order phase transition.