Quantum Phase Transitions in the Itinerant Ferromagnet ZrZn$_2$

TL;DR

This study investigates the ferromagnetic quantum phase transitions in ZrZn$_2$, revealing a first-order transition at a critical pressure and identifying a tricritical point, thus supporting the idea that QPTs in itinerant ferromagnets are inherently first order.

Contribution

It provides experimental evidence that quantum phase transitions in ZrZn$_2$ are first order, highlighting the role of a tricritical point and the sensitivity of the free energy landscape.

Findings

Discontinuous disappearance of ferromagnetism at p_c=16.5 kbar

Identification of a tricritical point separating first and second order transitions

Observation of rapid changes in magnetization derivatives up to 12 T

Abstract

We report a study of the ferromagnetism of ZrZn$_{2}$, the most promising material to exhibit ferromagnetic quantum criticality, at low temperatures $T$ as function of pressure $p$. We find that the ordered ferromagnetic moment disappears discontinuously at $p_c$=16.5 kbar. Thus a tricritical point separates a line of first order ferromagnetic transitions from second order (continuous) transitions at higher temperature. We also identify two lines of transitions of the magnetisation isotherms up to 12 T in the $p-T$ plane where the derivative of the magnetization changes rapidly. These quantum phase transitions (QPT) establish a high sensitivity to local minima in the free energy in ZrZn$_{2}$, thus strongly suggesting that QPT in itinerant ferromagnets are always first order.