Tricritical behavior in itinerant quantum ferromagnets

TL;DR

This paper explains the complex phase diagrams of certain itinerant ferromagnets using a mean-field theory, highlighting the transition from second to first order at a tricritical point and analyzing quantum critical behavior.

Contribution

It introduces a simple mean-field model to describe tricritical behavior and quantum critical points in itinerant ferromagnets, connecting phase diagram features to underlying physics.

Findings

Transition changes from second to first order at a tricritical point.

First-order transition surfaces emerge in magnetic fields, ending at quantum critical points.

Quantum critical behavior in nonzero fields is calculated exactly.

Abstract

It is shown that the peculiar features observed in the low-temperature phase diagrams of ZrZn_2, UGe_2, and MnSi can be understood in terms of a simple mean-field theory. The nature of the ferromagnetic transition changes from second order to first order at a tricritical point, and in a small external magnetic field surfaces of first-order transitions emerge which terminate in quantum critical points. This field dependence of the phase diagram follows directly from the existence of the tricritical point. The quantum critical behavior in a nonzero field is calculated exactly.