Magnetic hard-axis ordering near ferromagnetic quantum criticality

TL;DR

This paper demonstrates how quantum fluctuations near a ferromagnetic quantum critical point can cause magnetic moments to align along a hard axis, supported by a theoretical model and relevant experimental observations.

Contribution

It introduces a fermionic quantum order-by-disorder approach to explain hard-axis magnetic ordering driven by quantum fluctuations.

Findings

Quantum fluctuations can induce moments to align along the magnetic hard axis.

The phase transition becomes first-order below a tricritical point due to fluctuations.

Experimental observations support the theoretical prediction of hard-axis ordering.

Abstract

We investigate the interplay of quantum fluctuations and magnetic anisotropies in metallic ferromagnets. Our central result is that fluctuations close to a quantum critical point can drive the moments to point along a magnetic hard axis. As a proof of concept, we show this behavior explicitly for a generic two-band model with local Coulomb and Hund's interactions, and a spin-orbit-induced easy plane anisotropy. The phase diagram is calculated within the fermionic quantum order-by-disorder approach, which is based on a self-consistent free energy expansion around a magnetically ordered state with unspecified orientation. Quantum fluctuations render the transition of the easy-plane ferromagnet first-order below a tricritical point. At even lower temperatures, directionally dependent transverse fluctuations dominate the magnetic anisotropy and the moments flip to lie along the magnetic hard axis. We discuss our findings in the context of recent experiments that show this unusual ordering along the magnetic hard direction.