Observation of magnetic fragmentation in spin ice

TL;DR

This paper reports neutron scattering evidence of magnetic fragmentation in Nd$_2$Zr$_2$O$_7$, showing coexistence of magnetic order and a fluctuating Coulomb phase, revealing complex ground states in spin ice systems.

Contribution

It provides experimental confirmation of magnetic fragmentation in spin ice, demonstrating coexistence of order and fluctuations in a real material.

Findings

Observation of magnetic Bragg peaks and pinch point patterns

Coexistence of antiferromagnetic order and fluctuating Coulomb phase

Evidence of magnetic fragmentation in Nd$_2$Zr$_2$O$_7$

Abstract

Fractionalised excitations that emerge from a many body system have revealed rich physics and concepts, from composite fermions in two-dimensional electron systems, revealed through the fractional quantum Hall effect, to spinons in antiferromagnetic chains and, more recently, fractionalisation of Dirac electrons in graphene and magnetic monopoles in spin ice. Even more surprising is the fragmentation of the degrees of freedom themselves, leading to coexisting and a priori independent ground states. This puzzling phenomenon was recently put forward in the context of spin ice, in which the magnetic moment field can fragment, resulting in a dual ground state consisting of a fluctuating spin liquid, a so-called Coulomb phase, on top of a magnetic monopole crystal. Here we show, by means of neutron scattering measurements, that such fragmentation occurs in the spin ice candidate Nd$_2$Zr$_2$O$_7$. We observe the spectacular coexistence of an antiferromagnetic order induced by the monopole crystallisation and a fluctuating state with ferromagnetic correlations. Experimentally, this fragmentation manifests itself via the superposition of magnetic Bragg peaks, characteristic of the ordered phase, and a pinch point pattern, characteristic of the Coulomb phase. These results highlight the relevance of the fragmentation concept to describe the physics of systems that are simultaneously ordered and fluctuating.