Melting upon cooling in a quantum magnet

TL;DR

This paper reports the discovery of an inverse melting phenomenon in a quantum magnet, where cooling induces a transition from a long-range ordered state to a short-range correlated spin state, akin to a liquid.

Contribution

It demonstrates the occurrence of a spin Pomeranchuk effect in a frustrated quantum magnet, revealing a new type of phase transition driven by temperature changes.

Findings

Long-range magnetic order develops upon cooling and then melts into a spin-stripe state.

The phenomenon is analogous to inverse melting observed in liquid helium.

Strong competition between spin interactions in frustrated magnets can cause such phase transitions.

Abstract

Heating enhances thermal fluctuations and typically leads to melting of solids, but in exceptional cases, heating can also cause liquids to solidify. The paradigm of this counterintuitive phenomenon is solidification of liquid $^3$He upon increasing temperature, known as the Pomeranchuk effect. Here we show that such inverse melting also appears in quantum magnetism. We find that, on cooling, the Ising-like triangular-lattice antiferromagnet erbium heptatantalate first develops a three-sublattice long-range magnetic order -- analogous to a solid -- which then, unexpectedly, melts at even lower temperatures into a short-range correlated spin-stripe state -- analogous to a liquid. We propose that such an unprecedented ``spin Pomeranchuk effect" can generically arise from strong competition between spin-spin interactions in frustrated magnets, and provides a novel avenue to transformations between exotic magnetic phases.