Kinetically Inhibited Order in a Diamond-Lattice Antiferromagnet

TL;DR

This study reveals a first-order phase transition in a frustrated diamond-lattice antiferromagnet, where kinetic freezing inhibits long-range order, explaining glassy behavior in such systems.

Contribution

It provides the first comprehensive neutron scattering evidence that a frustrated antiferromagnet undergoes a first-order transition with kinetic freezing, challenging previous glassy behavior interpretations.

Findings

Observation of diffuse scattering peaking at Neel wavevectors

Emergence of spin-wave excitations below T*

Identification of T* as a first-order transition with kinetic freezing

Abstract

Frustrated magnetic systems exhibit highly degenerate ground states and strong fluctuations, often leading to new physics. An intriguing example of current interest is the antiferromagnet on a diamond lattice, realized physically in A-site spinel materials. This is a prototypical system in three dimensions where frustration arises from competing interactions rather than purely geometric constraints, and theory suggests the possibility of unusual magnetic order at low temperature. Here we present a comprehensive single-crystal neutron scattering study of CoAl2O4, a highly frustrated A-site spinel. We observe strong diffuse scattering that peaks at wavevectors associated with Neel ordering. Below the temperature T*=6.5 K, there is a dramatic change in the elastic scattering lineshape accompanied by the emergence of well-defined spin-wave excitations. T* had previously been associated with the onset of glassy behavior. Our new results suggest instead that T* signifies a first-order phase transition, but with true long-range order inhibited by the kinetic freezing of domain walls. This scenario might be expected to occur widely in frustrated systems containing first-order phase transitions and is a natural explanation for existing reports of anomalous glassy behavior in other materials.