Static and dynamic XY-like short-range order in a frustrated magnet with exchange disorder

TL;DR

This study investigates a frustrated Co-based pyrochlore magnet with exchange disorder, revealing short-range XY antiferromagnetic correlations and a frozen state without long-range order, through neutron scattering experiments.

Contribution

It provides new insights into the static and dynamic short-range order in a frustrated magnet with exchange disorder, showing the absence of order-by-disorder selection.

Findings

Diffuse elastic scattering indicates frozen XY antiferromagnetic clusters.

Short-range correlations have a length scale of 16 Å.

The system exhibits both relaxing XY-like and collinear antiferromagnetic fluctuations.

Abstract

A single crystal of the Co2+ based pyrochlore NaCaCo2F7 was studied by inelastic neutron scattering. This frustrated magnet with quenched exchange disorder remains in a strongly correlated paramagnetic state down to one 60th of the Curie-Weiss temperature. Below T_f = 2.4 K, diffuse elastic scattering develops and comprises 30 +/- 10% of the total magnetic scattering, as expected for J_{eff} = 1/2 moments frozen on a time scale that exceeds \hbar/\delta E=3.8 ps. The diffuse scattering is consistent with short range XY antiferromagnetism with a correlation length of 16 \AA. The momentum (Q) dependence of the inelastic intensity indicates relaxing XY-like antiferromagnetic clusters at energies below ~ 5.5 meV, and collinear antiferromagnetic fluctuations above this energy. The relevant XY configurations form a continuous manifold of symmetry-related states. Contrary to well-known models that produce this continuous manifold, order-by-disorder does not select an ordered state in NaCaCo2F7 despite evidence for weak (~12 %) exchange disorder. Instead, NaCaCo2F7 freezes into short range ordered clusters that span this manifold.