Spin liquid in a single crystal of the frustrated diamond lattice antiferromagnet CoAl2O4

TL;DR

This study investigates the spin liquid state in CoAl2O4 using neutron scattering, revealing short-range spiral correlations, magnetic field effects, and conventional spin wave excitations, contributing to understanding frustrated magnetism.

Contribution

It provides the first detailed neutron scattering analysis of the spin liquid state in CoAl2O4, highlighting the role of short-range correlations and magnetic field effects in a frustrated diamond lattice antiferromagnet.

Findings

Broad magnetic Bragg peaks indicate short-range correlations.

Magnetic field increases static magnetic moment and alters peak profiles.

Spin excitations are conventional spin waves at zero field.

Abstract

We study spin liquid in the frustrated diamond lattice antiferromagnet CoAl2O4 by means of single crystal neutron scattering in zero and applied magnetic field. The magnetically ordered phase appearing below TN=8 K remains nonconventional down to 1.5 K. The magnetic Bragg peaks at the q=0 positions remain broad and their profiles have strong Lorentzian contribution. Additionally, they are connected by weak diffuse streaks along the <111> directions. These observations are explained within the spiral spin liquid model as short-range magnetic correlations of spirals populated at these finite temperatures, as the energy minimum around q=0 is flat and the energy of excited states with q=(111) is low. The agreement is only qualitative, leading us to suspect that microstructure effects are also important. Magnetic field significantly perturbs spin correlations. The 1.5 K static magnetic moment increases from 1.58 mB/Co at zero field to 2.08 mB/Co at 10 T, while the magnetic peaks, being still broad, acquire almost Gaussian profile. Spin excitations are rather conventional spin waves at zero field, resulting in the exchange parameters J1=0.92(1) meV, J2=0.101(2) meV and the anisotropy term D=-0.0089(2) meV for CoAl2O4. The application of a magnetic field leads to a pronounced broadening of the excitations at the zone center, which at 10 T appear gapless and nearly featureless.