Spin-chain correlations in the frustrated triangular lattice material CuMnO$_2$

TL;DR

This study investigates the magnetic correlations in CuMnO$_2$, revealing persistent paramagnetic fluctuations, anisotropic correlations, and frustration effects in a triangular lattice, using advanced neutron scattering techniques.

Contribution

It provides a detailed neutron scattering analysis of magnetic correlations and fluctuations in CuMnO$_2$, clarifying the nature of frustration and anisotropy in this triangular lattice material.

Findings

Paramagnetic fluctuations persist up to ~25 meV above T_N

Two directions in the lattice are perfectly frustrated in the spin-liquid phase

Strong antiferromagnetic correlations are only along the b-axis

Abstract

The Ising triangular lattice remains the classic test-case for frustrated magnetism. Here we report neutron scattering measurements of short range magnetic order in CuMnO$_2$, which consists of a distorted lattice of Mn$^{3+}$ spins with single-ion anisotropy. Physical property measurements on CuMnO$_2$ are consistent with 1D correlations caused by anisotropic orbital occupation. However the diffuse magnetic neutron scattering seen in powder measurements has previously been fitted by 2D Warren-type correlations. Using neutron spectroscopy, we show that paramagnetic fluctuations persist up to $\sim$25 meV above TN= 65 K. This is comparable to the incident energy of typical diffractometers, and results in a smearing of the energy integrated signal, which hence cannot be analysed in the quasi-static approximation. We use low energy XYZ polarised neutron scattering to extract the purely magnetic (quasi)-static signal. This is fitted by reverse Monte Carlo analysis, which reveals that two directions in the triangular layers are perfectly frustrated in the classical spin-liquid phase at 75 K. Strong antiferromagnetic correlations are only found along the b-axis, and our results hence unify the pictures seen by neutron scattering and macroscopic physical property measurements.