Decoupling lattice and magnetic instabilities in frustrated CuMnO$_2$

TL;DR

This study investigates how high pressure influences the structural and magnetic properties of CuMnO$_2$, revealing decoupling of lattice and magnetic instabilities, and providing insights into frustration and inhomogeneity in antiferromagnets.

Contribution

It demonstrates that high pressure induces an isostructural phase transition and decouples lattice and magnetic degrees of freedom in CuMnO$_2$, highlighting the interplay between structure and magnetism.

Findings

High pressure causes a reversible collapse of the c-axis.

Non-zero anisotropic Cu displacements coexist with magnetic order.

High pressure activates Cu$^{+}$ cation instabilities, affecting magnetism.

Abstract

The $A$MnO$_{2}$ delafossites ($A$=Na, Cu), are model frustrated antiferromagnets, with triangular layers of Mn$^{3+}$~spins. At low temperatures ($T_{N}$=65 K), a $C2/m \rightarrow P\overline{1}$ transition is found in CuMnO$_2$, which breaks frustration and establishes magnetic order. In contrast to this clean transition, $A$=Na only shows short-range distortions at $T_N$. Here we report a systematic crystallographic, spectroscopic, and theoretical investigation of CuMnO$_2$. We show that, even in stoichiometric samples, non-zero anisotropic Cu displacements co-exist with magnetic order. Using X-ray/neutron diffraction and Raman scattering, we show that high pressures acts to decouple these degrees of freedom. This manifests as an isostuctural phase transition at $\sim$10 GPa, with a reversible collapse of the $c$-axis. This is shown to be the high pressure analog of the $c$-axis negative thermal expansion seen at ambient pressure. DFT simulations confirm that dynamical instabilities of the Cu$^{+}$ cations and edge-shared MnO$_{6}$ layers are intertwined at ambient pressure. However, high pressure selectively activates the former, before an eventual predicted re-emergence of magnetism at the highest pressures. Our results show that the lattice dynamics and local structure of CuMnO$_2$ are quantitatively different to non-magnetic Cu delafossites, and raise questions about the role of intrinsic inhomogeniety in frustrated antiferromagnets.