Decoupling Lattice and Magnetic Instabilities in Frustrated CuMnO₂

Area of Science:

• Condensed Matter Physics
• Materials Science
• Magnetism

Background:

• Delafossites with the chemical formula AMnO2 (A = Na, Cu) serve as model systems for studying frustrated antiferromagnetism due to their triangular manganese spin lattices.
• CuMnO2 undergoes a magnetic transition at 65 K, involving a structural change from C2/m to P1̅ symmetry, which resolves magnetic frustration and establishes long-range magnetic order.
• In contrast, NaMnO2 exhibits only short-range structural distortions at its magnetic transition temperature.

Purpose of the Study:

• To conduct a comprehensive investigation of CuMnO2 using crystallographic, spectroscopic, and theoretical methods.
• To elucidate the interplay between copper ion displacements, lattice dynamics, and magnetic ordering under varying pressure conditions.
• To explore the high-pressure behavior of CuMnO2 and its relationship to ambient pressure phenomena like negative thermal expansion.

Main Methods:

• X-ray and neutron diffraction were employed to analyze crystallographic structures under pressure.
• Raman scattering was used to probe lattice vibrations and phase transitions.
• Density functional theory (DFT) simulations were performed to model the electronic structure and predict material behavior.

Main Results:

• Anisotropic copper displacements were observed to coexist with magnetic order even in stoichiometric CuMnO2 samples.
• Applying hydrostatic pressure leads to an isostructural phase transition at approximately 10 GPa, characterized by a reversible collapse of the c-axis.
• DFT calculations indicate that dynamical instabilities of Cu+ cations and MnO6 octahedra are coupled at ambient pressure but are selectively activated by high pressure.

Conclusions:

• High pressure effectively decouples copper ion dynamics from the magnetic ordering, analogous to the negative thermal expansion observed at ambient pressure.
• The lattice dynamics and local structure of CuMnO2 are quantitatively distinct from nonmagnetic copper delafossites.
• The findings raise important questions regarding the role of intrinsic structural inhomogeneity in the behavior of frustrated antiferromagnetic materials.