Magnetic inhomogeneity on a triangular lattice: the magnetic-exchange versus the elastic energy and the role of disorder

Decoupling Lattice and Magnetic Instabilities in Frustrated CuMnO2

The AMnO₂ delafossites (A = Na, Cu) are model frustrated antiferromagnets, with triangular layers of Mn³⁺ spins. At low temperatures (T_N = 65 K), a C2/m → P1̅ transition is found in CuMnO₂, 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 CuMnO₂. We show that, even in stoichiometric samples, nonzero anisotropic Cu displacements coexist with magnetic order. Using X-ray/neutron diffraction and Raman scattering, we show that high pressures act to decouple these degrees of freedom. This manifests as an isostuctural phase transition at ∼10 GPa, with a reversible collapse of the c-axis. This is shown to be the high-pressure analogue of the c-axis negative thermal expansion seen at ambient pressure. Density functional theory (DFT) simulations confirm that dynamical instabilities of the Cu⁺ cations and edge-shared MnO₆ layers are intertwined at ambient pressure. However, high pressure selectively activates the former, before an eventual predicted reemergence of magnetism at the highest pressures. Our results show that the lattice dynamics and local structure of CuMnO₂ are quantitatively different from nonmagnetic Cu delafossites and raise questions about the role of intrinsic inhomogeneity in frustrated antiferromagnets.

Development of short and long-range magnetic order in the double perovskite based frustrated triangular lattice antiferromagnet Ba[Formula: see text]MnTeO[Formula: see text]

Frustrated magnets based on oxide double perovskites offer a viable ground wherein competing magnetic interactions, macroscopic ground state degeneracy and complex interplay between emergent degrees of freedom can lead to correlated quantum phenomena with exotic excitations highly relevant for potential technological applications. By local-probe muon spin relaxation ([Formula: see text]SR) and complementary thermodynamic measurements accompanied by first-principles calculations, we here demonstrate novel electronic structure and magnetic phases of Ba[Formula: see text]MnTeO[Formula: see text], where Mn[Formula: see text] ions with S = 5/2 spins constitute a perfect triangular lattice. Magnetization results evidence the presence of strong antiferromagnetic interactions between Mn[Formula: see text] spins and a phase transition at [Formula: see text] = 20 K. Below [Formula: see text], the specific heat data show antiferromagnetic magnon excitations with a gap of 1.4 K, which is due to magnetic anisotropy. [Formula: see text]SR reveals the presence of static internal fields in the ordered state and short-range spin correlations high above [Formula: see text]. It further unveils critical slowing-down of spin dynamics at [Formula: see text] and the persistence of spin dynamics even in the magnetically ordered state. Theoretical studies infer that Heisenberg interactions govern the inter- and intra-layer spin-frustration in this compound. Our results establish that the combined effect of a weak third-nearest-neighbour ferromagnetic inter-layer interaction (owing to double-exchange) and intra-layer interactions stabilizes a three-dimensional magnetic ordering in this frustrated magnet.

Thermal evolution of quasi-one-dimensional spin correlations within the anisotropic triangular lattice of α - NaMnO 2

Magnetic order on the spatially anisotropic triangular lattice of α - NaMnO 2 is studied via neutron diffraction measurements. The transition into a commensurate, collinear antiferromagnetic ground state with k = ( 0.5 , 0.5 , 0 ) was found to occur belowT N = 22 K . Above this temperature, the transition is preceded by the formation of a coexisting, short-range ordered, incommensurate state belowT IC = 45 K whose two-dimensional propagation vector evolves toward k = ( 0.5 , 0.5 ) as the temperature approachesT N . At high temperatures( T > T IC ) , quasielastic scattering reveals one-dimensional spin correlations along the nearest-neighbor Mn-Mn "chain direction" of the MnO6 planes. Our data are consistent with the predictions of a mean-field model of Ising-like spins on an anisotropic triangular lattice, as well as the predominantly one-dimensional Heisenberg spin Hamiltonian reported for this material.

Symmetry Reduction in the Quantum Kagome Antiferromagnet Herbertsmithite

Employing complementary torque magnetometry and electron spin resonance on single crystals of herbertsmithite, the closest realization to date of a quantum kagome antiferromagnet featuring a spin-liquid ground state, we provide novel insight into different contributions to its magnetism. At low temperatures, two distinct types of defects with different magnetic couplings to the kagome spins are found. Surprisingly, their magnetic response contradicts the threefold symmetry of the ideal kagome lattice, suggesting the presence of a global structural distortion that may be related to the establishment of the spin-liquid ground state.