Nanoscale degeneracy lifting in a geometrically frustrated antiferromagnet

Abstract

The local atomic and magnetic structures of the compounds AMnO2 (A = Na, Cu), which realize a geometrically frustrated, spatially anisotropic triangular lattice of Mn spins, have been investigated by atomic and magnetic pair distribution function analysis of neutron total scattering data. Relief of frustration in CuMnO2 is accompanied by a conventional cooperative symmetry-lowering lattice distortion driven by Néel order. In NaMnO2, however, the distortion has a short-range nature. A cooperative interaction between the locally broken symmetry and short-range magnetic correlations lifts the magnetic degeneracy on a nanometer length scale, enabling long-range magnetic order in the Na-derivative. The degree of frustration, mediated by residual disorder, contributes to the rather differing pathways to a single, stable magnetic ground state in these two related compounds. This study demonstrates how nanoscale structural distortions that cause local-scale perturbations can lift the ground state degeneracy and trigger macroscopic magnetic order. a arXiv:2001.09917v1 [cond-mat.str-el]

Decoupling Lattice and Magnetic Instabilities in Frustrated CuMnO2

“…However, neither our X-ray PDF measurements (Fig. 3), nor reported neutron PDFs 13 show any such effect for CuMnO 2 . These experiments used structure models in C2/m, with all symmetry allowed variables refined.…”

“…The nearly temperature independent values of the ADPs below T N =65 K, offer a possible explanation. Upon cooling NaMnO 2 , magnetic frustration drives local symmetry breaking, which manifests as short-range order in neutron scattering 13 , as well as increased lattice microstrain 9 . However, neither our X-ray PDF measurements (Fig.…”

“…In contrast, a bulk phase transition is seen for CuMnO 2 , with a well-ordered triclinic ground state [10][11][12] at the Néel temperature (T N ) of 65 K. This distortion lifts magnetic frustration, as the [110] in-plane couplings are no longer equivalent by symmetry. Curiously, neutron scattering 13 shows that short-range fluctuations of Mn-Mn distances persist well above T N for NaMnO 2 only. The interlayer chemistry has further subtle effects in CuMnO 2 .…”

“…This picture is also supported by theoretical estimates of the magnetoelastic coupling 16 . In contrast, Frandsen et al used pair distribution function analysis to show that the instantaneous structure of NaMnO 2 is actually more triclinic than CuMnO 2 on the local scale 13 . This was said to indicate that residual disorder explains the different structural ground states, and that the P 1 structure reflects the non-disordered limit.…”

Charge-ordered state satisfying the Anderson condition in LiRh2O4 arising from local dimer order

“…Although a regular pyrochlore lattice was expected above the transition, recent experimental results using the pair distribution function (PDF) analysis of the local structure have shown that tetragonal distortions appear in the shortrange length scales due to the realization of fluctuating d-orbital degeneracy lifting (ODL) [21,22]. ODL states do not only appear in special systems, but are universally found in transition metal compounds, linked to several important phenomena in condensed matter physics [21,[25][26][27][28][29]. For example, ODL states have been predicted in the superconductor FeSe due to local nematicity [25,26].…”