Incommensurate atomic and magnetic modulations in the spin-frustrated β−NaMnO2 triangular lattice

Abstract

The layered β-NaMnO 2 , a promising Na-ion energy-storage material has been investigated for its triangular lattice capability to promote complex magnetic configurations that may release symmetry restrictions for the coexistence of ferroelectric and magnetic orders. The complexity of the neutron powder diffraction patterns underlines that the routinely adopted commensurate structural models are inadequate. Instead, a single-phase superspace symmetry description is necessary, demonstrating that the material crystallizes in a compositionally modulated q= (0.077(1), 0, 0) structure. Here, Mn 3+ Jahn-Teller distorted MnO 6 octahedra form corrugated layer stacking sequences of the β-NaMnO 2 type, which are interrupted by flat sheets of the α-like oxygen topology. Spontaneous long-range collinear antiferromagnetic order, defined by the propagation vector k= (½, ½, ½), appears below T N1 = 200 K. Moreover, a second transition into a spatially modulated proper-screw magnetic state (kq) is established at T N2 = 95 K, with an antiferromagnetic order parameter resembling that of a two-dimensional (2D) system. The evolution of 23 Na NMR spin-lattice relaxation identifies a magnetically inhomogeneous state in the intermediate T-region (T N2 <T< T N1 ), while its strong suppression below T N2 indicates that a spin-gap opens in the excitation spectrum. High-resolution neutron inelastic scattering confirms that the magnetic dynamics are indeed gapped (Δ~5 meV) in the low-temperature magnetic phase, while simulations on the basis of the single-mode approximation suggest that Mn-spins residing on adjacent antiferromagnetic chains, establish sizable 2D correlations. Our analysis points that novel structural degrees of freedom promote, cooperative magnetism and emerging dielectric properties in this non-perovskite-type of manganite.

Nanoscale degeneracy lifting in a geometrically frustrated antiferromagnet

“…In a more general context, NMO appears to be a rare example of a geometrically frustrated magnet exhibiting long-range magnetic order coupled to a shortrange lattice distortion. Separately, the local triclinic distortion observed in NMO by PDF may explain various unusual experimental results in the literature, including the subtle anisotropic broadening of the low-temperature Bragg peaks seen in previous diffraction work [12], the nanoscale inhomogeneities of the ground state suggested by muon spin relaxation and nuclear magnetic resonance experiments [17,20], and the emergent magnetoelectric response [22].…”

“…The experimental evidence here and elsewhere indicates that NMO exhibits significantly more structural disorder than CMO. This is evidenced by the more pronounced distortion of the local structure in NMO than CMO, the greater percentage of site occupancy disorder (8% Na vacancies versus 2% excess Cu from antisite defects), and a higher density of planar defects (see Supplementary Information) [21,22,36]. Note that the Na vacancies and Cu impurities allow for conversion of Mn 3+ (S = 2) to Mn 4+ (S = 3/2) [37], which could also create magnetic disorder.…”

“…Recent work further suggests that the magnetic ground state is intrinsically inhomogeneous [20], adding to the unusual nature of the magnetism in this compound. Finally, this system exhibits polymorphism [21] and significant structural disorder [22] that may influence the magnetostructural properties, although how this may happen remains an open question. Clarifying these issues in NMO continues to be an important objective in the study of AMnO 2 systems, while also presenting a valuable opportunity for a more general study of magnetic frustration, disorder, and their interplay in TMOs.…”

Cluster glass behavior of the frustrated birnessites AxMnO2·yH2O(AKulish1, Scholtens2, Blake3 2019Phys. Rev. B70152View full textAdd to dashboardCite

“…15 In the case of β-NaMnO2, it was shown that a spontaneous long-range collinear AFM order appears below 200 K. Moreover, a transition to a spatially modulated proper screw magnetic state was found at 95 K. Between these two transitions a magnetically inhomogeneous state exists, and a spin gap (~ 5 meV) opens in the low-temperature state. 20 Summarizing current knowledge of these systems, a precise description of the magnetic properties of birnessite compounds remains obscure, specifically, the influence of the mixed oxidation state of the manganese ions, the distribution of Mn 3+/4+ in the matrix, as well as how the alkali cation deficiency and the number of water molecules can be controlled to tune the magnetic properties. Here we take one of the first steps to explore the structure and magnetic behavior of birnessite compounds.…”

Phase stability of the layered oxide, Ca2Mn3O8; probing interlayer shearing at high pressure

“…When B is a magnetic ion with unpaired electrons the triangular connectivity between these species often results in the realisation of exotic magnetic states. [11][12][13][14][15][16] More recently, interest is growing in the layered transition metal oxide, Ca 2 Mn 3 O 8 , first reported by Horowitz et al in 1978. 17 They showed that Ca 2 Mn 3 O 8 crystallises with a monoclinic C2/m layered structure described by Mn 3 O 8 4À sheets, formed from edge sharing MnO 6 octahedra and separated by trigonal bipyramidal CaO 6 sites as shown in Fig.…”