# Incommensurate atomic and magnetic modulations in the spin-frustrated $$β-NaMnO_2$$ triangular lattice

Orlandi, Fabio, Aza, Eleni, Bakaimi, Ioanna, Kiefer, Klaus, Klemke, Bastian, Zorko, Andrej, Arčon, Denis, Stock, Christopher, Tsibidis, George D., Green, Mark A., and others. (2018) Incommensurate atomic and magnetic modulations in the spin-frustrated $$β-NaMnO_2$$ triangular lattice. Physical Review Materials, 2 (7). Article Number 074407. ISSN 2475-9953. E-ISSN 2475-9953. (doi:10.1103/PhysRevMaterials.2.074407) (KAR id:67281)

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## 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 $$k = (_{1/2,1/2,1/2})$$, appears below $$T_{N1} = 200 K$$. Moreover, a second transition into a spatially modulated properscrew 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 out that novel structural degrees of freedom promote cooperative magnetism and emerging dielectric properties in this nonperovskite type of manganite.

Item Type: Article 10.1103/PhysRevMaterials.2.074407 Q Science Divisions > Division of Natural Sciences > School of Physical Sciences Mark Green 12 Jun 2018 15:49 UTC 16 Feb 2021 13:55 UTC https://kar.kent.ac.uk/id/eprint/67281 (The current URI for this page, for reference purposes) https://orcid.org/0000-0002-7886-6205