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Incommensurate atomic and magnetic modulations in the spin-frustrated ?-NaMnO2 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 ?-NaMnO2 triangular lattice. Physical Review Materials, 2 (7). ISSN 2475-9953. E-ISSN 2475-9953. (doi:10.1103/PhysRevMaterials.2.074407)

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The layered ?-NaMnO2, 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 Mn3+ Jahn-Teller distorted MnO6 octahedra form corrugated layer stacking sequences of the ?-NaMnO2 type, which are interrupted by flat sheets of the ?-like oxygen topology. Spontaneous long-range collinear antiferromagnetic order, defined by the propagation vector k = (1/2, 1/2, 1/2), appears below TN1 = 200 K. Moreover, a second transition into a spatially modulated properscrew magnetic state (k ± q) is established at TN2 = 95 K, with an antiferromagnetic order parameter resembling that of a two-dimensional (2D) system. The evolution of 23Na NMR spin-lattice relaxation identifies a magnetically inhomogeneous state in the intermediate T region (TN2 <T <TN1), while its strong suppression below TN2 indicates that a spin gap opens in the excitation spectrum. High-resolution neutron inelastic scattering confirms that the magnetic dynamics are indeed gapped (? ? 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
DOI/Identification number: 10.1103/PhysRevMaterials.2.074407
Subjects: Q Science
Divisions: Faculties > Sciences > School of Physical Sciences > Functional Materials Group
Depositing User: Mark Green
Date Deposited: 12 Jun 2018 15:49 UTC
Last Modified: 12 Jul 2019 14:45 UTC
Resource URI: (The current URI for this page, for reference purposes)
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