Orlandi, F.; Aza, E.; Bakaimi, I.; Kiefer, K.; Klemke, B.; Zorko, A.; Arcon, D.; Stock, C.; Tsibidis, G.; Green, M.; Manuel, P.; Lappas, A.: Incommensurate atomic and magnetic modulations in the spin-frustrated β-NaMnO2 triangular lattice. Physical Review Materials 2 (2018), p. 074407/1-14
10.1103/PhysRevMaterials.2.074407
Open Accesn Version
Abstract:
The layered β-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. HereMn3+ Jahn-Teller distorted MnO6 octahedra form corrugated layer stacking sequences of the β-NaMnO2 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 TN1 = 200K. Moreover, a second transition into a spatially modulated properscrew magnetic state (k ± q) is established at TN2 = 95K, with an antiferromagnetic order parameter resembling that of a two-dimensional (2D) system. The evolution of 23NaNMRspin-lattice relaxation identifies amagnetically 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 themagnetic dynamics are indeed gapped (∼5meV) 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.