Incommensurate atomic and magnetic modulations in the spin frustrated beta NaMnO2 triangular lattice

The layered amp; 946; 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 amp; 946; NaMnO2 type, which are interrupted by flat sheets of the amp; 945; 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 lt; T lt; 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 amp; 8764;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 manganit