  The layered {\beta}-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
{\beta}-NaMnO2 type, which are interrupted by flat sheets of the {\alpha}-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 proper-screw 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 inhomogene-ous 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 ({\Delta}~5 meV) in the low-temperature magnetic phase, while
simulations on the basis of the single-mode approximation suggest that Mn-spins
residing on ad-jacent antiferromagnetic chains, establish sizable 2D
correlations. Our analysis points that novel struc-tural degrees of freedom
promote, cooperative magnetism and emerging dielectric properties in this
non-perovskite-type of manganite.
