Commensurate helicoidal order in the triangular layered magnet Na₂MnTeO₆

The trigonal layered quaternary tellurate Na₂MnTeO₆ has been studied by means of various techniques to clarify its magnetic properties. The crystal structure of this compound is based on the triangular arrangement of all cations in the parallel layers with the space group P31c. By using symmetry analysis of the magnetic neutron scattering data, we have found that the solution for the magnetic structure corresponds to the magnetic Shubnikov group R3´c´ (No. 167.4.1337). Mn⁴⁺ ions in an octahedral environment form a triangular network where all spins are directed from the center of each triangle. Overall magnetic structure in Na₂MnTeO₆ is commensurate 120° spin helix with propagation vector k = (1/3, 1/3, 1/3) in variance with planar spin structure in structurally equivalent Li₂MnTeO₆ with magnetic propagation vector k = (1/3, 1/3, 0). The magnetization measurements show that Na₂MnTeO₆ experiences an antiferromagnetic order at T_N = 5.5 K. NMR, electron spin resonance, and thermodynamics experiments demonstrate the extended temperature region of 2D short-range correlations well above the ordering temperature.