Introducing the monoclinic polymorph of the Kitaev magnet Na$_{2}$Co$_{2}$TeO$_{6}$

TL;DR

This paper reports the discovery of a monoclinic polymorph of Na$_{2}$Co$_{2}$TeO$_{6}$ with a distinct magnetic order and transition temperature, expanding understanding of honeycomb cobaltates and their magnetic behaviors.

Contribution

It introduces a new monoclinic polymorph of Na$_{2}$Co$_{2}$TeO$_{6}$ with unique magnetic properties and detailed neutron diffraction analysis, highlighting differences from the previously known hexagonal form.

Findings

Monoclinic polymorph exhibits a single AFM transition at 9.6 K.

Ground state shows in-plane zigzag AFM order with canted spins.

Magnetic field of 6 T suppresses the zigzag order.

Abstract

Recent theoretical studies have suggested that the low-energy Hamiltonian of honeycomb cobaltate systems could be dominated by anisotropic Kitaev interactions. Motivated by the theory, a honeycomb layered material Na$_{2}$Co$_{2}$TeO$_{6}$ with a hexagonal unit cell has been studied and found to exhibit antiferromagnetic (AFM) ordering at 27 K with two spin reorientation transitions at 15 and 5 K. Here we report a monoclinic polymorph of Na$_{2}$Co$_{2}$TeO$_{6}$, also with honeycomb layered structure but with a single AFM transition at 9.6 K and without spin reorientation transitions at lower temperatures. Using neutron diffraction, we identify an in-plane zigzag AFM order in the ground-state with the spins canted out of the honeycomb planes and ferromagnetically coupled between them. The zigzag order is suppressed by a magnetic field of 6 T.