Field evolution of magnetic phases and spin dynamics in the honeycomb lattice magnet Na2Co2TeO6: 23Na NMR study

We report on the results of 23Na NMR in the honeycomb lattice magnet Na2Co2TeO6 , which has been nominated as a Kitaev material. Measurements of magnetic shift and width of the NMR line as functions of temperature and magnetic field show that a spin-disordered phase does not appear up to a field of 9 T. In the antiferromagnetic phase just below the Neel temperature TN , we find a temperature region extending down to similar to TN/2, where the nuclear spin-lattice relaxation rate 1/T1 remains enhanced and is further increased by a magnetic field. This region crosses over to a low-temperature region characterized by the rapidly decreasing 1/T1 , which is less field-sensitive. These observations suggest incoherent spin excitations with a large spectral weight at low energies in the intermediate temperature region transforming to more conventional spin-wave excitations at low temperatures. The drastic change of the low-energy spin dynamics is likely caused by strong damping of spin waves activated only in the intermediate temperature region, which may be realized for triple -q magnetic order possessing partially disordered moments as scattering centers of spin waves. In the paramagnetic phase near TN , dramatic field suppression of 1/T1 is observed. From analysis of the temperature dependence of 1/T1 based on the renormalized-classical description of a two-dimensional quantum antiferromagnet, we find the field-dependent spin stiffness constant that scales with TN as a function of magnetic field. This implies field suppression of the energy scale characterizing both two-dimensional spin correlations and three-dimensional long-range order, which may be associated with an increasing effect of frustration in magnetic fields.

Automated summary

The research reports on the results of 23Na NMR in the honeycomb lattice magnet Na2Co2TeO6, indicating the presence of different spin phases. In the antiferromagnetic phase, a transition between spin disordered phase and spin wave excitations was observed. Near the critical temperature, a field-dependent spin stiffness constant was found, reflecting the suppression of energy scales associated with magnetic field and frustration effects.