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

TL;DR

This study uses 23Na NMR to investigate magnetic phases and spin dynamics in Na2Co2TeO6, revealing a transition from incoherent to spin-wave excitations and field-dependent suppression of spin correlations.

Contribution

It provides new insights into the low-energy spin excitations and field effects in Na2Co2TeO6, a candidate Kitaev material, highlighting the role of magnetic frustration and damping of spin waves.

Findings

Absence of spin-disordered phase up to 9 T

Enhanced 1/T1 in the intermediate temperature region

Field-dependent suppression of spin stiffness

Abstract

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 ~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.