Field evolution of magnetic phases and spin dynamics in the honeycomb lattice magnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Na</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Co</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>TeO</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math>: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Na</mml:mi><mml:mprescripts/><mml:none/><mml:mn>23</mml:mn></mml:mmultiscripts></mml:math> NMR study
Jun Kikuchi, Takayuki Kamoda, Nobuyoshi Mera, Yodai Takahashi, Kouji Okumura, Yukio Yasui
Abstract
We report on the results of $^{23}\mathrm{Na}$ NMR in the honeycomb lattice magnet ${\mathrm{Na}}_{2}{\mathrm{Co}}_{2}{\mathrm{TeO}}_{6}$, 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 N\'eel temperature ${T}_{N}$, we find a temperature region extending down to $\ensuremath{\sim}{T}_{N}/2$, where the nuclear spin-lattice relaxation rate $1/{T}_{1}$ remains enhanced and is further increased by a magnetic field. This region crosses over to a low-temperature region characterized by the rapidly decreasing $1/{T}_{1}$, 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-$\mathbf{q}$ magnetic order possessing partially disordered moments as scattering centers of spin waves. In the paramagnetic phase near ${T}_{N}$, dramatic field suppression of $1/{T}_{1}$ is observed. From analysis of the temperature dependence of $1/{T}_{1}$ based on the renormalized-classical description of a two-dimensional quantum antiferromagnet, we find the field-dependent spin stiffness constant that scales with ${T}_{N}$ 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.