Anisotropic spin dynamics in the frustrated chainCa3Co2O6detected by single-crystalC59oNMR

Abstract

Static and dynamic magnetic properties are investigated by $^{59}\text{C}\text{o}$ NMR measurements on a single crystal of ${\text{Ca}}_{3}{\text{Co}}_{2}{\text{O}}_{6}$, a rare model of the ferromagnetic Ising chain coupled antiferromagnetically on a triangular lattice. Above a ferrimagnetic order transition temperature ${T}_{\text{N}}=25\text{ }\text{K}$, the static spin susceptibility probed by the $^{59}\text{C}\text{o}$ Knight shift shows anisotropic behavior in the temperature dependence, consistent with ferromagnetic interactions along the $c$-axis chain and antiferromagnetic interactions in the $ab$ plane. Correspondingly, the dynamical spin susceptibility obtained from the $^{59}\text{C}\text{o}$ nuclear spin-lattice relaxation rate is also highly anisotropic. The intrachain ferromagnetic correlation steeply develops below 300 K while the interchain antiferromagnetic one does below 120 K with the critical exponent expected in the two-dimensional Ising antiferromagnet. Below ${T}_{\text{N}}$, the low-energy excitation arising from the predominant spin-wave magnons has an anisotropic full gap, reflecting the strongly Ising nature. Clear microscopic evidence of the ferrimagnetic order is demonstrated in the $^{59}\text{C}\text{o}$ NMR spectra at 5 K. Rotation of the external magnetic field from the $ab$ plane to the $c$ axis shows the first-order transition from the ferrimagnetic to the fully polarized state. The ferrimagnetic state with the uniform magnitude of magnetic moment is observed even in the transverse field adjusted parallel to the $ab$ plane.