High magnetic anisotropy and magnon excitations in single crystals of the double spin chain compound PbMn2Ni6Te3O18

We have successfully grown single crystals of $\mathrm{Pb}{\mathrm{Mn}}₂{\mathrm{Ni}}₆{\mathrm{Te}}₃{\mathrm{O}}₁₈$ and present a comprehensive study of their magnetic, thermodynamic, and Raman spectroscopic properties. $\mathrm{Pb}{\mathrm{Mn}}₂{\mathrm{Ni}}₆{\mathrm{Te}}₃{\mathrm{O}}₁₈$ consists of a planar network of pairwise rotated $\mathrm{Ni}{\mathrm{O}}₆$ dimers coupled by corners. Similarities to ${\mathrm{Ni}}₃\mathrm{Te}{\mathrm{O}}₆$ exist, which forms honeycomb layers. The magnetic susceptibility \ensuremath{\chi} and heat capacity ${C}_p$ data reveal an antiferromagnetic phase transition around 84 K, which is evidently hysteretic on warming and cooling between 94 and 40 K with a loop width of about 1.83\ifmmode\pm\else\textpm\fi{} 0.41 K; thus the transition appears to be of first order. \ensuremath{\chi} is anisotropic, with larger values for in-plane fields over the entire measured temperature range. Raman spectroscopy has been employed to investigate the lattice and magnetic excitations of the $\mathrm{Pb}{\mathrm{Mn}}₂{\mathrm{Ni}}₆{\mathrm{Te}}₃{\mathrm{O}}₁₈$ from 5 to 300 K. Besides an anharmonic phonon behavior, i.e., a decay into acoustic phonons, we find a coupling to the spin system at ${T}_N=84\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, as well as weak anomalies at ${T}^*\ensuremath{\approx}200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This second characteristic temperature gives evidence for an instability of the coupled spin/lattice system, as here the phonon linewidths and intensities evidence similar behavior as at ${T}_N=84\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Furthermore, magnetic Raman scattering at $240\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ is used to estimate an exchange coupling of $J=--86\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in general agreement with the Curie-Weiss temperature.