Spin dynamics and magnetoelectric properties of the coupled-spin tetrahedral compoundCu2Te2O5Cl2

Abstract

We report on the spin dynamics and discovery of magnetoelectricity in the coupled-spin tetrahedral compound ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{\mathrm{Cl}}_{2}$. $^{125}\mathrm{Te}$ NMR measurements show an anomalous resonance frequency shift and a signal wipe-out phenomenon around the N\'eel temperature ${T}_{N}$ = 18.2 K, which could be attributed to the anomalous critical slowing down of the Cu spin fluctuations on the NMR time scale ($\ensuremath{\sim}10$--100 MHz). The critical exponent of ${({T}_{1}T)}^{\ensuremath{-}1}\ensuremath{\propto}{(T\ensuremath{-}{T}_{N})}^{\ensuremath{-}\ensuremath{\alpha}}$ is 0.40 \ifmmode\pm\else\textpm\fi{} 0.03, as compared to 0.5 for a three-dimensional mean-field model. This is in contrast to the Br compound [S.-H. Baek et al., Phys. Rev. B 86, 180405 (2012)], which exhibits pronounced singlet dynamics with a large spin gap. Electric polarization (${P}_{\mathrm{c}}$) is observed along the $c$ axis for temperatures below ${T}_{N}$ under finite magnetic field but not sensitive to the electric poling. ${P}_{\mathrm{c}}$ increases sharply over zero to 2 T and then reaches saturation. Below ${T}_{N}{}_{,}$ ${P}_{\mathrm{c}}$ changes its sign depending on the applied magnetic field direction, positive for the $H\ensuremath{\perp}c$ axis and negative for $H$ \ensuremath{\parallel} $c$ axis. We discuss possible explanations for the observed magnetoelectric (ME) behavior in terms of linear ME effect, spin-driven multiferroicity, and an exchange striction of intertetrahedral exchange paths involving the ${\mathrm{Te}}^{4+}$ lone-pair ions. Our results suggest that ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{\mathrm{Cl}}_{2}$ is a type of ME material whose properties are tuned by intertetrahedral exchange interactions involving polarizable ${\mathrm{Te}}^{4+}$ ions.