We report on the spin dynamics and discovery of magnetoelectricity in the coupled-spin tetrahedral compound Cu2Te2O5Cl2. 125Te NMR measurements show an anomalous resonance frequency shift and a signal wipe-out phenomenon around the Néel temperature TN = 18.2 K, which could be attributed to the anomalous critical slowing down of the Cu spin fluctuations on the NMR time scale (∼10–100 MHz). The critical exponent of (T1T)−1∝(T−TN)−α is 0.40 ± 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 (Pc) is observed along the c axis for temperatures below TN under finite magnetic field but not sensitive to the electric poling. Pc increases sharply over zero to 2 T and then reaches saturation. Below TN, Pc changes its sign depending on the applied magnetic field direction, positive for the H⊥c axis and negative for H ∥ 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 Te4+ lone-pair ions. Our results suggest that Cu2Te2O5Cl2 is a type of ME material whose properties are tuned by intertetrahedral exchange interactions involving polarizable Te4+ ions.
Physics, Astronomy, and Materials Science
© 2014 American Physical Society
Besara, T., E. S. Choi, K-Y. Choi, P. L. Kuhns, A. P. Reyes, P. Lemmens, H. Berger, and N. S. Dalal. "Spin dynamics and magnetoelectric properties of the coupled-spin tetrahedral compound Cu 2 Te 2 O 5 Cl 2." Physical Review B 90, no. 5 (2014): 054418.
Physical Review B