Single crystal growth and magnetoelectric effect of Co₄Nb₂O₉

Multiferroics have attracted a lot of attention as they exhibit coexistence of two or more switchable states such as electric polarization, magnetization or strain rendering them very interesting for both their intriguing fundamental physics and promising applications. In the route towards materials having strong magnetoelectric (ME) responses, Co₄Nb₂O₉ (CNO) is very attractive. Both the magnetically-induced electric polarization and the control of magnetization with an electric field were observed in polycrystalline CNO. In this work, we first grow successfully single crystal of CNO samples by using optical floating zone technique, and find that the spin flop occurred along a axis rather than c axis. Furthermore, detailed ME measurements reveal a nonlinear ME effect, with linear and quadratic coefficients. A pronounced response was observed under a small cooling magnetic field which cannot even cause the spin flop. We also found that the magnetization of every axis responds to electric field applied along a-axis, but fail to do so when the electric field is applied c-axis.