Compared to the previous report on $\mathrm{C}{\mathrm{u}}_3\mathrm{Bi}{\left(\mathrm{Se}{\mathrm{O}}_3\right)}_2{\mathrm{O}}_{2}X\left(X=\mathrm{Cl},\mathrm{Br}\right)$ [V. Gnezdilov et al., (arXiv:1604.04249)],where no dielectric anomaly was observed without magnetic field near $T_N\sim 25.6\mathrm{K}$, we further present the dielectric behaviors without and with magnetic field in $\mathrm{C}{\mathrm{u}}_3\mathrm{Bi}{\left(\mathrm{Se}{\mathrm{O}}_3\right)}_2{\mathrm{O}}_2\mathrm{Cl}$. At zero field $H=0$, an antiferromagnetic transition from magnetization and specific heat measurements is clearly established at $T_N$, while no dielectric anomaly was observed. Those results are similar to the previous report [V. Gnezdilov et al., (arXiv:1604.04249)]. Above the critical field $H_{\mathrm{c}}\sim 0.8\mathrm{T}$, a metamagnetic spin-flip transition from antiferromagnetic to ferrimagnetic order at $T\sim T_N$ is induced anisotropically only for $H\parallel c$. Meanwhile, a ferroelectric behavior from dielectric and pyroelectric current measurements is observed below $T\sim T_N$; then a corresponding type-II multiferroics emerges above $H_{\mathrm{c}}$. The key mechanism of the anisotropic spin-flip-induced multiferroicity in $\mathrm{C}{\mathrm{u}}_3\mathrm{Bi}{\left(\mathrm{Se}{\mathrm{O}}_3\right)}_2{\mathrm{O}}_2\mathrm{Cl}$ can be ascribed to the breaking of magnetic twofold symmetry in the $bc$ plane above $H_{\mathrm{c}}$.

• Received 15 August 2016
• Revised 2 February 2017

DOI:https://doi.org/10.1103/PhysRevB.95.125121