Magnetic Ordering-Induced Multiferroic Behavior in [CH3NH3][Co(HCOO)3] Metal–Organic Framework

We present the first example of magnetic ordering-induced multiferroic behavior in a metal–organic framework magnet. This compound is [CH3NH3]­[Co­(HCOO)3] with a perovskite-like structure. The A-site [CH3NH3]+ cation strongly distorts the framework, allowing anisotropic magnetic and electric behavior and coupling between them to occur. This material is a spin canted antiferromagnet below 15.9 K with a weak ferromagnetic component attributable to Dzyaloshinskii–Moriya (DM) interactions and experiences a discontinuous hysteretic magnetic-field-induced switching along [010] and a more continuous hysteresis along [101]. Coupling between the magnetic and electric order is resolved when the field is applied along this [101]: a spin rearrangement occurs at a critical magnetic field in the ac plane that induces a change in the electric polarization along [101] and [10-1]. The electric polarization exhibits an unusual memory effect, as it remembers the direction of the previous two magnetic-field pulses applied. The data are consistent with an inverse-DM mechanism for multiferroic behavior.

我们首次报道了金属有机框架磁体（metal–organic framework magnet）中由磁有序诱导产生的多铁性行为的首个实例。该化合物为[CH3NH3][Co(HCOO)3]，具有类钙钛矿结构（perovskite-like structure）。其A位阳离子[CH3NH3]+对框架产生强烈畸变，赋予体系各向异性的磁、电行为，并实现磁与电之间的耦合。
该材料在15.9 K以下为自旋倾斜反铁磁体，其弱铁磁分量可归因于德热纳-森重（Dzyaloshinskii–Moriya，DM）相互作用。当沿[010]方向施加磁场时，材料会发生不连续的磁场诱导磁滞翻转；沿[101]方向施加磁场时，则呈现更为连续的磁滞特性。
当沿[101]方向施加磁场时，磁有序与电有序之间的耦合效应得以显现：在临界磁场下，a-c晶面内发生自旋重排，进而诱导沿[101]与[10-1]方向的电极化发生改变。该材料的电极化表现出罕见的记忆效应，可记忆此前两次磁场脉冲的施加方向。
相关实验数据与多铁性行为的反DM相互作用机制相符。