Precise electrochemical control of ferromagnetism in a cyanide-bridged bimetallic coordination polymer

Magnetic coordination polymers can exhibit controllable magnetism by introducing responsiveness to external stimuli. This report describes the precise control of magnetism of a cyanide-bridged bimetallic coordination polymer (Prussian blue analogue: PBA) through use of an electrochemical quantitative Li ion titration technique, i.e., the galvanostatic intermittent titration technique (GITT). K _0.2Ni[Fe(CN) ₆] _0.74.7H ₂O (NiFe-PBA) shows Li ion insertion/extraction reversibly accompanied with reversible Fe ³⁺/Fe ²⁺ reduction/oxidation. When Li ion is inserted quantitatively into NiFe-PBA, the ferromagnetic transition temperature T _C gradually decreases due to reduction of paramagnetic Fe ³⁺ to diamagnetic Fe ²⁺, and the ferromagnetic transition is completely suppressed for Li _0.6(NiFe- PBA). On the other hand, T _C increases continuously as Li ion is extracted due to oxidation of diamagnetic Fe ²⁺ to paramagnetic Fe ³⁺, and the ferromagnetic transition is nearly recovered for Li ₀(NiFe-PBA). Furthermore, the plots of T _C as a function of the amount of inserted/extracted Li ion x are well consistent with the theoretical values calculated by the molecular-field approximation.