Precise Electrochemical Control of Ferromagnetism in a Cyanide-Bridged Bimetallic Coordination Polymer

Abstract

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]_0.7·4.7H₂O (NiFe-PBA) shows Li ion insertion/extraction reversibly accompanied with reversible Fe³⁺/Fe²⁺ reduction/oxidation. When Li ion is inserted quantitatively into NiFe-PBA, the ferromagnetic transition temperature <i>T</i>_C gradually decreases due to reduction of paramagnetic Fe³⁺ to diamagnetic Fe²⁺, and the ferromagnetic transition is completely suppressed for Li_0.6(NiFe-PBA). On the other hand, <i>T</i>_C increases continuously as Li ion is extracted due to oxidation of diamagnetic Fe²⁺ to paramagnetic Fe³⁺, and the ferromagnetic transition is nearly recovered for Li₀(NiFe-PBA). Furthermore, the plots of <i>T</i>_C as a function of the amount of inserted/extracted Li ion <i>x</i> are well consistent with the theoretical values calculated by the molecular-field approximation