Plasma-enhanced magnetic transition in 3D MnII–NbIV octacyanidometalate magnetic sponge

Abstract

A new magnetic phase with Curie temperature of T_C = 72 K is obtained by exposing a three-dimensional {[Mn^II(H₂O)₂]₂[Nb^IV(CN)₈]·4H₂O}_n coordination ferrimagnet (T_C = 49 K) to air, oxygen, nitrogen, and argon-based plasma. The X-ray powder diffraction and X-ray absorption spectroscopy reveal that the unit cell slightly contracts after plasma treatment. Shortening the distance between the magnetic ions results in a 20% enhancement of the superexchange couplings, as estimated by the mean-field approximation model. To explain how plasma modifies the magnetic properties of the studied system, several attempts with heating, UV exposure, and dehydration are used to reproduce the same post-plasma magnetic phase. However, none of them are successful. Samples subjected to heat and UV show partial decomposition of the original compound, while the dehydrated material undergoes a major structural reorganization. Ultimately, the observed changes are attributed to the removal of crystallization water molecules, which apparently could only be achieved through plasma treatment.