Crystallite size-dependent magnetic moment in hydrogen bridged manganese tetracyanonickelate based two-dimensional metal-organic frameworks

Abstract

Two-dimensional (2D) metal–organic frameworks (MOFs) are the coordination network of organic cyanide (CN) ligands and metal cyanometallates. They possess great potential as molecular magnets between subzero up to above room temperature. These complexes are greatly sought after due to the unique structural features that allow them to offer tunable magnetic properties by changing crystal entities and/or through external stimulation such as light and heat. Layered structure of tetracyanonickellate based MOFs T(H2O)2Ni(CN)4·.4H2O, T = Mn herein called Mn-MOF, have been synthesized by the chemical precipitation method with controlled particle size using tri-sodium citrate dihydrate (TSCD) as the chelating agent. This work involves the study of spontaneous magnetism in the layered solid, the effect of their crystallite sizes and morphology on the magnetic susceptibility. The magnetic interaction takes place in-plane between the pseudo-octahedral metal (T) ions through the diamagnetic Ni2+ center, and out-of-plane through the hydrogen bonding bridge formed in the interlayer spacing. Magnetic properties were measured using Vibrating Sample Magnetometry (VSM) and calculated for Curie temperatures with magnetic exchange energies using the Curie-Weiss mean field theory and total angular momentum of the involved metal ions. The MOFs exhibit a paramagnetic behavior above 50 K and show increased magnetic susceptibility on cooling with a paramagnetic to weak ferromagnetic (wFM) transition below 50 K. A field-dependent response of the magnetic ordering temperature from 34 to 44 K was also observed for the Mn(H2O)2Ni(CN)4·.4H2O complex. The value of magnetic exchange coupling between interlayer metal centers increases though the hydrogen bonding network which results increased magnetic moment with decreased crystallite surface area to volume ratio.