Strengthening the Magnetic Interactions in Pseudobinary First-Row Transition Metal Thiocyanates, M(NCS)2.

Abstract

Understanding the effect of chemical composition on the strength of magnetic interactions is key to the design of magnets with high operating temperatures. The magnetic divalent first-row transition metal (TM) thiocyanates are a class of chemically simple layered molecular frameworks. Here, we report two new members of the family, manganese (II) thiocyanate, Mn(NCS)₂, and iron (II) thiocyanate, Fe(NCS)₂. Using magnetic susceptibility measurements on these materials and on cobalt (II) thiocyanate and nickel (II) thiocyanate, Co(NCS)₂ and Ni(NCS)₂, respectively, we identify significantly stronger net antiferromagnetic interactions between the earlier TM ions—a decrease in the Weiss constant, θ, from 29 K for Ni(NCS)₂ to −115 K for Mn(NCS)₂—a consequence of more diffuse 3d orbitals, increased orbital overlap and increasing numbers of unpaired t2g electrons. We elucidate the magnetic structures of these materials: Mn(NCS)₂, Fe(NCS)₂ and Co(NCS)₂ order into the same antiferromagnetic commensurate ground state, whilst Ni(NCS)₂ adopts a ground state structure consisting of ferromagnetically ordered layers stacked antiferromagnetically. We show that significantly stronger exchange interactions can be realised in these thiocyanate frameworks by using earlier TMs.