Magnetic anisotropy through cooperativity in multinuclear transition metal complexes: theoretical investigation of an anisotropic exchange mechanism

Abstract

To be a candidate for a good single-molecule magnet, a compound should have a reasonably large zero-field splitting, a reasonably large total spin, and preferably axial symmetry. We investigate how this can be achieved in multi-heteronuclear d-Block transition metal complexes with both 3d and 5d centres. While spin-orbit coupling is large at the 5d centre, the 3d centres contribute many unpaired electrons. If the heavy-element building block does not have a single-ion anisotropy it might still transfer its spin-orbit effects to the 3d centre(s) through anisotropic exchange. We present spin-orbit configuration interaction calculations on a bi- and trinuclear compound with one OsIII and one or two MnII centres to demonstrate the mechanism. The interaction between the metal centres brings about zero-field splitting that is entirely due to anisotropic exchange. Because of strong spin-orbit coupling, the Os centre behaves as a pseudospin with that shows antiferromagnetic and anisotropic exchange coupling with the Mn centres. The ground state of the binuclear complex has a pseudo S = 2 ground state with an axial zero-field splitting parameter D = +0.59 cm−1, while the trinuclear complex has a pseudo ground state with a negative D = −0.32 cm−1. Coordination of the second Mn centre thus increases total spin and magnetic anisotropy, and reverses the sign of D.