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 Os^III and one or two Mn^II 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.

Keywords:

• magnetic anisotropy
• osmium
• spin-orbit
• anisotropic exchange
• single molecule magnet

Acknowledgements

This work has been supported by the Deutsche Forschungsgemeinschaft (DFG) through the transregional collaborative research centre SFB/TRR 88 ‘3MET’. We are also grateful for computer time (project acronym TUKL-MagCalc) provided by the Allianz für Hochleistungsrechnen Rheinland-Pfalz (AHRP).