Oscillatory exchange coupling in magnetic molecules

Abstract

Recently, first-principles calculations based on the spin-dependent density functionaltheory (DFT) have revealed that the magnetic ground state of a finite linearcarbon chain capped by two transition metal (TM) atoms alternates betweenferromagnetic and antiferromagnetic configurations depending on the number ofcarbon atoms. The character of indirect exchange coupling in this nanoscale,quasi-zero-dimensional system is different from those analogous extended structuresconsisting of magnetic layers separated by a non-magnetic spacer (or magnetic impuritiesin a non-magnetic host material) and a formulation based on an atomic picture is needed.We present a tight-binding model which provides a theoretical framework to theunderlying mechanism of the exchange coupling in molecular structures. Themodel calculations are capable of reproducing the essential features of the DFTresults for the indirect exchange coupling and the atomic magnetic moments in theTM–Cn–TM structures as functions of the number of carbon atoms. In nanostructures consisting of afew atoms the concepts of extended wavefunctions and the band theory lose their validity,and hence the oscillatory exchange coupling turns out to be a consequence ofquantum interference effects due to the spin-dependent onsite and hopping energies.