Regensburg 2004 – scientific programme

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MA: Magnetismus

MA 21: Bio- und molekularer Magnetismus

MA 21.4: Talk

Thursday, March 11, 2004, 11:00–11:15, H22

Charge-induced modulation of magnetic interactions in a [2 × 2] metallo-organic grid-complex — •Christian Romeike¹, Maarten R. Wegewijs¹, Wolfgang Wenzel², and Herbert Schoeller¹ — ¹Institut für Theoretische Physik A, RWTH Aachen, 52045 Aachen — ²Institut für Nanotechnologie, Forschungszentrum Karlsruhe, Postfach 3640, 76021 Karlsruhe

We study the electronic spin and charge degrees of freedom of a [2×2]-transition metal-grid complex cation at different stages of its reduction[1] in a phenomenological low temperature model. We show that extra electrons on the bridging ligands can drastically change the coupling of the spins on the metal sites and lead to maximal total spin states of the molecular complex (spin of unpaired electrons on the metals and ligands) due to the Nagaoka mechanism [2]. For a low-spin Fe²⁺-[2×2]-grid electrons can move between the ligands using an empty orbital on the metal as a bridge. In contrast to this, for a low-spin Co²⁺-[2×2]-grid the metal bridge contains a localized electron which can interact or exchange with the passing electrons. The presence of a localized spin on each bridging metal-ion in the case of Co²⁺ has two main effects: (1) the total spin achievable in the fully polarized Nagaoka state is larger and (2) the threshold Coulomb interaction required to achieve this state can be diminished to values close to those of Fe only if direct exchange coupling between the metal-ions and ligands is taken into account. It has to be of the same order as the exchange coupling obtained in perturbation theory.

1ex [1] M.Ruben et al., Chem.Eur.J. 2003, 9, No.1
Y. Nagaoka, Phys. Rev.147, 392 (1966)