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Microscopic model for superexchange interactions and photomagnetism in binuclear transition metal complexes

Ramasesha, S and Raghunathan, R and Ducasse, L and Sutter, JP and Mathonieere, C (2006) Microscopic model for superexchange interactions and photomagnetism in binuclear transition metal complexes. In: Phase Transitions, 79 (8). 637- 654.

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Recent experiments show that the superexchange interaction in molecular clusters containing transition metal ions $A = Ni^{II}$ and $B = W^V, Nb^{IV} or Mo^V$ in some cases is antiferromagnetic, contrary to the conventional superexchange rules. To understand this anomaly, we develop a quantum many-body model Hamiltonian and solve it exactly using a valence bond (VB) approach. We identify the various model parameters which control the ground state spin in different clusters of the A-B system. We present quantum phase diagrams that delineate the high and low-spin ground states in the parameter space. We fit the spin gap to a spin Hamiltonian and extract the effective exchange constant within the experimentally observed range, for reasonable parameter values. We also find a region of intermediate spin ground state in the parameter space, in clusters of larger size. The spin spectrum of the microscopic model cannot be reproduced by a simple Heisenberg exchange Hamiltonian. The above microscopic model is generic and can also be employed to explain photomagnetism in the $MoCu_6$ system. We solve the model for $MoCu_6$ and find that ground state is degenerate and is spanned by the S = 0, 1, 2 and 3 manifolds with doubly occupied Mo site corresponding to Mo(IV) and singly occupied Cu sites corresponding to Cu(II) configurations. In each of these spin spaces, we observe that there exist charge-transfer (CT) states at \approx 3 eV above the ground state which are dipole coupled to the ground state. The transition dipole in the S = 3 manifold is the largest for the CT excitations. Coupled with the fact that the density of states of the S = 3 manifold is sparse, compared to other spin manifolds, we expect that the S = 3 CT excited state to be long-lived, thereby explaining the experimentally observed photomagnetism in the $MoCu_6$ system.

Item Type: Journal Article
Publication: Phase Transitions
Publisher: Taylor and Francis
Additional Information: Copyright of this article belongs to Taylor and Francis.
Keywords: Superexchange interactions;Transition metal clusters;Photomagnetism
Department/Centre: Division of Chemical Sciences > Solid State & Structural Chemistry Unit
Date Deposited: 29 May 2008
Last Modified: 27 Aug 2008 13:24
URI: http://eprints.iisc.ac.in/id/eprint/14088

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