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Microscopic model for high-spin versus low-spin ground state in $[Ni_2M(CN)_8](M=Mo^V,W^V,Nb^I^V)$ magnetic clusters

Raghunathan, Rajamani and Sutter, Jean Pascal and Ducasse, Laurent and Desplanches, Cédric and Ramasesha, S (2006) Microscopic model for high-spin versus low-spin ground state in $[Ni_2M(CN)_8](M=Mo^V,W^V,Nb^I^V)$ magnetic clusters. In: Physical Review B: Condensed Matter and Materials Physics, 73 (10). 104438-1-104438-8.

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Conventional superexchange rules predict ferromagnetic exchange interaction between Ni(II) and $M(M=Mo^V, W^V, Nb^I^V)$. Recent experiments show that in some systems this superexchange is antiferromagnetic. To understand this feature, in this paper we develop a microscopic model for Ni(II)-M systems and solve it exactly using a valence bond approach. We identify the direct exchange coupling, the splitting of the magnetic orbitals and the interorbital electron repulsions, on the M site as the parameters which control the ground state spin of various clusters of the Ni(II)-M system. We present quantum phase diagrams which delineate the high-spin 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 in the parameter space where an intermediate spin state is the ground state. These results indicate that the spin spectrum of the microscopic model cannot be reproduced by a simple Heisenberg exchange Hamiltonian.

Item Type: Journal Article
Publication: Physical Review B: Condensed Matter and Materials Physics
Publisher: American Physical Society
Additional Information: Copyright of this article belongs to American Physical Society.
Department/Centre: Division of Chemical Sciences > Solid State & Structural Chemistry Unit
Date Deposited: 26 Apr 2006
Last Modified: 19 Sep 2010 04:26
URI: http://eprints.iisc.ac.in/id/eprint/6478

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