Anisotropic mixing of spin levels in spin-admixed tetrameric iron–sulfur clusters with spin frustration

Abstract

Mixing of the S=1/2 and S=3/2 levels in the ground state of the spin-admixed [4Fe–4S] + cluster by anisotropic local interactions is considered in the model with taking into account the Heisenberg exchange and double exchange couplings. In the area of crossing and anisotropic mixing of levels, the ground state represents strong mixture of some S=1/2 and S=3/2 levels with different intermediate spins. Small variation of the exchange parameters results in essential change of the spin of the ground state and intermediate spins – spin frustration effect in clusters. Anisotropic mixing of the S=1/2 and S=3/2 states strongly contributes to zero-field splittings (ZFS) of levels. ZFS essentially depends on individual ZFS parameters and isotropic exchange interactions. g-Factors of the lowest doublets are determined by anisotropic mixing of the S=1/2 and S=3/2 levels. g-Values strongly depend on the sign and value of local ZFS parameters, the Heisenberg exchange and double exchange parameters, intermediate spins. Anisotropic mixing of levels results in strong anisotropy of g-factors. The model with anisotropic mixing provides qualitative explanation of experimentally observed g-factors of spin-admixed cluster compound. Effective hyperfine constants of the spin-mixed system essentially depend on the local ZFS parameters and on the exchange interactions in cluster. Mixing of the S=1/2 and S=3/2 levels leads to increase or reduction of hyperfine constants in the area of mixing depending on the sign of individual ZFS parameters. Anisotropic mixing of levels reduces the magnetic moment in comparison with magnetic moment of the pure S gr=3/2 cluster.