Description of spin interactions in model [Fe_6S_6]~(4+) supercluster

摘要

A theoretical study of Heisenberg exchange and double exchange interactions in iron-sulphur superclusters is presented. The clusters can play an important role in biological systems (proteins and enzymes) acting as so-called active centers. The cluster with valence 4+ can be modeled by four Fe(III) and two Fe(II) ions. An idealized structure of double cubane has been considered instead of a more realistic defected double cubane structure of lower symmetry. Possible spin ground states (S = 0,1,2,3,4,5,6) have been predicted. The total spin S = 6 is preferred within a partially ferromagnetic region if a Heisenberg exchange parameter [between the nearest Fe(III) ions] J_2 < 0 although increase of a double exchange parameter b leads to narrowing of those region in favor of the states with lower spins. If J_2 > 0, domination of the spin states with S = 0 and S = 4 is expected for larger values of the parameter b. The state of S = 4 is predicted for J_2 > 0 in agreement with experimental data.