Antiferromagnetic clusters

Cheng H S and Wang L S 1996 Dimer growth, structure transition and antiferromagnetic ordering in small chromium clusters Phys. Rev. Lett. 77 51... 

Viitala E, Merikoski J, Manninen M and Timonen J 1997 Antiferromagnetic order and frustration in small clusters Phys. Rev. B 55 11 541... 

The two iron ions of the Rieske cluster are antiferromagnetically coupled therefore, the ground state has a spin S = while excited states of the spin ladder S = I, i, I, and, are at energies -3J, 8J,... 

For all known cases of iron-sulfur proteins, J > 0, meaning that the system is antiferromagnetically coupled through the Fe-S-Fe moiety. Equation (4) produces a series of levels, each characterized by a total spin S, with an associated energy, which are populated according to the Boltzmann distribution. Note that for each S level there is in principle an electron relaxation time. For most purposes it is convenient to refer to an effective relaxation time for the whole cluster. 

Strongly supporting this spectroscopic data, Mossbauer spectroscopy of the as-isolated Rr shows the presence of two types of iron centers a magnetic component that can be well simulated by the parameters of Rd, and a diamagnetic component attributed to the diiron-oxo cluster and resulting from the antiferromagnetic coupling of the two irons. 

---