Antiferromagnetic effects

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. 

A plot of the reciprocal of the measured susceptibility 1 /imol v.v. T is a straight line with slope 1/C, and which crosses the abscissa at T = 0 (Fig. 19.6). For 0 = 0 the equation is simplified to the classic Curie law mol = C/T. Generally, values of 0 (j arc found when cooperative effects arise at low temperatures (ferro-, ferri- or antiferromagnetism). The straight line then has to be extrapolated from high to low temperatures (dashed lines in Fig. 19.6). 

---