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The molecular fleld equations for the allowed spin directions for dimer and trimer molecular species have been solved numerically for a range of values of the exchange and anisotropy interactions and of the applied magnetic fleld. The results may be used for the evaluation of the exchange and anisotropy constants from high fleld Mossbauer spectra or for selecting the best conditions for proposed experiments in order to determine these parameters. Many molecular species contain coupled iron ions in the form of dimers, trimers or more complicated arrangements. In trying to deduce the bridging ligands for such species, determination of the exchange and anisotropy constants can be an important piece of information. Although application of a magnetic fleld can break the magnetic coupling, the measurement of bulk properties, such as the magnetisation, does not provide an easy method of evaluating these microscopic magnetic parameters, particularly on powder samples. However, Mossbauer spectra in applied magnetic flelds provide two independent pieces of information on the internal magnetic arrangements, namely the measured hyperflne fleld and the intensities of the ¢m = 0 lines. Together, these should be su-cient to determine the exchange and anisotropy constants, at least when the results from several spectra at difierent flelds are used. A recent study which illustrates this point was for the iron(III) sites in ferreascidin (Taylor et al. 1995) in which three iron species, two dimeric and one trimeric, were found but could not be completely identifled at that stage. The magnetic properties of simple two sublattice antiferromagnets in an applied magnetic fleld are well known. In the case of strong anisotropy, one obtains a spin ∞ip at some critical value of the applied fleld when it is strong enough to break the exchange coupling. In the case of weak anisotropy, one obtains a spin ∞op when the applied fleld overrides the anisotropy, provided the applied fleld direction is su-ciently close to the anisotropy direction, followed by a linear approach to saturation as the directions of the two sublattices come closer together. ⁄ Refereed paper based on a contribution to the International Workshop on Nuclear Methods

Evaluation of Exchange and Anisotropy Constants of Molecular Species from High Field M&ouml;ssbauer Spectra

Evaluation of Exchange and Anisotropy Constants of Molecular Species from High Field Mössbauer Spectra