On the use of Mossbauer spectroscopy for characterisation of iron oxides and oxyhydroxides in soils

An empirical expression for the dependence of the magnetic hyperfine field on the aluminium content and the particle size in iron oxides and oxyhydroxides is often used in Mossbauer studies of soil samples. According to this expression, the reduction of the hyperfine field in nanometer-sized particles should be proportional to the inverse particle diameter. This is apparently in disagreement with the expression for the influence of collective magnetic excitations on the magnetic hyperfine field, which predicts a reduction proportional to the inverse volume. It is shown that this apparent discrepancy in the case of non-interacting hematite nanoparticles (accidentally) can be explained by the size dependence of the magnetic anisotropy constant. However, it is also pointed out that a third parameter, namely the strength of the inter-particle interactions, can have a significant influence on the magnetic hyperfine splitting in Mossbauer spectra of magnetic nanoparticles. Therefore, an analysis of data, based on the empirical expression, which only takes into account the particle size and the aluminium content, can give erroneous results.