Mossbauer spectra of antiferromagnetic powders in applied fields

Abstract

The problem of interpreting Mossbauer spectra of antiferromagnetic powders subject to applied magnetic fields is considered. A mean-field model for systems with uniaxial anisotropy is used to fit simultaneously spectra taken with applied fields of different strengths. The model predicts a variety of asymmetric broad line profiles depending on the strengths of the exchange and anisotropy terms. The method is applied to the 4.2 K spectra of goethite and haematite subject to longitudinal applied fields of 0, 3, 6 and 9 T. Favourable agreement is obtained with the results from single crystals, showing that the method can be reliably used for a variety of antiferromagnetic materials.

Structural and Magnetic Properties of Ferrihydrite

“…It should be noted that the apparent splitting does not prove, per se, that the magnetic states are not simply antiferromagnetic, since even pure antiferromagnets can show a similar effect in applied fields (Pankhurst and Pollard, 1990). Quantitative analysis is required in order to determine whether the magnetic structures of the two samples are different.…”

“…This model has been described in detail by Pankhurst and Pollard (1990) and by Pankhurst (1991). The main feature of the model is that it incorporates the competition between the relative strengths of the exchange, anisotropy and local fields, the ratio of the sublattice moments, and the powder summation of the angles between the applied field and the easy anisotropy axis of each crystallite.…”

Characterization of FeOOH polymorphs and ferrihydrite using low-temperature, applied-field, Mössbauer spectroscopy

“…The behaviour can be understood from the response of the antiferromagnetically coupled moments to the applied field (Pankhurst & Pollard, 1990). Depending on the relative strengths of the exchange interaction and the magnetocrystalline anisotropy, the moments will be partially reoriented by the applied field.…”

“…Goethite is characterized by strong antiferromagnetic exchange and reasonably large anisotropy (Meagher et al, 1986;Pankhurst & Pollard, 1990). Replacement of Fe by A1 in goethite leads to an increased asymmetry of the outer lines of applied-field spectra, and is thought to indicate Al-induced ferrimagnetism .…”

“…In one, the magnetocrystalline anisotropy is assumed to be uniaxial and a mean-field model is used to determine the equilibrium orientations of the moments. Spectra taken with different strength applied fields are simultaneously fitted (Pankhurst & Pollard, 1990;Pankhurst, 1991). The influence of the applied field is considered for representative orientations of the particles (-20) and the weighted subspectra are summed.…”