The distinctive Mossbauer spectra of fine-particle goethite (a-FeOOH) have been attributed by many authors to superparamagnetism.However, measurements of the magnetic anisotropy energy and particle volume show that the superparamagnetic blocking temperature for most samples is much greater than the Neel temperature. A model involving magnetic ordering of clusters created by high concentrations of vacancy defects is proposed, in which the cluster moments slowly relax, thus producing a Boltzmann distribution in the z component of the magnetization. The model provides excellent fits to the temperature-dependent hyperfine-field distributions observed in the Mossbauer spectra, and may have wider applicability to other diamagnetically substituted iron oxide materials. A linear relation is observed between Tn and the inverse mean crystallite dimension in the [111]direction of -1060 (130) K nm, and it is shown that goethite particles must have a volume of less than 1000 nm' in order to show superparamagnetic Mossbauer spectra at room temperature.

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The response of finepanicle mametic oxides to applied magnetic fields is of primary importance in several areas of applied physics, such as magnetic separation. magnetic fluid devices, and magnetic recording. It is also of fundamental interest. In this article contemporary reswch on the important properties of magnetic oxide partides is reviewed. including both applied and basic work. The microscopic and macroscopic propeliies of ferrimagnetic and antiferromagnetic fine-particle systems are considered Special attention is paid to lhe physics of magnetization reversal mechanisms, surface effects, and the intrinsic p r o p d e s of fine-parrick magnetic oxides

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.

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