MAGNETIC-PROPERTIES OF MICROCRYSTALLINE IRON(III) OXIDES AND RELATED MATERIALS AS REFLECTED IN THEIR MOSSBAUER-SPECTRA

Iron(III) oxides are common constituents of geologic materials, they a re products and by-products of many industrial processes, they are inv olved in biological processes, and they are the outcome of iron and st eel corrosion. In many of these examples the iron oxides are - fortuit ously or intentionally - of small particle size, and as a consequence difficult, if not impossible, to characterize by standard physicochemi cal techniques. Fe-57 Mossbauer spectroscopy is suitable for this purp ose because it can serve as a probe of the electric and magnetic condi tions in the vicinity of iron nuclei in solid samples, no matter how t he iron may be bound. Deviations of the magnetic properties of iron ox ides of small particle size from those of their bulk counterparts lead to radical changes in the appearance of their Mossbauer spectra. Dive rse models that have been put forward to account for such changes are discussed in this paper, including superparamagnetism, collective magn etic excitations, anomalous recoil-free fractions, superferromagnetism , spin canting and speromagnetism, reduced hyperfine field supertransf er, and Neel temperature reductions and distributions. Specific trampl es of microcrystalline iron(III) oxides and related minerals originati ng from different natural environments, resulting from technical proce sses, and being studied as planetary analogs are presented and discuss ed in the light of present-day knowledge on the properties of such mat erials.