Field dependence of the Morin transition and Mossbauer spectroscopy on fine hematite particles


Hematite or \(\alpha\)-ferric oxide is the most stabile form of iron oxides (citation not found: Morrish:1994wf). It crystallizes in a rhombohedral structure [Bragg, 1924] with two sublattices and equal moment. Below the Morin-transition temperature (\(T_m\)) the lattices are antiferromagnetically coupled and are aligned along the crystallographic c-axis. Above the Morin transition - 262K for pure, bulk Fe\(_2\)O\(_3\) - the magnetocrystalline anisotropy changes sign, rotating the spins by 90° into the basal plane. A slight canting produces a weak ferromagnetism.
The use of red beds is common practice in paleomagnetism. Their remanence is carried to a large extent by fine grained α-Fe2O3 particles [Dunlop, 1971]. Hematite has been identified to be an important magnetic mineral on Mars by the rovers Opportunity and Spirit using Mössbauer spectroscopy [Klingelhöfer et al., 2004]. [1]
It’s magnetic properties have been studied thoroughly in the past. In recent years sub-micrometer particles became the focus of intensive research due to their unique properties and broad technological applications. Cation substitution generally lowers the Morin Transition temperature (T_M) [De Grave [1983]; Morin [1950]; Morrish et al. [1963]; Vandenberghe et al. [1986]] with the exception of Ru, Rh and Ir [Kren and Szabo, 1965; Liu, 1986; Liu and Fan, 1984], which increased T_M. The grain-size dependence of T_m has been studied by numerous workers [Gee et al., 2004; Jacob and Khadar, 2010; Vandenberghe et al., 2001; Zysler et al., 2003]. Even though a lot of research has been done on ultra-fine <alpha>-Fe<_2>O<_3> particles, there are still aspects of their magnetic properties that are not understood completely [2]. The field dependence of the transition for example has mostly been studied on large samples (citation). Also Mössbauer is commonly used to characterize hematite crystals and their transition, however rarely in combination with the study of the associated magnetic parameters. We try to fill this gap in studying differently sized Fe2O3 particles, their Tm-field dependence and combining the results with room temperature Mössbauer spectra.
[1] In nature, however, it is not unlikely that surface rocks have been repeatedly cycled through the Morin transition, especially haematite-bearing rocks occurring at high latitudes and altitudes. DeBoer2001 [2] Bodker:2000uy

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