Magnetite and maghemite

Magnetite is ferrimagnetic at room temperature and has a Curie temperature of 850 K. The two different cation sites in the structure - tetrahedral (A) occupied by 

Fe and octahedral (B) occupied by Fe and Fe (see Chap. 2) - form the basis for two interpenetrating magnetic sublattices. Below Tc, the spins on the A and B sites 

Maghemite is ferrimagnetic at room temperature. Measurement of Tc is difficult because maghemite transforms to hematite at temperatures above ca. 7-800 K. The Curie temperature has been estimated as lying between 820 K and 986 K (Murad, 1988). 

Two sextets assigned to the A and B sites, can be fitted to a broadened Mossbauer spectrum. For maghemite formed via reduction of hematite to magnetite, followed 

Incorporation of Mn + ions into the structure of magnetite was monitored by Mossbauer spectroscopy and XRD [272]. Mn-for-Fe substitution affected the increase of the unit cell parameters and reduction in the HMF. At a low content, Mn ions substituted iron mainly in octahedral sites, whereas at higher concentrations substitutions at tetrahedral sites also occurred. 

Substitution of Fe with Cd and Zn ions in tetrahedral sites of magnetite reduced the HMF of both the octahedral and tetrahedral iron [340]. Substitution with Cr ions at octahedral sites in magnetite reduced the HMF values of iron at octahedral as well as at tetrahedral sites [250]. This substitution also reduced Curie temperature from 

850 to 795 K [341]. Substitution of Fe by Ga ions in magnetite (Fe2.9Gao.1O4) reduced Curie temperature from 850 to 785 K [341], whereas the substitution of Fe Curie temperature from 850 to 770 K [318]. 

The authors wish to thank Elsevier Publ. Co. for permission to reproduce Figs. 23.2-23.19. 

Kundsen, E Larsen, J.E. Moreira, O. Faurskov Nielsen, Acta Chem. Scand. A 1975, 29, 833-839. 

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Substitution of a large range of cations can be readily induced in magnetite and maghemite because tetrahedral as well as octahedral positions are available. Sidhu et al. 

For all three types of anisotropy the coercivity can be calculated (Tab. 7.7). For SD magnetite and maghemite, shape anisotropy, dominates over strain and crystal anisotropy, whereas for hematite and goethite, morphology has little influence on coer-civity. 

The oxide film formed in dry air at room temperature consists of a spinel phase, probably a solid solution of magnetite and maghemite. Such films form on magnetic tapes. They are around 1.5-2.0 nm thick, and in a dry atmosphere, can provide indefinite protection (e.g. the Delhi pillar). Ali and Wood (1969) found that with time and at a relative humidity of 46%, some hematite developed as well. At higher temperatures (200-300 °C) well defined duplex films with an inner layer of magnetite... 

Hunt, C. P., Singer, M. J., Kletetschka, G., TenPas, J. Verosub, K. L. (1995). Effect of citrate-bicarbonate-dithionite treatment on fine-grained magnetite and maghemite. Earth and Planetary Science Letters, 130, 87-94. 

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