Magnetite Verwey transition

A recent review of the experimental situation has been given by Honig(1985). It is pointed out that the electrical properties, particularly near to the transition, are very sensitive to purity and specimen preparation, and that much of the extensive experimental work is therefore open to doubt. None the less, the broad features of the behaviour of this material are clear. The history of the so-called Verwey transition in this material goes back to 1926, when Parks and Kelly (1926) detected an anomalous peak near 120 K in the heat capacity of a natural crystal of magnetite. The first detailed investigations were those of Verwey and co-workers (Verwey 1939, Verwey and Haayman 1941, Verwey et al. 1947), who showed that there was a near discontinuity in the conductivity at about 160K. The conductivity as measured by Miles et al. (1957) is shown in Fig. 8.1. 

Verble, J. L. (1974) Temperature-dependent lightscattering studies of the Verwey transition and electron disorder in magnetite. Phys. Rev. (B), 9,5236—48. 

The application of Mossbauer studies to magnetic materials is well illustrated by the spectra of magnetite (Fe304) shown in Fig. 2.48. Although this ferrimagnetically ordered material is an inverse spinel with nominally tetrahedral Fe + and both Fe and Fe on octahedral sites, at temperatures above 119 K (the Verwey transition temperature) only... 

Fig. 2.48. The Fe Mossbauer spectra of magnetite (Fe304) at temperatures (a) above and (b) below the Verwey transition (after Sawatzky et al., 1969).

Gasparov LV, Tanner DB, Romero DB, Berger H, Margaritondo G, Forro L (2000) Infrared and Raman studies of the Verwey transition in magnetite. Phys Rev B 62 7939-7944... 

Holland T, Powell R (1996a) Thermodynamics of order-disorder in minerals I. Syimnetric formahsm applied to minerals of fixed composition. Am Mineral 81 1413-1424 Holland T, Powell R (1996b) Thermodynamics of order-disorder in minerals II. Syimnetric formahsm applied to solid solutions. Am Mineral 81 1425-1437 Honig JM (1995) Analysis of the Verwey transition in magnetite. J Ahoys Compounds 229 24-39 lida S (1980) Stractme of Fe304 at low temperatmes. Philos Mag 42 349-376... 

Moloni K, Moskowitz BM, Dahlberg ED (1996) Domain structures in single-crystal magnetite below the Verwey transition as observed with a low-temperature magnetic force microscope. Geophys Res Lett 23 2851-2854... 

A simple model for the Verwey transition has been proposed (Honig, Spalek Gopalan, 1990) octahedral sites in magnetite were represented by a site pair, with a ground energy state (an electron trapped), a first excited state (the electron resonating between the two components of the site pair) and a second excited state (two electrons in the site pair). An important characteristic of this model was that the Verwey transition was driven by the coulomb repulsive interaction between electrons in the site pair. 

Honig, J. M., Spalek, J. Gopalan, P. (1990). Simple interpretation of the Verwey transition in magnetite. Jourrud of the American Ceramic Society, 73, 3225-30. 

Kakol, Z. (1990). Magnetic and transport properties of magnetite in the vicinity of the Verwey transition. Journal of Solid State Chemistry, 88, 104-14. 

M. Soiescu, L. Dia mandescu, R. A. Brand, T. Mihaila, Mossbauer study of manganese-doped magnetite below the Verwey transition. Mater. Lett. 58 (2004) 885-888. 

Brabers V. A. M., Walz F.and Kronmiiller H. (1998) Impurity effects upon the Verwey transition in magnetite, Phys. Rev. B, Volume 58, pp 14163-14166. 

Below the Verwey transition, at about 120 K, the Mossbauer spectrum of magnetite can be properly decomposed into five components (Table 20.2), one related to Fe ions on the tetrahedral sites and four corresponding to Fe and Fe " " ions on two nonequivalent octahedral sites [59,60]. This methodology was successfully used to analyze the MS at 77 K of a magnetite as a corrosion product [61,62]. 

As far as the magnetic susceptibility is concerned (Figure 10), we find an important reduction of the magnetic moment at relatively low temperature (Tblocking temperature (T -100 K) which may be related to the Verwey transition ( 118 K) met in bulk magnetite-shifted to relatively lower-T here maybe due to smaller particle-size. 

At low temperatures, the spectrum of pure magnetite is very complex (Fig. 3.14a) and may be described by at least five subspectra [118]. This is due to the 3d electron localization below the so-called Verwey transition at about 125 K leading to discrete Fe " and Fe " spectral contributions of the B sites. However, this transition temperature is lowered in the case of substitution or partial oxidation [119-121]. This is illustrated in Fig. 3.14b where oxidized magnetite (Fe2.94404)... 

Fig. 3.14 Spectra below the Verwey transition a spectrum of stoichiometric magnetite at 100 K with visible Fe lines (indicated by arrows), and b spectrum of non-stoichiometric magnetite Fe2.94404 at 100 K with the two typical Fe " and Fe " sextets...

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