Verwey transition in magnetite

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... 

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. 

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. 

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. 

The samples were also measured at low temperature i.e. at 130 K in order to remain above the Verwey transition of magnetite (Fig. 3.47). In these spectra the AF and WF phase of hematite were simultaneously present demonstrating a better crystallinity for this hematite phase than for the hematite observed in the total soil and loess sample, for which at 80 K only the WF state was revealed. 

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... 

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... 

Magnetite is a spinel ferrite which can be written as Fe [Fe Fe ]04. Unlike y-Fe203 it has no cation vacancies on the octahedral sites, but these sites contain equal numbers of Fe + and Fe + ions. A transition in many of its physical properties takes place between 110 and 120 K, and Verwey postulated a fast electron-transfer process (electron hopping) between the Fe + and Fe ions on the octahedral B sites above this temperature. The low-temperature form has discrete valence states and orthorhombic symmetry. 

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. 

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

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)... 

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