The Verwey Transition

At room temperature, magnetite has the cubic-spinel structure is a good electronic conductor, and exhibits a spontaneous magnetism. 

Scientific awareness of a low-temperature transition in magnetite began in 1929 with the observation of a A-type anomaly in the specific heat at about 120 K. The anomaly was typical of an order-disorder transition, but it was well below the magnetic-ordering temperature Tc = 850 In 1931, Okamura observed an abrupt semiconductor-semiconductor transition near 120 K. The transition exhibits no thermal hysteresis, but the transition temperature is sensitive to the oxygen stoichiometry. More recent specific-heat measurements show the presence of two resolvable specific-heat peaks at the transition temperature the lower-temperature peak near 110 K appears to be due to a spin reorientation. 

In 1939, Verwey noted that the existence of a mixed valence on energetically equivalent sites permitted electronic conduction as the charge-transfer states 

Subsequently the transition has been called the Verwey transition and the transition temperature the Verwey temperature Ty. Verwey also guessed that, below Ty, the mobile electrons order as Fe on [110] rows and Fe on [llO] rows of B-site cations to produce a distortion to orthorhombic symmetry with lattice parameters approximately y/2 + 6)ao X (V2 - 6)ao x ao, where 5 is a small fraction and ao is the cubic lattice parameter. Although Bickford was able to confirm that Fc304 is magnetically orthorhombic at temperatures T Ty, it is now known that the low-temperature structure is in fact monoclinic with lattice parameters V2ao x y/lz x 2ao and that the electronic ordering is more complex than originally proposed by Verwey. 

In 1948, N6el postulated the existence of antiferromagnetic coupling between two cation sublattices in order to account for materials having a magnetic susceptibility 

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In contrast to Fe3, 04 and Fe304 jF the Verwey transition disappears even for small concentrations of zinc in the system ZniFe3 x04 l. This observation indicates that... 

The assumption that the carriers are small or intermediate polarons in no way militates against discussions of ths band structure of the ground state (see e.g. Camphausen et al. 1972, Cullen and Callen 1971,1973). The absence of Jahn-Teller distortion (Goodenough 1971) also, in our view, indicates not the absence of a polaron mass-enhancement but rather a value.of V0jB not too far from the critical value. These conclusions seem to be in agreement with the considerations of Sokoloff (1972), who used a description in terms of a degenerate band of small polarons. Samara (1968) showed that pressure lowers the temperature of the Verwey transition. If this depended only on e2/ ca then the opposite should be the case. But pressure will increase B, and push the substance nearer to the critical value for the metal-insulator transition. 

Fig. 8.2 Conductivity of Fe304 xFx for various values of x as a function of reciprocal temperature (Whall et al 1978) (1) x=0.025 (2) 0.05 (3) 0.1 (4) 0.15. It will be seen that the Verwey transition disappears somewhere between x = 0.025 and 0.05.

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

The Verwey transition in Fe304 is associated with a marked jump in conductivity, but the material remains a semiconductor both above and below the transition temperature (123 K) below 123 K, there is... 

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... 

Marin IVL, Ortuno M, Hernandez A, Abelian J (1990) Peicolative treatment of the Verwey transition in cobalt-iron and nickel-iron ferrites. Physica Status Solidi (B) 157 275-280 McElhiimy MW, McFadden PL (1999) Paleomagnetism. Academic Press, New York McVitie S, Chapman JN (1995) Coherent Lorentz imaging of soft thin-film magnetic materials. Mater Res Soc Bull, October 1995 55-58... 

Figure 9. Temperature dependenee of saturation renianenee Msirm, produeed by a 2.5 T field along [001] at 300 K, during zero-field eooling from 300 K to 10 K and zero field warming baek to 300 K (after Ozdemir and Dunlop 1999). An irreversible loss of remanence occurs on cooling from room temperature to the isotropic point (Ti o = 130 K). A small but stable remanence remains between Ti o and the Verwey transition (71, = 120 K). Below Tv there is a reversible jump in remanence due to the changes in magnetocrystalline anisotropy and microstructure associated with the cubic to monoclinic phase transition.

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... 

O Reilly W (1984) Rock and Mineral Magnetism. Blackie, Glasgow, London O Reilly W, Baneqee SK (1965) Cation distribution in titanomagnetites. Phys Lett 17 237-238 Otsuka N, Sato H (1986) Observation of the Verwey transition in Fe304 by high-resolution electron microscopy. J Solid State Chem 61 212-222... 

Detailed measurements in the vicinity of the Verwey transition have shown... 

Fig. 10.8 Spectra for single crystal Fej04 below the Verwey transition (a) cooled through the transition in zero applied field to 82 K so that c twinning occurs, and polarised by a 2-7-kG field applied transversely (b) cooled in a 9-kG magnetic field aligned along the [100] axis to 82 K and polarised similarly (c) cooled as in (b) but with a longitudinal field of 20-3 kG at 30 K. In all cases the [100] direction was parallel to the applied field. Note the dramatic improvement in resolution when V twinning is eliminated. [Ref. 31, Fig. 1]...

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