This review encompasses the story of the Verwey transition in magnetite over a period of about 90 years, from its discovery up to the present. Despite this long period of thorough investigation, the intricate multi-particle system Fe3O4 with its various magneto-electronic interactions is not completely understood, as yet - although considerable progress has been achieved, especially during the last two decades. It therefore appeared appropriate to subdivide this retrospect into three eras: (I) from the detection of the effect to the Verwey model (1913-1947), being followed by a period of: (II) checking, questioning and modification of Verwey's original concepts (1947-1979). Owing to prevailing under-estimation of the role of crystal preparation and qualitiy control, this period is also characterized by a series of uncertainties and erroneous statements concerning the reaction order (one or two) and type of the transition (multi-stage or single stage). These latter problems, beyond others, could definitely be solved within era (III) (1979 to the present) - in favour of a first-order, single-stage transition near 125 K - on the basis of experimental and theoretical standards established in the course of a most inspiring conference organized in 1979 by Sir Nevill Mott in Cambridge and solely devoted to the present topic. Regarding the experimental field of further research, the remarkable efficiency of magnetic after-effect (MAE) spectroscopy as a sensitive probe for quality control and investigation of low-temperature (4 K Tv) into a Wigner crystal (T<Tv), describes most adequately the various low-temperature mechanisms in Fe3O4 in terms of tunnelling and variable range hopping of small polarons. On the other hand, the well-elaborated Ihle-Lorenz model, assuming a superposition of polaron-band and -hopping conductivity, is in better agreement with the high-temperature data (Tv<T<600 K).

https://iopscience.iop.org/article/10.1088/0953-8984/14/12/203/pdf

The Verwey transition - a topical review

The electronic structure of charge-ordered magnetite (Fe 3 O 4 ) below the Verwey transition and Mn-, Co-, or Ni-substituted Fe 3 O 4 are investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as with the so-called LSDA+ U approach, for which the charge ordering is found to be a stable solution in contrast to a metallic state given by the LSDA. The x-ray absorption spectra as well as the x-ray magnetic circular dichroism spectra at the K, L 2 , 3 , and M 2 , 3 edges for transition metal sites are calculated. A good agreement between theory and experiment is obtained.

Electronic structure and x-ray magnetic circular dichroism in Fe3O4 and Mn-, Co-, or Ni-substituted Fe3O4

Publisher Summary This chapter discusses the progress in spinel ferrite research. A large number of oxides with a metal–oxygen ratio of 0.75 as composition is known to crystallize into the spinel structure. Among these oxides, magnetite Fe 3 O 4 is an important compound, from which the spinel ferrites can be derived by partial substitution of the iron ions by other cations. Between most of the spinel oxides solid solutions can be formed, which means that a great variety of spinel oxides is possible. The simple spinel ferrites are those ferrites in which x = 0 and the bivalent metal ion is Mg or Cd or one of the bivalent transition metal elements Mn, Fe, Co, Ni, Cu, and Zn. Partial solid solutions of the simple ferrites are also possible with compounds that either do not exist or that do not have the spinel structure. A great variety in the chemical composition of spinel ferrites is possible. The diversity in composition results into a large range of physical properties that permits the tuning of the properties for specific applications and makes spinel ferrites of particular interest.

Chapter 3 Progress in spinel ferrite research

The utilization of solar light for the photoelectrochemical and photocatalytic production of molecular hydrogen from water is a scientific and technical challenge. Semiconductors with suitable properties to promote solar-driven water splitting are a desideratum. A hitherto rarely investigated group of semiconductors are ferrites with the empirical formula MFe2O4 and related compounds. This contribution summarizes the published results of the experimental investigations on the photoelectrochemical and photocatalytic properties of these compounds. It will be shown that the potential of this group of compounds in regard to the production of solar hydrogen has not been fully explored yet.

Research Update: Photoelectrochemical water splitting and photocatalytic hydrogen production using ferrites (MFe2O4) under visible light irradiation

The optical and magneto-optical spectra of charge-ordered magnetite $({\mathrm{Fe}}_{3}{\mathrm{O}}_{4})$ below the Verwey transition and ${\mathrm{Mg}}^{2+}$- and ${\mathrm{Al}}^{3+}$-substituted ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ are investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as with the so-called $\mathrm{LSDA}+U$ approach for which the charge ordering is found to be a stable solution with an energy gap value of 0.19 eV (the experimental value is 0.14 eV) in contrast to a metallic state given by LSDA. The origin of the Kerr rotation realized in the compounds is examined.

Electronic structure and magneto-optical Kerr effect of Fe 3 O 4 and Mg 2 + - or Al 3 + -substituted Fe 3 O 4

Polar Kerr rotation and ellipticity of the synthetic single crystal magnetite are measured at room temperature in the 400 to 2400 nm spectral region. Using these magnetooptical measurements together with the earlier optical data the diagonal and the off-diagonal tensor elements of the dielectric function are deduced. In the infrared region the dependences are dominated by the presence of the charge carriers while the behaviour in the visible part of the spectra is mainly due to the 3d crystal field transitions of the tetrahedral and octahedral iron ions.



La rotation et l'ellipticite Kerr polaire d'un monocristal de magnetite synthetique ont ete mesurees dans la bande spectrale de 400 a 2400 nm a la temperature ambiante. En utilisant ces mesures magnetooptiques combines avec les resultats optiques recents on a deduit les elements diagonaux et non-diagonaux du tenseur dielectrique. Dans la bande infrarouge les spectres sont domines par la presence des porteurs de charge electrique tandis que le comportement dans la partie visible du spectre est du principalement aux transitions des electrons entre des niveaux 3d des ions de fer occupant des sites tetraedriques et octaedriques.

Polar Kerr Effect of Magnetite in the Visible and Near Infrared Spectral Regions

The near normal reflectance spectrum together with the data on the index of refraction and the extinction coefficient are combined in a Kramers-Kronig analysis to obtain the spectral dependences of the real and the imaginary parts of the diagonal elements of the dielectric function of the single crystal series MnxFe3−xO4 for 0 ≦ x ≦ 1.6. One-oscillator fit to optical constants is used to show that the main rise in absorption at about 2 eV is due to the 3d-crystal field transitions of the Fe3+ and/or Mn2+ ions in tetrahedral sites. The anomalous behaviour of optical constants below 1 eV in iron rich Mn-ferrites is ascribed to the presence of charge carriers (Fe2+) in octahedral sites.



Das bei fast normalem Lichteinfall gemessene Reflexionsspektrum wird zusammen mit den Werten des Brechungsindex und des Extinktionskoeffizienten fur die Kramers-Kronig-Analyse benutzt, um die Spektralabhangigkeit der Real- und Imaginarteile der Diagonalelemente der Dielektrizitatsfunktion von MnxFe3−xO4-Einkristallen (0 ≦ x ≦ 1,6) zu erhalten. Die Anpassung der optischen Konstanten an das Ein-Oszillator-Modell zeigt, das fur den Hauptanstieg der Absorption um etwa 2 eV die 3d-Kristallfeldubergange der Fe3+- und/oder Mn2+-Kationen in den Tetraederplatzen verantwortlich sind. Das anomale Vehalten der optischen Konstanten unterhalb 1 eV in eisenreichen Manganferriten wird der Anwesenheit von Ladungstragern (Fe2+) in Oktaederplatzen zugeschrieben.

Optical properties of manganese ferrites

The optical properties of Ga-substituted magnetoplumbites are studied by ellipsometry and by light reflectivity measurements. Ellipsometrical analysis is presented supposing that the samples are homogeneous absorbing anisotropic crystals with smooth surfaces. Data obtained from experiments are combined in a Kramers-Kronig analysis to deduce the spectral dependences of the real and imaginary parts of the permittivity. The discussion is based on the assumption that the optical spectra are given by a simple sum of the ionic contributions.



Die optischen Eigenschaften von Ga-substituierten Magnetoplumbiten werden durch Ellipsometrie und durch Licht-Reflexionsmessungen untersucht. Die ellipsometrische Analyse wird unter der Annahme vorgeschlagen, das die Proben homogen absorbierende anisotrope Kristalle mit glatter Oberflache sind. Die experimentell erhaltenen Egebnisse werden mit einer Kramers-Kronig-Analyse kombiniert. Daraus folgt die spektrale Abhangigkeit des realen und imaginaren Teils der Permittivitat. Die Diskussion basiert auf der Annahme, das das optische Spektrum durch eine einfache Summe der Ionenbeitrage gegeben wird.

P. Široký

Papers

Polar Kerr Effect of Magnetite in the Visible and Near Infrared Spectral Regions

Optical properties of manganese ferrites

Optical properties of Ga-substituted magnetoplumbites