Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics

Systematic trends in the geometry of 149 oxide and 80 sulfide binary and ternary spinels have been examined from the standpoint of ionic radius and electronegativity. The mean ionic radii of the octahedral and tetrahedral cations, taken together, account for 96.9 and 90.5% of the variation in the unit cell parameter, a, of the oxides and sulfides, respectively, with the octahedral cation exerting by far the dominant influence in sulfides. The mean electronegativity of the octahedral cation exerts an additional, but small, influence on the cell edge of the sulfides. The equation a=(8/3√d)d
tet+(8/3)d
oct, where d
tet and d
oct are the tetrahedral and octahedral bond lengths obained from the sum of the ionic radii, accounts for 96.7 and 83.2% of the variation in a in the oxides and sulfides, respectively, again testifying to the applicability of the hard-sphere ionic model in the case of the spinel structure. Comparison of observed and calculated u values for 94 spinels indicates that up to 40% of the experimentally measured anion coordinates may be significantly in error. In addition to these compounds, u values are given for 52 spinels for which no data have previously been determined. Diagrams are presented for the rapid interpretation of the internal consistency of published data and the prediction of the structural parameters of hypothetical or partially studied spinels.

Systematics of the spinel structure type

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

Systematic trends of the 57Fe Mössbauer isomer shifts in (FeOn) and (FeFn) polyhedra. Evidence of a new correlation between the isomer shift and the inductive effect of the competing bond T-X (→ Fe) (where X is O or F and T any element with a formal positive charge)

Multiferroic bismuth ferrite-based materials for multifunctional applications: Ceramic bulks, thin films and nanostructures

Magnetic anisotropy and saturation of LaFe12O19 and some related compounds

CuCr1+xRh1−xSe4 with −1 < x ≦ 1 and CuCr1+xTi1−xS4 with 0 < x ≦ 1 show p‐type metallic conduction and are ferromagnetic, while CuCr1+xTi1−xS4 with x<0 show n‐type metallic conduction and no magnetic ordering at 4°K. From the magnetic and electrical properties, which indicate that Cu is monovalent in these compounds, a model for the electronic structure of copper‐containing sulfospinels is deduced. Ferromagnetism and p‐type conduction are attributed to holes in a broad valence band, n‐type conduction to electrons in a broad conduction band. Susceptibility measurements on CuCo2S4, CuTi2S4, CuCoTiS4, CuRh2S4, and CuRh2Se4 are reported. The main contribution to the temperature‐independent susceptibility observed is attributed to the Van Vleck susceptibility of Co3+ or Rh3 ions in the low spin state t26.

Magnetic Properties and Electrical Conduction of Copper‐Containing Sulfo‐ and Selenospinels

Magnetic properties, conductivity and ionic ordering in Fe1−xCuxCr2S4

Mössbauer effect in LaFe12O19

Electron exchange between Fe2+ and Fe3+ ions on octahedral sites in spinels studied by means of paramagnetic Mössbauer spectra and susceptibility measurements

F.K. Lotgering

Papers

Magnetic anisotropy and saturation of LaFe12O19 and some related compounds

Magnetic Properties and Electrical Conduction of Copper‐Containing Sulfo‐ and Selenospinels

Magnetic properties, conductivity and ionic ordering in Fe1−xCuxCr2S4

Mössbauer effect in LaFe12O19

Electron exchange between Fe2+ and Fe3+ ions on octahedral sites in spinels studied by means of paramagnetic Mössbauer spectra and susceptibility measurements