Domain aspects of multi-ferroics are reviewed, i.e. of materials, in which two or all three of the properties ‘ferroelectricity.’ ‘ferromagnetism’ and ‘ferroelasticity’ occur simultaneously in the same phase, and in which the magnetic point group has been reliably established by magnetoelectric, optical, dielectric, magnetic and related studies on single crystals and single domains. Nearly only members of the boracite crystal family are concerned, whereas for the perovskite family and other classes of material there is great paucity of data on single crystals. Polarized light microscopy is shown to be an indispensable tool for the study of multi-ferroics, in particular when ferroelasticity is involved. Potential multi-ferroic magnetoelectrics, so far only known in ceramic form, are excluded from the survey.

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Multi-ferroic magnetoelectrics

Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin‐dependent electron scattering due to spin canting of the manganese oxide.

31p-S-4 Giant Magnetoresistance in Magnetic Manganese Oxide with Intrinsic Antiferromagnetic Spin Structure

A review of new developments in theoretical and experimental electronic-structure investigations of half-metallic ferromagnets (HMFs) is presented. Being semiconductors for one spin projection and metals for another, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMFs by the peculiarities of their electronic structure and chemical bonding is discussed. The effects of electron-magnon interaction in HMFs and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Special attention is paid to the appearance of nonquasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-the-art electronic calculations for correlated d-systems are discussed, and results for specific HMFs (Heusler alloys, zinc-blende structure compounds, CrO2, and Fe3O4) are reviewed.

/pdf/half-metallic-ferromagnets-from-band-structure-to-many-body-1hrl8mp5lm.pdf

Half-metallic ferromagnets: From band structure to many-body effects

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 porous Fe3O4/carbon core/shell nanorods were fabricated via a three-step process. α-Fe2O3 nanorods were first obtained, and α-Fe2O3/carbon core/shell nanorods were subsequently fabricated using glucose as a carbon source by a hydrothermal method, in which the thickness of the carbon coating was about 3.5 nm. Fe3O4/carbon core/shell nanorods were synthesized after an annealing treatment of the product above under a mixture of Ar/H2 flow. After the H2 deoxidation process, the Fe3O4 core exhibited a character of porosity; the thickness of the carbon shell was decreased to about 2.5 nm, and its degree of graphitization was enhanced. The interesting core/shell nanostructures are ferromagnetic at room temperature, and the Verwey temperature was about 120 K. Electromagnetic properties of the core/shell nanorod–wax composite were investigated in detail. The maximum reflection loss was about −27.9 dB at 14.96 GHz for the composite with a thickness of 2.0 mm, and the absorption bandwidth with the reflection los...

Porous Fe3O4/Carbon Core/Shell Nanorods: Synthesis and Electromagnetic Properties

Magnetite single crystals Fe32xMxO4 doped with M5Ni, Co, Mg, Al, Ga, and Ti were grown and annealed under a controlled atmosphere to produce homogeneous and oxygen stoichiometric samples. The cation vacancy concentration of the samples was proved to be lower than 10 26 by means of magnetic disaccommodation spectroscopy. The Verwey temperature shift as function of the substituent concentration was determined from the temperature dependence of the resistivity. The systematics of the transition temperature shift as function of the concentration and nature of the substituents is indicative that the mechanism of the transition is related to the second-neighbor Coulomb interaction of the cations on the octahedral sites. @S0163-1829~98!00846-7#

/pdf/impurity-effects-upon-the-verwey-transition-in-magnetite-ohbwv6pk7x.pdf

Impurity effects upon the Verwey transition in magnetite

A versatile automatized measuring technique is developed with special respect to the investigation of magnetic after-effects (MAEs) occurring in ferro- and ferrimagnetic materials. The method is based on a voltage-controled LC-oscillator, the resonance conditions of which allow to determine quantitatively both components of coil-impedances. By various types of measurements performed on a series of different Fe samples it is demonstrated that the knowledge on both components may be helpful for the analysis of MAEs, particularly for a discrimination between different relaxation mechanisms.



Ein vielseitiges, automatisches Mesverfahren zur Untersuchung der magnetischen Nachwirkung in ferro- und ferrimagnetischen Materialien wird weiterentwickelt und quantitativ gepruft. Grund-lage des Verfahrens ist ein spannungsgeregelter LC-Oszillator aus dessen Resonanzbedingungen die beiden Komponenten komplexer induktiver Widerstande mit groser Genauigkeit bestimmt werden konnen. Durch verschiedenartige Messungen an einer Reihe unterschiedlich vorbehandelter Eisen-Proben wird gezeigt, das die Zeitabhangigkeit beider Komponenten fur die Analyse magne-tischer Nachwirkungsmessungen wichtig sein kann, insbesondere zur Unterscheidung zwischen verschiedenen Nachwirkungsmechanismen.

High-sensitivity two-component measuring technique of inductances as applied to the investigation of magnetic after-effects

The magnetic relaxation spectra of different types of single crystalline magnetite are investigated in the temperature range 4 K < T < 450 K and compared with corresponding spectra obtained on stoichiometric (Fe3O4) and vacancy-doped polycrystals (Fe3−ΔO4). Three basic relaxation mechanisms are revealed within the following temperature ranges 4 K < TI < 35 K, 40 K < < TII < 130 K and 200 K < TIII < 450 K, which may be associated to either electronic (TI, TII) or ionic (TIII) hopping processes. In crystals with systematically reduced stoichiometry characteristic modifications in the structure of these spectra are observed from which additional information for a theoretical understanding may be obtained.



Im Temperaturbereich 4 K < T < 450 K werden die Relaxationsspektren in unterschiedlich hergestellten Megnetit-Einkristallen untersucht und mit den entsprechenden Spektren verglichen, die in polykristallinem stochiometrischem (Fe3O4) und Leerstellen-dotiertem Material (Fe3−ΔO4) auftreten. In den folgenden Temperaturbereichen werden drei verschiedene Relaxationsmechanismen beobachtet: 4 K < TI < 35 K, 40 K < TII < 130 K und 200 K < TIII < 450 K, die entweder elektronischen (TI, TII) oder ionischen (TIII) Sprungprozessen zugeordnet werden konnen. Mit systematisch verringerter Stochiometrie der Kristalle andert sich die Struktur der Nachwirkungsspektren in charakteristischer Weise, wodurch weitere Ruckschlusse auf die zugrundeliegenden Relaxationsmechanismen ermoglicht werden.

Magnetic After-Effects in Single- and Poly-Crystalline Magnetite

Suszeptibilitaten und insbesondere zeitliche Anderungen von Suszeptibilitaten wurden bislang vorwiegend mit Hilfe komplexer Widerstandsbrucken gemessen. In dieser Arbeit wird vorgeschlagen, diese Mesgrosen uber die Eigenfrequenz eines LC-Oszillators zu bestimmen, dessen Induktivitat durch das zu untersuchende Ferromagnetikum – als Kern der Schwingkreisspule – beeinflust wird. Der entscheidende Vorteil dieses Verfahrens besteht darin, das mit seiner Hilfe der gesamte Mesvorgang automatisiert werden kann; auserdem konnen die Mesdaten sofort auf Lochstreifen in computergerechter Form erfast werden. Die Zweckmasigkeit des vorgeschlagenen Mesverfahrens wird am Beispiel der Kohlenstoffnachwirkung in Eisen ausfuhrlich diskutiert.



Magnetic susceptibilities and especially variations of susceptibilities were measured hitherto mostly by means of complex bridge methods. This work suggests to measure these values via the resonance frequency of a LC oscillator, the inductivity of which is strongly influenced by the ferromagnetic crystal under investigation. The advantage of this method consists in the possibility of building up a completely automatic measuring device, displaying all datas immediately by a tape punch recorder in a computer-compatible way. The usefulness of the suggested measuring method is demonstrated for the magnetic after-effect of C-atoms in α-Fe (Richternachwirkung).

F. Walz

Papers

The Verwey transition - a topical review

Impurity effects upon the Verwey transition in magnetite

High-sensitivity two-component measuring technique of inductances as applied to the investigation of magnetic after-effects

Magnetic After-Effects in Single- and Poly-Crystalline Magnetite

Ein automatisiertes Meßverfahren zur Bestimmung der magnetischen Nachwirkung in Ferromagnetika