In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

13. 벼잎선충의 약제처리 방법별 방제 효과

The article deals with coordination compounds of iron(II) that may exhibit thermally induced spin transition, known as spin crossover, depending on the nature of the coordinating ligand sphere. Spin transition in such compounds also occurs under pressure and irradiation with light. The spin states involved have different magnetic and optical properties suitable for their detection and characterization. Spin crossover compounds, though known for more than eight decades, have become most attractive in recent years and are extensively studied by chemists and physicists. The switching properties make such materials potential candidates for practical applications in thermal and pressure sensors as well as optical devices. The article begins with a brief description of the principle of molecular spin state switching using simple concepts of ligand field theory. Conditions to be fulfilled in order to observe spin crossover will be explained and general remarks regarding the chemical nature that is important for the occurrence of spin crossover will be made. A subsequent section describes the molecular consequences of spin crossover and the variety of physical techniques usually applied for their characterization. The effects of light irradiation (LIESST) and application of pressure are subjects of two separate sections. The major part of this account concentrates on selected spin crossover compounds of iron(II), with particular emphasis on the chemical and physical influences on the spin crossover behavior. The vast variety of compounds exhibiting this fascinating switching phenomenon encompasses mono-, oligoand polynuclear iron(II) complexes and cages, polymeric 1D, 2D and 3D systems, nanomaterials, and polyfunctional materials that combine spin crossover with another physical or chemical property.

/pdf/spin-state-switching-in-iron-coordination-compounds-42yuqvvfsi.pdf

Spin state switching in iron coordination compounds

The perovskites and perovskite-related structures exhibit several features of technical as well as fundamental interest. Technically useful properties include oxide-ion conduction with/without electronic conduction, oxidation catalysis, ferroic displacements in classic and relaxor ferroelectrics, half-metallic ferromagnetism and high-temperature superconductivity. Of more fundamental interest is the ability to tune, by chemical substitution on the large-cation subarray, transition-metal oxides through the crossover on the transition-metal array from localized dn configurations to itinerant d-electron behaviour without/with changing the valence state of that array. The localized-electron configurations may exhibit cooperative Jahn–Teller distortions that introduce anisotropic exchange interactions. At crossover, bond-length fluctuations may segregate into an ordered array of alternating covalent and ionic bonding in a single-valent perovskite; multicentre polarons or correlation bags may replace small polarons in a mixed-valent system. Bond-length fluctuations at crossover give vibronic conduction and suppression of the phonon contribution to the thermal conductivity; the fluctuations may order, to give high-temperature superconductivity, or transform to quantum–critical-point behaviour at lowest temperatures. Crossover of σ-bonding electrons in the presence of localized spins associated with π-bonding electrons gives rise to the colossal magnetoresistance phenomenon above a ferromagnetic Curie temperature.

https://iopscience.iop.org/article/10.1088/0034-4885/67/11/R01/pdf

Electronic and ionic transport properties and other physical aspects of perovskites

Rhombohedral $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ has been studied by using density-functional theory (DFT) and the generalized gradient approximation (GGA). For the chosen supercell all possible magnetic configurations have been taken into account. We find an antiferromagnetic ground state at the experimental volume. This state is 388 meV/(Fe atom) below the ferromagnetic solution. For the magnetic moments of the iron atoms we obtain $3.4{\ensuremath{\mu}}_{\mathrm{B}},$ which is about $1.5{\ensuremath{\mu}}_{\mathrm{B}}$ below the experimentally observed value. The insulating nature of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ is reproduced, with a band gap of 0.32 eV, compared to an experimental value of about 2.0 eV. Analysis of the density of states confirms the strong hybridization between Fe $3d$ and O $2p$ states in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}.$ When we consider lower volumes, we observe a transition to a metallic, ferromagnetic low-spin phase, together with a structural transition at a pressure of 14 GPa, which is not seen in experiment. In order to take into account the strong on-site Coulomb interaction U present in ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ we also performed $\mathrm{D}\mathrm{F}\mathrm{T}+U$ calculations. We find that with increasing U the size of the band gap and the magnetic moments increase, while other quantities such as equilibrium volume and Fe-Fe distances do not show a monotonic behavior. The transition observed in the GGA calculations is shifted to higher pressures and eventually vanishes for high values of U. Best overall agreement, also with respect to experimental photoemission and inverse photoemission spectra of hematite, is achieved for $U=4\mathrm{eV}.$ The strength of the on-site interactions is sufficient to change the character of the gap from $d\ensuremath{-}d$ to $\mathrm{O}\ensuremath{-}p\ensuremath{-}\mathrm{Fe}\ensuremath{-}d.$

http://www.thp.uni-duisburg.de/Paper/entel/PRB69-107-rollmann-fe2o3.pdf

First-principles calculation of the structure and magnetic phases of hematite

Electronic and structural properties of the high-pressure phase of Fe{sub 2}O {sub 3} were determined by combining the methods of M{umlt o}ssbauer spectroscopy, x-ray diffraction, and electrical resistance, R(P,thinspT) , to 80thinspthinspGPa. Because of a first-order phase transition taking place in the 50{endash}75thinspthinspGPa range and accompanied by a volume decrease of {approximately}10{percent} , a breakdown of the electronic d-d correlation occurred, leading to a Mott transition, a metallic and a nonmagnetic single Fe{sup 3+} electronic state. The high-pressure structure is of the distorted Rh{sub 2}O {sub 3}- II type. The accommodation of the denser phase within this six-coordinated structure is attributable to the metallic state. {copyright} {ital 1999} {ital The American Physical Society}

Breakdown of the Mott-Hubbard State in Fe 2 O 3 : A First-Order Insulator-Metal Transition with Collapse of Magnetism at 50 GPa

Structural studies and a full-profile refinement of the high-pressure phases of hematite $({\mathrm{Fe}}_{2}{\mathrm{O}}_{3})$ were carried out to 76 GPa using x-ray synchrotron powder diffraction. It was found that pressure induces a progressive distortion of the corundum-like hematite structure (HP1), culminating in a structural phase transition (HP2) at \ensuremath{\sim}50 GPa. At first sight the powder diffraction data obtained for HP2 could be equally explained in terms of either an orthorhombic perovskite or a ${\mathrm{Rh}}_{2}{\mathrm{O}}_{3}(\mathrm{II})$-type structure, but by a comparative analysis of the O-O bond length for both structures, recent M\"ossbauer spectroscopy results, and ab initio calculations allowed for the unambiguous assignment of the HP2 phase to the ${\mathrm{Rh}}_{2}{\mathrm{O}}_{3}(\mathrm{II})$-type structure. As a result of the phase transition the following changes are observed: (i) a substantial decrease in the Fe-O distances with a slight increase in Fe-Fe distances which led to a reduced cell volume, (ii) a diminution of the Fe-O-Fe bond distortion, and, (iii) a reduction in the distortion of the ${\mathrm{FeO}}_{6}$ octahedron. The structural transition coincides with a previously reported insulator-metal transition due to the electronic Mott transition. It is suggested that the unusual volume reduction of 10% is accounted by the combined crystallographic and electronic phase transition, the latter resulting into a substantial reduction of the ionic radii and consequently of the Fe-O bond lengths due to electron delocalization attributed to a charge-transfer gap closure. The mechanism of the combined structural, electronic, and magnetic transformations is discussed.

High-pressure structural studies of hematite Fe 2 O 3

Pressure-induced breakdown of a correlated system: The progressive collapse of the Mott-Hubbard state in R FeO 3

G. Kh. Rozenberg,1 Y. Amiel,1 W. M. Xu,1 M. P. Pasternak,1 R. Jeanloz,2 M. Hanfland,3 and R. D. Taylor4 1School of Physics and Astronomy, Tel Aviv University, 69978 Tel Aviv, Israel 2Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA 3European Synchrotron Research Facility, Boite Postale 220, 38043 Grenoble, France 4MPA-10, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA Received 14 July 2006; revised manuscript received 4 December 2006; published 11 January 2007

Structural characterization of temperature- and pressure-induced inverse↔normal spinel transformation in magnetite

Comprehensive x-ray powder diffraction studies were carried out in magnetite in the 80-150 K and 0-12 GPa ranges with a membrane-driven diamond anvil cell and helium as a pressure medium. Careful data analyses have shown that a reversible, cubic to a distorted-cubic, structural transition takes place with increasing pressure, within the (P,T) regime below the Verwey temperature TV(P). The experimental documentation that TV(P)=Tdist(P) implies that the pressure-temperature-driven metal-insulator Verwey transition is caused by a gap opening in the electronic band structure due to the crystal-structural transformation to a lower-symmetry phase. The distorted-cubic insulating phase comprises a relatively small pressure-temperature range of the stability field of the cubic metallic phase that extends to 25 GPa.

G. Kh. Rozenberg

Papers

Breakdown of the Mott-Hubbard State in Fe 2 O 3 : A First-Order Insulator-Metal Transition with Collapse of Magnetism at 50 GPa

High-pressure structural studies of hematite Fe 2 O 3

Pressure-induced breakdown of a correlated system: The progressive collapse of the Mott-Hubbard state in R FeO 3

Structural characterization of temperature- and pressure-induced inverse↔normal spinel transformation in magnetite

Origin of the Verwey transition in magnetite.