Colossal magnetoresistance (CMR) phenomena are observed in the perovskite-type hole-doped manganites in which the double-exchange ferromagnetic metal phase and the charge–orbital ordered antiferromagnetic phase compete with each other. The quenched disorder arising from the inherent chemical randomness or the intentional impurity doping may cause major modifications in the electronic phase diagram as well as in the magnetoelectronic properties near the bicritical point that is formed by such a competition of the two phases. One is the phase separation phenomenon on various time-scales (from static to dynamic) and on various length-scales (from glass-like nano to grain-like micron). The other is the enhanced phase fluctuation with anomalous reduction in the transition temperatures of the competing phases (and hence in the bicritical-point temperature). The highly effective suppression of such a phase fluctuation by an external magnetic field is assigned here to the most essential ingredient of the CMR physics. Such profound and dramatic features as appearing in the bicritical region are extensively discussed in this paper with ample examples of the material systems specially designed for this purpose. The unconventional phase-controls over the competing phases in terms of magnetic/electric fields and photo-excitations are also exemplified.

https://iopscience.iop.org/article/10.1088/0034-4885/69/3/R06/pdf

Critical features of colossal magnetoresistive manganites

A2B′B″O6 perovskites: A review

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

We have studied electron transport properties of benzenedithiol and benzenedimethanethiol covalently bonded to gold electrodes by repeatedly creating a large number of molecular junctions. For each molecule, conductance histogram shows peaks at integer multiples of a fundamental conductance value, which is used to identify the conductance of a single molecule. The conductance values of a benzenedithiol and benzenedimethanethiol are 0.011 G0 and 0.0006 G0 (G0 = 2e2/h), respectively. The conductance peaks are broad, which reflects variations in the microscopic details of different molecular junctions. We have also studied electrochemical gate effect.

Measurement of Single Molecule Conductance: Benzenedithiol and Benzenedimethanethiol

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

A single crystal of ordered double perovskite oxide ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6},$ a conducting ferromagnet with ${T}_{C}\ensuremath{\sim}420\mathrm{K},$ has been prepared by the floating zone method, and the magnetic and electronic properties have been investigated. It has been found that the ordinary and anomalous Hall coefficients are negative and positive, respectively, and that the density of the conduction electrons is estimated at $1.1\ifmmode\times\else\texttimes\fi{}{10}^{22}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3},$ corresponding to nearly one electron-type carrier per a pair of Fe and Mo. The electronic specific heat is about 8 mJ/(mol ${\mathrm{K}}^{2}$) in accord with the result of the band-structure calculation by the local density functional (LDA) method. In the optical conductivity spectrum, interband transitions from Fe ${e}_{g}$ to Mo ${t}_{2g}$ band with up spin across the Fermi level and from the O ${2}_{p}$ to the Mo/Fe ${t}_{2g}$ band with down spin are observed at 0.5 eV and 3.9 eV, respectively, which confirms the half-metallic electronic structure predicted by the LDA calculation.

Magnetic and electronic properties of a single crystal of ordered double perovskite Sr 2 FeMoO 6

Structural and electronic properties have been investigated for a series of Re-based ordered double perovskites, ${A}_{2}M{\mathrm{ReO}}_{6}$ $(A$=Sr,Ca; $M$=Mg,Sc,Cr,Mn,Fe,Co,Ni,Zn), which contain some prospective candidates of the high-temperature half-metals for future spin-electronic use. Neutron diffraction measurements have revealed the variation of the Re-O bond length with the change of its effective valence states depending on the counter M ion, namely, ${\mathrm{Re}}^{6+}$- or ${\mathrm{Re}}^{5+}$-based insulating states, or otherwise mixed-valent metallic (or barely Mott-insulating) states. Magnetotransport and specific heat studies have clarified metal-insulator phenomena for $M$=Cr and Fe compounds with the change of A site as well as the intergrain tunneling magnetoresistance characteristic of the ceramics of half-metal oxides.

Structural and electronic properties of the ordered double perovskites A 2 M ReO 6 ( A =Sr,Ca; M =Mg,Sc,Cr,Mn,Fe,Co,Ni,Zn)

The global phase diagram for the spin and orbital ordering has been investigated for perovskite-type $R{\mathrm{VO}}_{3}$ $(R$ being a rare-earth ion or Y) by measurements of specific heat, magnetization, and Raman-scattering spectra. $R{\mathrm{VO}}_{3}$ $(R=\mathrm{Lu}\ensuremath{-}\mathrm{Pr}),$ in which the large tilting of ${\mathrm{VO}}_{6}$ tends to stabilize the collective Jahn-Teller distortion, shows a structural phase transition concomitantly with the G-type orbital ordering (OO) at a temperature far above the C-type magnetic transition temperature. By contrast the C-type spin correlation appears to develop primarily and then induce the G-type OO in ${\mathrm{CeVO}}_{3}$ and ${\mathrm{LaVO}}_{3}$ with smaller lattice distortion.

Y. Okimoto

Papers

Magnetic and electronic properties of a single crystal of ordered double perovskite Sr 2 FeMoO 6

Structural and electronic properties of the ordered double perovskites A 2 M ReO 6 ( A =Sr,Ca; M =Mg,Sc,Cr,Mn,Fe,Co,Ni,Zn)

Spin-orbital phase diagram of perovskite-type R VO 3 ( R = rare -earth ion or Y)

Charge/orbital ordering in perovskite manganites

強磁性秩序2重ペロブスカイト(Sr1?yCay)2FeReO6の金属‐絶縁体転移