http://sces.phys.utk.edu/mostcited/mc1_1114.pdf

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

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

Colossal magnetoresistive manganites

A2B′B″O6 perovskites: A review

Colossal-magnetoresistance materials: manganites and conventional ferromagnetic semiconductors

Temperature variation of the crystal structure has been investigated for ${\mathrm{TbBaCo}}_{2}{\mathrm{O}}_{5+\ensuremath{\delta}} (\ensuremath{\delta}=0.5),$ which shows a metal-insulator (MI) transition at ${T}_{\mathrm{MI}}\ensuremath{\approx}340 \mathrm{K}.$ The compound has an oxygen-deficient perovskite structure, with alternating ${\mathrm{CoO}}_{6}$ octahedral and ${\mathrm{CoO}}_{5}$ pyramidal chains along the crystallographic $b$ axis. We have found that both the pyramids and octahedra are significantly distorted in the insulating phase below ${T}_{\mathrm{MI}},$ suggesting a ${d}_{{3x}^{2}\ensuremath{-}{r}^{2}}{/d}_{{3y}^{2}\ensuremath{-}{r}^{2}}\ensuremath{-}\mathrm{type}$ orbital ordering.

Metal-insulator transition induced by a spin-state transition in TbBaCo 2 O 5 + δ ( δ = 0.5 )

Antiferromagnetic metallic state in the heavily doped region of perovskite manganites

Electronic structure has been investigated by reflectivity measurement for ordered-perovskite transition-metal oxides, ${\mathrm{Sr}}_{2}M{\mathrm{MoO}}_{6}$ $(M=\mathrm{Cr},$ Mn, Fe, and Co). The ground states of the Co and Mn compounds are paramagnetic insulators, making a sharp contrast with the ferrimagnetic metallic ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}.$ For all the compounds, we have observed intense optical transition at $\ensuremath{\sim}2\mathrm{eV},$ which has been ascribed to the charge-transfer excitation from the $O2p$ state to the $\mathrm{Mo}4d$ level. Based on the optical spectra, we argue the electronic structure of this class of compounds.

Electronic structure of double-perovskite transition-metal oxides

Electronic and magnetic properties are systematically investigated for doped manganites ${R}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ $(R={\mathrm{La}}_{1\ensuremath{-}z}{\mathrm{Nd}}_{z})$ by changing the nominal hole concentration $x$ and the averaged ionic radius of the perovskite $A$ site $(z),$ or the one-electron bandwidth $W.$ In the heavily doped region $(xg~0.50),$ we observed a layered-type ($A$-type) antiferromagnetic state with metallic conductivity ($\ensuremath{\rho}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\ensuremath{\Omega}\mathrm{cm}$ at 5 K for ${\mathrm{La}}_{0.46}{\mathrm{Sr}}_{0.54}{\mathrm{MnO}}_{3}$). This observation suggests that the antiferromagnetic metallic (AFM) state is the alternative ground state for doped manganites, besides the ferromagnetic metallic (FM) state.

Antiferromagnetic metallic state in doped manganites

Effects of the electron doping on the magnetic and electronic properties have been investigated in the conducting double perovskite ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}.$ Judging from the enhanced moment ${\ensuremath{\mu}}_{\mathrm{Mo}}$ at the Mo site, the doped electrons are considered to occupy mainly the down-spin $\mathrm{Mo}4d$ band. Filling dependence of the saturation magnetization ${M}_{\mathrm{s}},$ the spin-wave stiffness coefficient ${D}_{\mathrm{s}}$ and the $\ensuremath{\gamma}$ coefficient are discussed on the basis of a $\mathrm{LDA}+U$ band calculation. We further discuss possible origins for the magnetism.

T. Akimoto

Papers

Metal-insulator transition induced by a spin-state transition in TbBaCo 2 O 5 + δ ( δ = 0.5 )

Antiferromagnetic metallic state in the heavily doped region of perovskite manganites

Electronic structure of double-perovskite transition-metal oxides

Antiferromagnetic metallic state in doped manganites

Electron doping effects in conducting Sr 2 FeMoO 6