A single crystal of ordered double perovskite oxide ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6,$ a conducting ferromagnet with $T_C\sim 420\mathrm{K},$ 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 $1.1\times {10}^{22}{\mathrm{cm}}^{\mathrm{-}3},$ 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.

Optical spectra and their temperature dependence have been investigated for a prototypical double-exchange ferromagnet ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ ( $x=0.175\mathrm{}$). The optical conductivity spectrum above $T_c$ is characterized by interband transitions between the exchange-split bands and gradually changes into intraband excitations with decrease of temperature, accompanied by conspicuous transfer of spectral weight over a wide energy region of 0–3 eV. The results indicate a large variation of the electronic structure with the spin polarization of the conduction carriers.

Electric, thermal, optical, and magnetic properties have been investigated for ferromagnetic (ferrimagnetic) ordered double perovskites, ${\mathrm{A}}_2{\mathrm{CrReO}}_6$ $(\mathrm{A}=\mathrm{Sr}$ and Ca). ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ is found to be a metallic ferromagnet with a high Curie temperature ${\mathrm{(T}}_C\text{=635\hspace{0.17em}}\mathrm{K}\mathrm{),}$ possibly the highest-$T_C$ half metal among the perovskite family. By contrast, ${\mathrm{Ca}}_2{\mathrm{CrReO}}_6$ is a ferromagnetic Mott insulator with $T_C$ of 360 K.

The spin gap energy (≈ 30 meV) associated with the low-spin ($S=0\mathrm{}$) to high-spin ($S=2\mathrm{}$) transition in LaCo${\mathrm{O}}_3$ was proved to be considerably smaller than the charge gap energy (≈ 0.1 eV) which was estimated by optical spectroscopy. Hole doping in the low-spin ground state of LaCo${\mathrm{O}}_3$ leads to formation of localized magnetic polarons with unusually high spin number ($S=10-16\mathrm{}$), which can be viewed as a precursor of the doping-induced ferromagnetic metallic state.

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.