The double-perovskite oxides Ca2FeMoO6, Sr2FeMoO6 and Ba2FeMoO6 show evidence of ferromagnetic metallic behaviour, with Curie temperatures of 345-426 K and saturation magnetization of 3.5-3.9 µB/formula unit. Ceramics show little anisotropic magnetoresistance, but the low-field isotropic negative magnetoresistance of Sr2FeMoO6 is 7% in 1 T at room temperature and reaches 15% at 77 K. An effect of about 1% is found in 20 mT at room temperature in both ceramics and pressed powders of this compound.

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Magnetization curves up to 25 T at 4.2 K are reported for mixed-valence manganites (R0.7A0.3)MnO3. All the R=La compounds show an extrapolated ferromagnetic moment of ∼3.4 μB/f.u. The A=Sr and Ca compounds show essentially no high-field slope, dM/dB<0.001 μB/T whereas the Pb and Ba compounds show slopes of 0.0047 and 0.0025 μB/T, suggesting that electrons of both spins are present at the Fermi level. All Pr compounds show much greater slopes, dM/dB≈0.02 μB/T associated with a singlet ground state for Pr3+. All except the Ca compound have extrapolated moments of 3.6±0.3 μB/f.u. but (Pr0.7Ca0.3)MnO3 exhibits two first-order magnetization processes, at 5 and 6 T, due respectively to melting of charge order as the compound becomes ferromagnetic and a level-crossing transition for the Pr3+ ion. All the Nd compounds, except for the charge ordered, Ca containing compound, have a magnetization which is ∼1 μB greater than their La counterparts, due to ferromagnetic alignment of Nd and Mn moments.

The double perovskite Ca2FeMoO6 is a poor metal which orders magnetically at 345 K. The Fe57 Mössbauer hyperfine field and isomer shift at 4.2 K (48 T and 0.70 mm/s relative to α-Fe) indicate an Fe3+ configuration 3d5↑ which is strongly hybridized with the Mo 4d↓t2g orbitals. The saturation magnetization of 3.6μB/formula is consistent with neutron diffraction measurements which indicate iron and molybdenum moments of 4.0(1) and −0.2(3)μB per iron or molybdenum, respectively. The sample exhibits approximately 6% cation disorder relative to the ideal NaCl-type order of Fe and Mo in the double perovskite structure.

Treating off-stoichiometric (LaMn) 0.95 O 3 in hydrogen transforms its electrical and magnetic properties. The untreated material is a ferromagnetic narrow-gap semiconductor (T c = 125 K) whereas the near-stoichiometric reduced material is an antiferromagnetic insulator T N = 110 K). Although the off-stoichiometric manganite has formally the same mixed manganese valency as compounds such as (La 0.7 Ca 0.3 )MnO 3 , there is no resistivity peak at T c and no giant magnetoresistance. The difference may be due to potential fluctuations associated with cation vacancies.

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Polycrystalline samples of the mixed-valence perovskite-type manganites (Sm0.7A0.3)MnO3 (A=Ca2+,Sr2+,Ba2+,Pb2+) have been synthesized and their transport properties studied. The electrical conductivity of the series involves a variable-range hopping mechanism in the paramagnetic state, which extends well below TC for three of the samples. Only (Sm0.7Sr0.3)MnO3 shows a metal-insulator transition. There is a peak in the resistivity at a temperature Tp, just below the Curie point, TC=70 K with a transition from thermally activated type to metallic conductivity at low temperature with a large residual resistivity, ρ0=6×10−4 Ω m. By comparison with the (Ln0.7A0.3)MnO3 compounds, Ln=La, Pr, Nd the samarium compounds are more localized. The decrease of both Tp and TC with decreasing lanthanide radius and increasing mass depends on the nature of the substituted divalent cation, but the data cannot be scaled on a universal curve with respect to average ionic radius or tolerance factor.

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