Transport and magnetic properties are significantly influenced by the substitution of various divalent (Mg, Zn, Ca) and trivalent (Cr, Mn, In, Y) cations for iron in the double-perovskite Ba2FeMoO6. An increase of the resistivity and a spectacular decrease of ferromagnetism, especially of T C, are observed as the substitution level increases. Both intragrain magnetoresistance (MR), characterized by a peak of the resistance ratio around T C, and intergrain tunnel magnetoresistance (TMR), at low temperature, are observed. The substitution does not kill the TMR because the TMR effect is obtained for the substitution level as high as 40% on the Fe site. More importantly, high TMR magnitudes are observed for zinc-substituted oxides.

The double perovskite Ba 2 FeMoO 6 has been studied by neutron powder diffraction, Mössbauer spectrometry and x-ray absorption spectroscopy, and is compared with Sr 2 FeMoO 6 . It is shown that the size of the A-site cation has a large effect upon the nature of the low-temperature state, favouring the majority Fe 2+ /Mo 6+ pair in the case of barium and the Fe 3+ /Mo 5+ pair in the case of strontium. In both cases, an electronic transfer appears as T decreases, interpreted as the existence of a minority t 2g ↓ narrow band, where the electron is itinerant.

No abstract