We have investigated effects of the partial substitution of trivalent La for divalent Ba on the magnetic properties of the double perovskite Ba2FeMoO6. Polycrystalline Ba2−xLaxFeMoO6 samples have been prepared by the conventional solid-state reaction in a stream of 5% H2/Ar gas. Magnetization (15 K, 5 kOe) is 2.6μB/f.u. for x=0.5 which is smaller than the value of 3.8μB/f.u. for x=0. The partial substitution of La3+ for Ba2+ considerably enhances the Curie temperature TC. The TC increases from 316 K for x=0 to 336 K for x=0.5.
Electronic structures of La-doped Ba 2 FeMoO 6 double perovskite oxides have been investigated using photoemission spectroscopy and soft x-ray absorption spectroscopy. The ground states of Ba 2−x La x FeMoO 6 are found to be in the Fe 2+ -Fe 3+ mixed-valent states and the Fe valence state increases toward 3+ with increasing x. The states close to E F consist of the degenerate Mo-Fe t 2g ↓ states. The LSDA + U calculations for Ba 2−x La x FeMoO 6 (x = 0, 1) show a larger occupied bandwidth of the Mo t 2g ↓ states for x = 1 than for x = 0, which is consistent with the higher T C value predicted in a type of the double exchange mechanism.Large magnetoresistance (MR) has been observed in the ordered double-perovskite oxides of A 2 FeMoO 6 (A = Sr, Ba) with the very high magnetic transition temperature T C ( 330-450 K) [1]. A metal-insulator transition occurs simultaneously with the ferromagnetic transition in A 2 FeMoO 6 . Magnetization data for Sr 2 FeMoO 6 indicated the ferrimagnetic coupling between Fe 3+ and Mo 5+ ions, and large MR was interpreted as being due to intergrain tunnelling with the half-metallic electronic structure. When La 3+ ions were substituted for Ba 2+ ions in Ba 2 FeMoO 6 , T C was enhanced significantly [2]. The substitution of La 3+ in Ba 2−x La x FeMoO 6 has two effects: (i) the reduction of the average ionic radius without distorting the cubic symmetry, and (ii) the change of the valence states and magnetic moments of Fe/Mo ions via electron doping. The exact mechanism of the enhanced T C with increasing x in Ba 2−x La x FeMoO 6 has not been clarified yet.
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