"ordered" sample with ∼70% of cationic ordering and a nominally "disordered" sample with ∼18% of cationic ordering) have been examined by magnetic measurements and neutron powder diffraction (NPD) techniques in the 15-500K temperature range. Our main finding is that the "disordered" sample exhibits a strong magnetic scattering (noticeable even at 500K), comparable to that displayed by the "ordered" one below T C = 415 K. For the "disordered" sample, the magnetic scattering exhibited on low angle Bragg positions, is not to be ascribed to a (non-existent) ferrimagnetic ordering: our results suggest that it originates upon naturally-occurring groups of Fe cations in which strong antiferromagnetic (AFM) Fe-O-Fe superexchange interactions are promoted, similar to those existing in the LaFeO 3 perovskite. These Fe groups are not magnetically isolated, but coupled by virtue of Fe-O-Mo AFM interactions, which maintain the longrange coherence of this AFM structure. Susceptibility measurements confirm the presence of AFM interactions below 770 K. * Corresponding author. Electronic mail: ja.alonso@icmm.csic.es IntroductionSome of the oxides of the double perovskites family A 2 B'B''O 6 (A= alkali-earth; B', B''= heterovalent transition metals) have been recently described to exhibit ferromagnetism and half-metallicity with a high spin polarization at the Fermi level, making them promising candidates as materials suitable for spin devices. The case of the half-metallic ferromagnet (ferrimagnet) Sr 2 FeMoO 6 is paradigmatic; with a Curie temperature above room temperature (T C = 415 K), it can be considered as a serious alternative to manganese perovskites for practical applications (1-5).The ideal structure of Sr 2 FeMoO 6 can be viewed as a regular arrangement of cornersharing FeO 6 and MoO 6 octahedra, alternating along the three directions of the crystal, with the voluminous Sr cations occupying the voids in between the octahedra. In a simple picture, the ferrimagnetic structure can be described as an ordered array of parallel Fe 3+ (S=5/2) magnetic moments, antiferromagnetically coupled with Mo 5+ (S=1/2) spins. In this ideal model, the saturation magnetization, at low temperature, would be of 4 µ B per formula unit (f.u.). In the real world, such a large magnetization value has not been obtained for bulk Sr 2 FeMoO 6 up to date; instead smaller values bellow 3.7 µ B /f.u. have been reported (1,5,6). The origin of this difference with the theoretical magnetization can be found in the so-called anti-site B-cation disorder, implying that some Mo 5+ cations occupy the positions of Fe 3+ cations, and viceversa.The problem of the order-disorder of B cations in double perovskites A 2 B'B''O 6 is a well-known one, and it has been previously addressed (7,8). If the charge difference between B' and B'' is greater than two, complete ordering of these cations is found, for instance in perovskites of the type A 2 B 2+ B 6+ O 6 and A 2 B + B 7+ O6. For them, a perfect rock-salt like structure is obtained for the B-cations sublattice...

We have succeeded in the preparation of a broad set of epitaxial Sr2FeMoO6 thin films by pulsed laser deposition. We have studied the effect of various deposition conditions, on both substrate-induced strain and antisite disorder (ASD), which translate into a variety of magnetic and transport properties of the films. We find that the saturation magnetization and the magnetoresistance are dominated by the degree of cationic order, and the strain effects are clearly evidenced in a lower TC. We characterize the ASD induced magnetoresistance, which arises from a moderate concentration of antisite defects and vanishes for massive ASD levels.

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Electron doping effects on Sr 2 FeMoO 6 have been investigated in the title series by X-ray diffraction (XRD), neutron powder diffraction (NPD) and magnetic measurements in polycrystalline samples. XRD patterns show that the anti-site defects concentration increases with x, giving rise to highly disordered samples at the B positions, for the highest doping levels. The study of the evolution of structural parameters and valence state of Fe and Mo cations with doping, followed by NPD, sheds some light on the observed changes in the magnetic properties, in particular the ferromagnetic ordering temperature (T c ). An important fraction of the injected electrons is localized at Mo sites, which is harmful concerning the potential applicability, since it promotes the occurrence of anti-site disordering between Fe and Mo cations. Our results suggest that the competition between steric and electronic effects gives rise to a non-monotonic behavior of T c , which tends to decrease at low doping levels and increases in highly doped samples. However, we have found that a higher T c is accompanied by a frustrated ferromagnetism probably associated with the AFM correlations emerging at the anti-sites.