The role of the synthesis conditions on the cationic Fe/Mo ordering in Sr 2 FeMoO 6 double perovskite is addressed. It is shown that this ordering can be controlled and varied systematically. The Fe/Mo ordering has a profound impact on the saturation magnetization of the material. Using the appropriate synthesis protocol a record value of 3.7µ B /f.u. has been obtained. Mössbauer analysis reveals the existence of two distinguishable Fe sites in agreement with the P4/mmm symmetry and a charge density at the Fe m+ ions significantly larger than 3d 5 suggesting a Fe contribution to the spin-down conduction band. The implications of these findings for the synthesis of Sr 2 FeMoO 6 having optimal magnetoresistance response are discussed. PACs: 75.30.Cr; 75.50.Gg;76.80.+y;81.40.Rs § To whom all correspondence should be send. 14-07-00 SFMO-01 2/13Although oxides of the type A 2 BB'O 6 where A is an alkaline earth (A=Sr, Ca, Ba) and B, B' are heterovalent transition metals such as B=Fe, Cr, .. and B'= Mo, W, Re,... , have known since long ago [1,2] they are receiving a renewed great deal of attention. This is motivated by the recent report that Sr 2 FeMoO 6 is a half-metallic ferromagnet with a relatively high Curie temperature (about 410-450K) [3]. Its half metallic nature leads to an ideal full polarization of the itinerant carriers and thus these materials are viewed as a serious alternative to the much investigated manganese perovskites but with the added advantage of having a wider temperature range for practical applications as magnetoresistive materials.The structure is built up by ordering perovskite blocks in a rock salt superlattice and the properties of the material are thought to critically depend on this ordering. Sr 2 FeMoO 6 is believed to be ferrimagnetic-like, i.e. the B and B' sublattice are antiferromagnetically coupled. In the simplest ionic picture Fe 3+ (3d 5 , S=5/2) ions in B sites are antiferromagnetically coupled to its six Mo 5+ (4d 1 , S=1/2) neighbors occupying the B' sites and thus a saturation magnetization M S =4µ B is predicted. Accordingly, it is expected that M S should be sensitively dependent on the ordering of Fe/Mo ions among the B/B' sublattices. Indeed, the M S values reported so far are systematically much smaller (3.1µ B [3], 3.5µ B [4] 3.2 µ B [5]), than the predicted 4µ B value. It is commonly thought that this significant decrease is due to antisite defects resulting from the partial disorder of Fe and Mo ions among the B/B' sublattices. Montecarlo simulations have indeed predicted a reduction of M S as a function of the antisite disorder that could account for the experimental observations [6].However, there are no strong experimental evidences that Fe/Mo disorder is the reason for the observed reduction of M S and thus strategies to enhance M S are lacking.We also note that the simple Fe 3+ /Mo 5+ ionic picture needs to be validated as neutron

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We report a detailed study of the magnetic and magnetotransport properties of Sr2FeMoO6 ceramics having a controlled concentration of antisite (AS) defects. It is found that a high-field differential susceptibility exists in all samples, which increases with AS. Similarly, a high-field magnetoresistivity develops and mimics the differential susceptibility. These observations suggest that antisite defects promote some magnetic frustration. High-resolution electron microscopy studies have allowed observation of the existence of antiphase boundary defects in the Sr2FeMoO6 structure.

By means of high resolution neutron powder diffraction at low temperature we have characterized the structural details of LaxSr2−xFeMoO6 (0 ≤ x ≤ 0.5) and CaxSr2−xFeMoO6 (0 ≤ x ≤ 0.6) series of compounds. This study reveals a similar variation of the mean bond-angle θFe−O−Mo in both series. In contrast, the mean bond-distance d (Fe,Mo)−O increases with La but not with Ca substitution. Both series also present a different evolution of the Curie temperature (TC), which raises in the La series and slightly decreases in the Ca one. We thus conclude that the enhancement of TC in the La series is due to the electron filling of the conduction band and a concomitant rising of the density of states at the Fermi level.

Magnetic dc susceptibility (χ) and electron spin resonance (ESR) measurements in the paramagnetic regime, are presented. We found a Curie-Weiss (CW) behavior for χ(T) with a ferromagnetic Θ = 446(5) K and µ ef f = 4.72(9)µ B /f.u., this being lower than that expected for either F e 3+ (5.9µ B ) or F e 2+ (4.9µ B ) ions. The ESR g-factor g = 2.01(2), is associated with F e 3+ .We obtained an excellent description of the experiments in terms of two interacting sublattices: the localized F e 3+ (3d 5 ) cores and the delocalized electrons. The coupled equations were solved in a mean-field approximation, 1 assuming for the itinerant electrons a bare susceptibility independent on T .We obtained χ 0 e = 3.7 10 −4 emu/mol. We show that the reduction of µ ef f for F e 3+ arises from the strong antiferromagnetic (AFM) interaction between the two sublattices. At variance with classical ferrimagnets, we found that Θ is ferromagnetic. Within the same model, we show that the ESR spectrum can be described by Bloch-Hasegawa type equations. Bottleneck is evidenced by the absence of a g-shift. Surprisingly, as observed in CMR manganites, no narrowing effects of the ESR linewidth is detected in spite of the presence of the strong magnetic coupling. These results provide evidence that the magnetic order in Sr 2 F eM oO 6 does not originates in superexchange interactions, but from a novel mechanism recently proposed for double perovskites.PACS: 75.10. 76.60.Es, 75.30.Vn

95, 97 Mo NMR experiments have been performed on a series of Sr 2 FeMoO 6 and electron-doped Sr 2-x La x FeMoO 6 ceramics. Detailed analysis of the NMR spectra from pristine Sr 2 FeMoO 6 conclusively shows that the Mo hyperfine field is mainly due to atomic Mo magnetic moments. No contribution of transferred hyperfine field has been observed, confirming the absence of s-electrons in the conduction band. Upon La doping, the NMR frequency (hyperfine field) gradually increases proving that the concentration of spin polarized electrons at Mo ion is enhanced by the La substitution.A simple linear correlation between magnetic moment at Mo sites and the Curie temperature of the system has been found. Implications for understanding the electronic structure and the ferromagnetic coupling in these systems are underlined. PACS: 76.60.Lz , 75.20.Hr

We report here on the electronic structure of electron-doped half-metallic ferromagnetic perovskites such Sr2−xLaxFeMoO6 (x=0-0.6) as obtained from high-resolved valence-band photoemission spectroscopy (PES). By comparing the PES spectra with band structure calculations, a distinctive peak at the Fermi level (EF ) with predominantly (Fe+Mo) t ↓ 2g character has been evidenced for all samples, irrespectively of the x values investigated. Moreover, we show that the electron doping due to the La substitution provides selectively delocalized carriers to the t ↓ 2g metallic spin channel. Consequently, a gradual rising of the density of states at the EF has been observed as a function of the La doping. By changing the incoming photon energy we have shown that electron doping mainly rises the density of states of Mo parentage. These findings provide fundamental clues for understanding the origin of ferromagnetism in these oxides and shall be of relevance for tailoring oxides having still higher TC .