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Structural and magnetic properties of γ-Fe2O3 have been studied in isometric nanoparticles ranging from 3 to 14 nm with a narrow particle size distribution. Cation vacancy order is observed for particles larger than 5 nm in diameter giving rise to a cubic superstructure, while for the smallest particles these vacancies are disordered. All magnetic properties measured showed a strong dependence on the average crystallite size. For the ordered samples, saturation magnetization was found to decrease linearly with decreasing crystallite size due to a surface spin canting effect. However, a stronger decrease was observed in the disordered samples, suggesting that also an internal spin canting (cation vacancy order−disorder) has to be taken into account to explain the magnetic properties of nanoparticles. The room-temperature coercive field decreases with decreasing crystallite size; however at low temperatures, the coercivity increases as the size decreases, reaching values larger than 3000 Oe. A model to explain the magnetic properties of these particles considering both surface and order−disorder effects is proposed.

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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