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Supporting Information Available: Detailed synthesis and experimental procedures, UV-vis, XRD, and XAS of the iron/iron oxide nanoparticles, SQUID of the final iron oxide shells, histograms of the particle sizes, and analysis of the electron beam influence during TEM imaging and of the high-temperature oxidation of the particles in solution. This material is available free of charge via the Internet at

Cold-pressed powders of the half-metallic ferromagnet CrO 2 are dielectric granular metals. Hysteretic magnetoresistance with maxima at the coercive field arises from interparticle contacts. Dilution with insulating antiferromagnetic Cr 2 O 3 powder reduces the conductivity by 3 orders of magnitude, but enhances the magnetoresistance ratio which reaches 50% at 5K. The negative magnetoresistance is due to tunneling between contiguous ferromagnetic particles along a critical path with a spin-dependent Coulomb gap. [S0031-9007(98)05996-1] PACS numbers: 72.15. Gd, 73.40.Gk, 75.50.Cc, 81.20.Ev Negative magnetoresistance has been widely investigated in ferromagnetic metals and heterostructures. Effects intrinsic to a material are distinguished from extrinsic effects which depend on the direction of magnetization in adjacent ferromagnetic regions. Examples of the former include the anisotropic magnetoresistance of permalloy [1] or the colossal magnetoresistance of nonstoichiometric EuO [2] and mixed-valence manganites [3]. Examples of the latter are the giant magnetoresistance of multilayers [4] and granular metals [5,6] or the behavior of spin-dependent tunnel junctions [7], where resistivity is greatest at the coercive or switching field and decreases as the sample reaches technical saturation. Recent experiments on epitaxial manganite films with a single grain boundary have allowed the high-field, colossal magnetoresistance to be separated from the low-field effect due to heterogeneous magnetization distribution in adjacent grains [8,9]. A characteristic but unexplained feature of the low-field magnetoresistance in manganite ceramics [10], polycrystalline films [11,12], and tunnel junctions [13,14] is its rapid decay with increasing temperature.Here we report a new type of extrinsic magnetoresistance. It is studied in pressed powders of CrO 2 , where it arises from contacts between particles. Chromium dioxide is an ideal material for spin-polarized electron tunneling, as it is a half-metallic ferromagnet where complete spin polarization of the conduction electrons is maintained up to the surface [15]. There are two 3d electrons in spinsplit t 2g subbands, one localized and the other in a halffilled band [16]. The two electrons are strongly coupled by the on-site exchange interaction J H ഠ 1 eV. The intrinsic metallic nature of the oxide is illustrated by the resistivity of an oriented film grown on TiO 2 , shown in Fig. 1(a). It follows Matthiessen's rule with a residual resistivity of 0.1 mV m ͑10 mV cm͒ and a room-temperature value about 30 times greater. The slope dr͞dT remains positive above the Curie temperature ͑T C 396 K͒ [17]. The films exhibit only a small linear intrinsic magnetoresistance effect, ͑1͞m 0 r͒dr͞dH ϳ1%͞T at room temperature.Our samples were made from a commercial CrO 2 powder used for magnetic recording. The powder is composed of acicular single-domain particles with an average length of 300 nm and an aspect ratio of about 8:1. Coercivity is 59 mT (590 G) at room temperature, rising up to ...

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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