"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...
A new reverse Monte Carlo (RMC) method for modelling both lattice and magnetic disorder in powder crystalline materials by direct calculation of the structure factor has been developed. The method, the program and the basic theory are described in some detail. Initial results from modelling the lattice and magnetic structure of MnO around the Néel temperature are also presented.
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