Origin of neutron magnetic scattering in antisite-disorderedSr2FeMoO6double perovskites

Abstract

"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...

Phase diagram and influence of defects in the double perovskites

“…[3][4][5][6][7][8][9][10][11][12][13][14][15] The magnetism of these compounds arises from the Fe 3ϩ , Sϭ5/2 core spin, while the charge state of the Mo ion is 5 ϩ. Spatially, the Mo and Fe ions occupy two interleaving fcc lattices ͑sodium chloride structure͒. The conduction band contains one electron per unit cell, which tends to be antiparallel to the Fe spin.…”

Local Magnetic and Electronic Properties of the A2FeM'O6 (A = Ba, Sr, Ca, M' = Mo, Re) Double Perovskites

“…The existence of defect sites in the sample would explain why the theoretical value of the magnetisation is not reached. A comparison of these data and the corresponding NMR spectra shows that the larger the amount of defects in the sample is the lower the magnetisation is observed [48,12,8]. In order to distinguish between the two types of defects (antisite ions and antiphase boundaries), NMR experiments under applied magnetic field (up to 0.9 T) on Ba 2 FeMoO 6 and Sr 2 FeMoO 6 compounds have been carried out.…”

Coherent Fe-rich nano-scale perovskite oxide phase in epitaxial Sr2FeMoO6 films grown on cubic and scandate substrates

“…In turn, this leads to a reduction in the values of saturation magnetization and Curie temperature of this material because of the antiferromagnetic nature of the anti-site Fe Mo -O-Fe Mo bonds. [14][15][16] Oxygen vacancies happen abundantly in SFMO because, in most cases, the films are grown in an argon environment at elevated temperatures. Earlier studies demonstrated that, with increasing concentration of oxygen vacancies, the saturation magnetization and T C decrease simultaneously.…”