Local structure and magneto-transport in Sr 2 FeMoO 6 oxides F. Liscio a , F. Bardelli a,b , C. Meneghini a, * , S. Mobilio a,b,c , Sugata Ray d , D.D. Sarma d a Dipartimento di Fisica, Universita ´ di ‘‘Roma Tre’’, Via della vasca navale 84, I-00146 Roma, Italy b OOG-GILDA, c/o ESRF Grenoble, France c INFN Laboratori Nazionali di Frascati, Via E. Fermi 40, I-00044 Frascati, Italy d Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India Available online 25 January 2006 Abstract Double perovskite oxides Sr 2 FeMoO 6 have attracted a great interest for their peculiar magneto-transport properties, and, in partic- ular, for the large values of low-field magneto-resistance (MR) which remains elevated even at room temperature, thanks to their high Curie temperature (T c > 400 K). These properties are strongly influenced by chemical cation disorder, that is by the relative arrangement of Fe and Mo on their sublattices: the regular alternation of Fe and Mo enhances the MR and saturation magnetization. On the contrary the disorder generally depresses the magnetization and worsen the MR response. In this work the X-ray absorption fine structure (XAFS) technique has been employed in order to probe the cation order from a local point of view. XAFS spectra were collected at the Fe and Mo K edges on Sr 2 FeMoO 6 samples with different degree of long-range chemical order. The XAFS results prove that a high degree of short-range cation order is preserved, despite the different long-range order: the Fe–Mo correlations are always preferred over the Fe–Fe and Mo–Mo ones in the perfectly ordered as well as in highly disordered samples. Ó 2005 Elsevier B.V. All rights reserved. PACS: 61.10.Ht; 72.25.b; 75.40.s Keywords: Double-perovskites; XAFS; Structure 1. Introduction Sr 2 FeMoO 6 oxides are half-metallic ferromagnetic materials with elevated Curie temperature (T c > 400 K) and high low-field magneto-resistance (LFMR) that remains elevated even at room temperature [1]. These pecu- liar features on one hand bring promising expectations for spin devices, on the other stimulated fundamental research in the field of strongly correlated systems. The crystallo- graphic structure of Sr 2 FeMoO 6 oxides belong to the dou- ble perovskite system having the general formula A 2 BB 0 O 6 and is made of tetragonally distorted pseudo-cubic units with Sr ions at the centres and B(B 0 )O 6 octahedra at the corners [2]. The regular alternation of Fe and Mo cations on B and B 0 sublattices (cation chemical order) produces ferromagnetic coupling between Fe 3+ localized spins whereas the charge carriers (minority spins) are coupled antiferromagnetically with the Fe localized moments. In this picture defects (structural and/or in cation chemical order) play a relevant role in determining magnetic and magneto-resistive responses: the sudden drop of resistivity in low magnetic field is associated to the granular nature of the samples [3–5] and commonly ascribed to spin tunnel- ling across grain boundaries [5]. On the other hand the antisite defects (AS), i.e. the occupation of Fe(Mo) in Mo(Fe) sites, is generally believed to be the primary cause of the reduced saturation magnetization and of the worsen- ing of LFMR [5–8] due to decreasing the spin polarization at the Fermi level. Finally the low remanence and small coercitivity observed on B–H hysteresis loop in some poly- crystalline Sr 2 FeMoO 6 samples has been attributed to the formation of antiphase boundaries (APB) at the interface 0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.12.033 * Corresponding author. Tel.: +39 06 55177054; fax: +39 06 5579303. E-mail address: meneghini@fis.uniroma3.it (C. Meneghini). www.elsevier.com/locate/nimb Nuclear Instruments and Methods in Physics Research B 246 (2006) 189–193 NIM B Beam Interactions with Materials & Atoms