Electronic structure and x-ray magnetic circular dichroism ofSr2FeMoO6:Ab initiocalculations

Abstract

Theoretical investigations of the electronic structure, x-ray absorption, and x-ray magnetic circular dichroism ͑XMCD͒ at the Fe L 2,3 and Mo L 2,3 edges of Sr 2 FeMoO 6 are carried out by means of the generalized gradient approximation. The magnetic coupling between Fe and Mo is found to be antiparallel, which gives direct confirmation of ferrimagnetic ordering and settles controversies existing between the earlier experimental reports. This is also confirmed by our good agreement of the Mo L 2,3 edges with experiment. Using our theoretical spectra, we recalculate the spin and orbital magnetic moments by means of the XMCD sum rules and compare the results with a direct self-consistent calculation and experiment. Soft x-ray magnetic circular dichroism ͑XMCD͒, which initially was viewed as quite an exotic technique, has now developed into a standard experimental tool in the exploration of magnetic and electronic properties, with atomic and orbital selectivity. With the help of the sum rules derived by Thole and co-workers, 1 the spin and orbital moments for all inequivalent sites in a crystal can be computed directly from the XMCD spectra. The results from XMCD measurements can be combined in several useful ways with density functional theory. For example, the calculation by Antonov et al. 2 of the XMCD spectra of magnetite Fe 3 O 4 , which is a mixed valent material with both Fe 2+ and Fe 3+ ions present, showed that Fe atoms occupy two types of sites, tetrahedral ones, called A, and octahedral ones, called B. All the A sites are occupied by Fe 3+ , whereas at the B sites there is a mix of Fe 2+ and Fe 3+ . By computing the XMCD spectra from density functional theory, it becomes possible to distinguish between the Fe signals from different sites, aiding the interpretation of the experimental XMCD spectra. Further, Kanchana et al. 3 have investigated the effect of magnetic anisotropy on XMCD spectra of CrO 2 . They found that the computed XMCD anisotropy is much smaller than the measured one.Sr 2 FeMoO 6 ͑SFMO͒ is a case of special interest due to its technological potential as a spintronics material, and due to the many diverging reports, both theoretical and experimental, on its electronic and magnetic structure. SFMO is a magnetic metal with a gap in one spin channel, i.e., it is a socalled half metal with a Curie temperature T C of 418 K, exceeding room temperature. The half metallicity causes total spin polarization of the charge carriers. This, in turn, may give rise to a low-field magnetoresistive effect based on intergrain tunneling, which can be explained in a simple model as follows. In a system with a microstructure consisting of half-metallic monodomain grains dispersed in an insulating matrix, the magnetic moments of the magnetic grains are randomly ordered, and the tunneling from grain to grain becomes low, since the moments of two adjacent grains in general are not aligned. As the grain magnetic moments align due to an increasing external magnetic field, the resistance is reduced. Note that there is...

Evidence of kinetic-energy-driven antiferromagnetism in double perovskites: A first-principles study of La-dopedSr2FeMoO6

“…9 shows the transition temperatures plotted as a function of carrier concentration. The ferromagnetic transition temperature at carrier concentration of 1, which corresponds to SFMO compound, is found to be 360 K in comparison to measured value of 410 K 19 . The ferromagnetic T c is found to decrease upon increasing La concentration, and nally becomes zero.…”

An investigation of the Fe and Mo oxidation states in Sr2Fe2−xMoxO6 (0.25⩽x⩽1.0) double perovskites by X-ray absorption spectroscopy

“…While there are a wide variety of studies on the valence states of Fe and Mo, no consensus has been reached [11,15,26,[31][32][33][34][35]. Although no definitive values have been reported, the oxidation state of Fe is generally found to be between 2+ and 3+ while the oxidation state of Mo is found to be between 5+ and 6+ [15,21,18,[31][32][33][34][35].…”

The effects of thermal treatment on the physical properties of Sr2FeMo1−xMxO6 perovskite with M=W, Ta and x≤0.3

“…As example, in highly ordered Sr 2 FeMoO 6 has been shown iron mixed valence states 30% Fe 3+ -Mo 5+ and 70% Fe 2+ -Mo 6+ [10]. One possible reason for these discrepancies may be complex intra-and intergrain structure on Sr 2 FeMoO 6 interfaces [12,13]. It has been shown also that Sr 2 FeMoO 6 decomposes gradually with time into SrMoO 4 , leading to a structural transformation of the system [14].…”