Structural, electronic and magnetic properties of the series of double perovskites
Graphical abstract
The Weiss constant as a function of La doping for the series, indicating changes in Fe–Ir magnetic coupling on both families.
Introduction
Perovskite-type transition metal oxides with the general formula AB O3 have been subject of intense scientific investigation since the discovery of magnetoresistance in manganese-based compounds [1], [2]. A variant of the perovskite structure, namely the double perovskite (DP) also presents interesting physical phenomena. These materials can be seen as a duplication of the simple perovskite unit cell with cation A occupying the cube vertices while cations and sit alternately at the center of the oxygen octahedron. Recently, tunneling-type magnetoresistance at room temperature in Sr2FeMoO6, Sr2FeReO6 and related compounds has attracted considerable interest due to its potentiality as magneto-electronic devices, such as magnetic tunnel junctions or low-field magnetoresistive sensors [3], [4], [5], [6], [7]. These materials are believed to be ferrimagnetic half-metals with highly spin polarized conduction electrons, which are understood as d-like electrons and strongly connected with the structural and magnetic properties. Actually, the level of delocalization of the cation electrons (e.g., Re and Mo), which generally present 4d or 5d character, is thought to be the key ingredient for the magneto-electronic properties of these families [4], [5], [6], [8], [9]. For instance, the large unquenched Re 5d orbital moments are crucially connected with the magneto-electric properties of the half-metallic Ba2FeReO6 [5].
Intense research in these compounds has been dedicated to study the effects due to the variation of the metallic/magnetic ions on the and sites as well as charge carrier doping, where the divalent alkaline earth ions (e.g., Ca, Sr and Ba) on the A site are partially replaced by a trivalent rare earth ion such as La [10]. In this way one expects to understand the electronic structure and the magnetic exchange interaction in these DPs in detail. Despite the recent progress there is still an open debate about the particularities of the interplay among the structural, electronic and magnetic degrees of freedom in these materials. Since changing the number of conduction electrons, achieved by partially doping on the A site, is accompanied by significant steric effects as anti-site disorder, it is difficult to distinguish between electronic and structural influences [10], [11], [12], [13].
To further explore the open issues above and the real potential of DP compounds as magneto-electronic devices, investigation of other DPs with different 4d or 5d transition metal on the cation site is necessary. In this context, Ir, like Re and Mo, is a transition metal ion that can have strongly hybridized 5d electrons and exist in five different integer valence states (2+ to 6+). Therefore, it is interesting to study both the influence of structural changes (in bond length and bond angles) and the effect of charge carrier doping in other examples of DPs. In our previous reports [14], [15], we showed some preliminary results for the and series. Like other Ir-based DPs [16], these compounds were fitted well with space group with a significant Fe/Ir cationic disorder. However, different studies in Sr2FeIrO6 suggested two distinct symmetries, [17] and [18]. Thus, there is an open debate about the structure of these compounds and new information concerning the Fe/Ir-based DP's is important to shed some light on the understanding of these systems.
In this work, we report studies of X-ray powder diffraction, magnetic susceptibility, magnetization as a function of applied magnetic field, Mössbauer spectroscopy, specific heat and electrical resistivity experiments in the series of compounds . We discuss the Rietveld refinement of Sr2FeIrO6 and SrLaFeIrO6 in three different space groups, , and . We also report new results (magnetic, specific heat, and Mössbauer spectroscopy) for the analog compounds and explore the connection among the structural, electronic and magnetic properties to bring some light on the role of cationic disorder in the microscopic magnetic interactions in these materials. Also, we investigate the very high Curie temperature observed for intermediate regions () of the Sr-based series.
Section snippets
Experimental details
Polycrystalline samples of (x=0.0, 0.5, 0.8, 1.0, 1.2, 1.6 and 2.0) were synthesized by solid-state reaction. Stoichiometric amounts of SrCO3 and La2O3, Fe2O3 and Ir metallic powder were mixed and heated in air at 900 °C for 24 h in a conventional tubular furnace. After the first step the material was re-grounded and then pressed into pellets, which were subsequently heated at 1100 °C for 24 h in air. The compounds were synthesized at the same temperatures, as reported
Results and analysis
The Sr2FeIrO6 XRD pattern and its corresponding Rietveld analysis with the space group are shown in Fig. 1. For comparison, some XRD data were also refined in (reported in Battle et al. neutron powder diffraction (NPD) and XRD studies [17]) and in (reported in Qasim et al. synchrotron XRD and NPD studies [18]) space groups. The main results are listed in Table 1, where one can see that despite the fact that XRD data could be fitted well in three symmetries, the , Rp and Rwp
Conclusions
We have successfully synthesized polycrystalline samples of the series. We shown the results of XRD, magnetic, thermal and electrical measurements for this series and also for . The XRD patterns can be fitted well in a monoclinic structure and despite the fact that our XRD results were slightly better fitted in the space group, other studies using NPD and synchrotron XRD reported the Sr2FeIrO6 structure to have [17] and [18] symmetries. It would be
Acknowledgments
This work was supported by CAPES, FAPESP-SP and CNPq (Brazil).
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