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

Abstract

The results of a combined NMR, X-ray absorption spectroscopy and X-ray magnetic circular dichroism study of the AA FeMoO6 and AA FeReO6 double perovskites are presented. They revealed a dependence of electronic and magnetic properties, including a d-electron transfer between Fe and Mo sites, on the structural tolerance factor. The maximum value of the 4d Mo electron occupation and the corresponding Mo moment is obtained for the tolerance factor of unity. This corresponds to the maximum strength of the magnetic interaction and, respectively, to the Curie temperature. The dominant T 5/2 type temperature dependence of the Mo hyperfine field reveals the half-metallicity of the AA FeMoO 6 compounds. Antisite defects and antiphase boundaries have been identified in NMR measurements and the strength of their magnetic coupling have been determined. A considerable orbital contribution to the Re and Fe magnetic moments were found in the NMR and X-ray magnetic circular dichroism measurements on the AA FeReO6 compounds. Its magnitude decreases with increasing structural tolerance factor and is correlated with their magnetic anisotropy. IntroductionDouble perovskites (DP) of a general formula A 2 MM O 6 , where A is an alkaline earth, and M and M are 3d, 4d, and/or 5d transition metals belong to a very broad family of oxide compounds crystallising in the perovskite structure. They exhibit intriguing magnetic and electronic properties including half metallicity, a considerable low field magnetoresistance (LFMR) and a variety of magnetic structures, see review paper [1]. Magnetic and electronic properties are closely interrelated, due to the same (d) character of the "magnetic" and "conduction" electrons. Exchange interactions can occur between ions of the same element, as e.g. in manganese perovskites, as well as between ions of different elements, as e.g. in double perovskites. The interplay of lattice, charge, and spin degrees of freedom causes a variety of electronic and magnetic properties ranging from a metallic ferro-or ferrimagnetism to antiferromagnetic or paramagnetic insulating behaviour. This also brings about the magnetoresistive properties of the compounds.The interplay between magnetic and electronic transport properties in these systems can be controlled by substitution of alkaline earths (e.g. Ba, Sr, Ca) or lanthanides (e.g. La, Pr, Nd) in the A-site, as well as by the substitution of transition metals (e.g. Fe, Cr, Mn, Mo, Rh, Ru, W, Re). Among the double perovskites, the Fe-Mo and Fe-Re compounds, which are the subject of the study presented in this paper, have recently attracted a great deal of interest among the researchers. Their relatively high magnetic ordering temperature (in most cases higher than the room temperature) compared to that of manganese perovskites, and a significant LFMR make them especially interesting for applications, e.g. in magnetoelectronics or spintronics. The LFMR is explained in terms of intergranular tunneling MR (ITMR), and is related to the half-metallic nature of the ground...

Temperature dependence of magnetization under high fields in Re-based double perovskites

“…The original T 3/2 Bloch law does not give a satisfactory fit to our results in contrast to the results given by Lofland et al for the Sr 2 FeMoO 6 double perovskite (see [36]). On the other hand, the detailed analysis of the nuclear magnetic resonance (NMR) data done by Zajac et al (see [37]) reveals a T 5/2 dependence of the Mo hyperfine field in this compound, as would be expected in half-metallic compounds [38]. Consequently, the model with the exponential parameter free and fixed saturation magnetization, i.e.…”

Magnetoelastic coupling in Sr2(Fe1−xCrx)ReO6double perovskites

“…The reason might be that when the B-O-Re hybridization increases, so does the coupling of the localized B spin moment with the crystal electric field at the Re site through the anisotropic Re(t 2g ) charge distribution. Indeed, a highly enhanced electronic coupling between Re and B t 2g -states could even induce some orbital moment at the Cr-site, as is seemingly the case of the Fe atom in Ca 2 FeReO 6 , in which an orbital to spin moment ratio of 0.18 (6) has been recently reported [32]. Sr 2 CrReO 6 is an appealing compound for spin electronics because of its presumed large spin polarization [9] and high Curie temperature [25,33].…”

Tuning magnetic coercivity with external pressure in iron-rhenium based ferrimagnetic double perovskites

“…A case in point is that of double perovskites A 2 FeReO 6 (A = Ba, Sr and Ca) and Sr 2 FeMoO 6 , which are ferrimagnets with high degree of spin polarization of conduction/valence electrons and high magnetic ordering tem-perature (300-550 K) [29][30][31][32][33][34] , hence good candidate materials for use as spin polarizers/analyzers in spintronic devices [35][36][37][38] . A major difference in magnetic properties in going from Mo (second row, 4d element) to Re (third row 5d element) at B sites is that the Re-based double perovskites are magnetically hard while the Mo-based double perovskites are magnetically soft 29,[39][40][41] . This points to the importance of spin-orbit interactions in the heavy Re ions and identifies the Re sublattice as the dominant source of magnetic anisotropy in the Re-based double perovskites which are the focus of the current study.…”