Spin and orbital magnetic moments of rhenium in AAЈFeReO 6 double perovskites ͑A , AЈ = Ba, Sr, and Ca͒ have been directly probed employing x-ray magnetic circular dichroism at the Re L 2,3 edges. A considerable orbital magnetic moment is observed in all the compounds studied, which confirm theoretical predictions of unquenched Re orbital moment despite its octahedral coordination. Relative orbital-to-spin moment ratio alters with lattice distortion from m L / m S = −0.285 to − 0.337 from Ba 2 FeReO 6 to Ca 2 FeReO 6 , respectively. Moreover, the spin moment of Re ion scales with Curie temperature, the most relevant property in spin electronics application of the compounds studied. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2234292͔ Ordered double perovskites have recently attracted great interest due to their large spin polarization and Curie temperature ͑T C ͒ much higher than room temperature. These properties are strongly desired in order to realize reasonable magnetoresistance effects at room temperature, which is not only a challenging subject of fundamental science but also an important phenomenon for potential applications in spin electronics. Therefore the first observation of substantial magnetoresistance at room temperature in Sr 2 FeMoO 6 ͑Ref. 1͒ quickly led to production of magnetic tunnel junctions and magnetoresistive potentiometers.Currently, the other ordered double perovskites AAЈBBЈO 6 ͑A , AЈ = Ca, Sr, Ba, La, etc.; BBЈ = FeMo, FeRe, CrRe, CrW, etc.͒ are being intensively studied in order to find a material with optimal performance. 2-7 Among them the Re-based double perovskites are the most promising compounds in terms of high Curie temperature, e.g., 538 and 635 K in Ca 2 FeReO 6 and Sr 2 CrReO 6 , respectively.3 Moreover, Re-based double perovskites are magnetically hard 4,6,8 and reveal large magnetoelastic effects, 9 which can only be explained by a substantial magnetocrystalline anisotropy due to the anisotropy of an unquenched orbital moment of Re.

A systematic study of the valence states of Mn and Co in the perovskite series LaMn 1−x Co x O 3 ͑x = 0 to 1͒ by means of x-ray absorption near edge spectroscopy ͑XANES͒ at the K-edges is presented. The chemical shift and the evolution of the pre-edge features reveal a gradual increase of the average oxidation level of both, Mn and Co ions, with Co doping, which suggests that mixed valence states of Co 2+ /Co 3+ and Mn 3+ /Mn 4+ exist in the whole solid solution range. The relation of the results to the magnetic properties of the compounds is discussed.

Two new ferromagnetic organic-inorganic hybrid materials [Co(II)3(H2O)6(pyz)3[W(V)(CN)8]2].3.5H2O (1) and [Co(II)3(H2O)4(4,4'-bpy)3[W(V)(CN)8]2].1.5(4,4'-bpy).6H2O (2) have been synthesised and characterised. The structure of the compounds have been investigated combining EXAFS (extended X-ray absorption fine structure), ES-MS (electrospray mass spectrometry), IR (infrared spectroscopy), UV-VIS electronic spectroscopy and TGA (thermogravimetric analysis) coupled with QMS (quadrupole mass spectrometer) experiments. The studies reveal that both compounds consist of Co(II)-NC-W(V) and Co(II)-L-Co(II) linkages (L = pyrazine (1) or 4,4-bipyridine (2)). Both networks are created by cyano-bridged Co(II)3W(V)2 chains joined by organic linkers into a 2D architecture. A difference of cobalt coordination numbers in both compounds derived from EXAFS study is consistent with the ES-MS conclusion. The ac magnetic characterisation exhibits the transition to the ferromagnetic phase at T(C) = 26 K (1) and to the spin glass-like phase at T(G) = 16 K (2). The frequency dependent chi'(T) and chi''(T) signals indicate the presence of some disorder in spin alignment below ordering temperatures. Both networks are also characterised a by magnetic hysteresis loop of coercive field H(c) = 750 Oe (1) and 1200 Oe (2) at T = 4.2 K.

EXAFS and QEXAFS experiments were carried out at Hasylab laboratory in DESY center (X1 beamline, Hamburg, Germany) to monitor the course of the hydrolysis reactions of [AuCl(4)](-) complex ions as well as their reduction using glucose. As a result, changes in the spectra of [AuCl(4)](-) ions and disappearance of absorption Au-L(3) edge were registered. From the results of the experiments we have carried out, the changes in bond lengths between Au(3+) central ion and Cl(-) ligands as well as the reduction of Au(3+) to metallic form (colloidal gold was formed in the system) are evident. Good quality spectra obtained before and after the reactions gave a chance to determine the bond length characteristic of Au-Cl, Au-OH and Au-Au pairs. Additionally, the obtained results were compared with the simulated spectra of different gold (III) complex ions, possibly present in the solution. Finally, the mechanism of these reactions was suggested. Unfortunately, it was not possible to detect the changes in the structure of gold (III) complex ions within the time of reaction, because of too high rates of both processes (hydrolysis and reduction) as compared with the detection time.

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...