The Fe electronic structure and magnetism in (i) monoclinic Ca2FeReO6 with a metal-insulator transition at T-MI similar to 140 K and (ii) quasicubic half-metallic Ba2FeReO6 ceramic double perovskites are probed by soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). These materials show distinct Fe L-2,L-3 XAS and XMCD spectra, which are primarily associated with their different average Fe oxidation states (close to Fe3+ for Ca2FeReO6 and intermediate between Fe2+ and Fe3+ for Ba2FeReO6) despite being related by an isoelectronic (Ca2+/Ba2+) substitution. For Ca2FeReO6, the powder-averaged Fe spin moment along the field direction (B = 5 T), as probed by the XMCD experiment, is strongly reduced in comparison with the spontaneous Fe moment previously obtained by neutron diffraction, consistent with a scenario where the magnetic moments are constrained to remain within an easy plane. For B = 1 T, the unsaturated XMCD signal is reduced below T-MI consistent with a magnetic transition to an easy-axis state that further reduces the powder-averaged magnetization in the field direction. For Ba2FeReO6, the field-aligned Fe spins are larger than for Ca2FeReO6 (B = 5 T) and the temperature dependence of the Fe magnetic moment is consistent with the magnetic ordering transition at T-C(Ba) = 305 K. Our results illustrate the dramatic influence of the specific spin-orbital configuration of Re 5d electrons on the Fe 3d local magnetism of these Fe/Re double perovskites.

C. Azimonte,1,2,* E. Granado,1,2,† H. Terashita,1,2 S. Park,3 and S-W. Cheong3 1Instituto de Física “Gleb Wataghin,” Universidade Estadual de Campinas (UNICAMP), Caixa Postal 6165, 13083-970 Campinas, SP, Brazil 2Laboratório Nacional de Luz Síncrotron, Caixa Postal 6192, 13083-970 Campinas, SP, Brazil 3Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA Received 25 October 2009; published 22 January 2010

Page s39 s39a combination of scientific methods that had never been applied jointly to explore and clarify the thermal decomposition behaviour of this material. 1Although monoalkylamines are usually employed as templates for the synthesis of titanium phosphate metastable phases, the structural features of γ-titanium phosphate alkylamineintercalated compounds only recently have been reported. 2Key to the full elucidation of the structure of the materials was the combination of XRD and NMR evidence with theoretical calculations of 1 H NMR chemical shifts.Since the great success of carbon nanotubes in 1991, interest in lowdimensional nanomaterials has fuelled a spectacular and unusual activity.Nowadays the nanotubes are not only made of carbon but also of inorganic materials, many of which are related with previously well-known layered structures.Organic-inorganic hybrid nanotubes based on the γ-titanium phosphate structure spaced with trialkylamines have been prepared by using microemulsion-mediated solvothermal and microwave-assisted methods.The interlayer distance in the inorganic sheets of the nanotube can be controlled by both the alkyl chain length and the amount of the amine template.All nanotubes obtained are open-ended with concentric cylinders, i.e., layers that fold and close within themselves.The possible reason for having this morphology seems to be related to the combination of two factors: i) the interaction between the nitrogen of trialkylamine molecules and the hydrogen belonging to inorganic skeleton that seeks to be maximum, and ii) the pseudo-conical geometry of the trialkylamine molecules that limits the number of accessible acid centers.In this way, the number of H-N links will depend on the length of the alkyl chain. 3In conclusion, studies using diffraction methods in tandem with high-resolution NMR spectroscopy should provide valuable information on the local environment in the inorganic-organic materials, including new γ-titanium phosphate based nanotubes.

The magnetostrictive atomic strain in a pure Fe single crystal was measured by differential x-ray absorption spectroscopy. The obtained tetragonal magnetostriction constant, (3/2)λ100, was determined to be 45 ppm, consistent with the previously reported theoretical value calculated from a spin-orbit coupling theory. These results provide a foundation for understanding the origin of magnetostriction in pure Fe as well as Fe-based binary alloys.

In this work, we studied the resonant x-ray diffraction of single EuTe anti ferromagnetic films and EuTe/PbTe(SnTe) superlattices. Below the Neel temperature half order peaks appear, whose magnetic origin was proved by a polarization analysis. A resonant diffraction enhancement of almost three orders was found near the Eu L-II and L-III absorption edges. Unexpectedly high count rates of 40,000 cps were obtained for the 1.5 mu m EuTe film. The magnetic diffraction signal from several EuTe/SnTe and EuTe/PbTe superlattices was also recorded. Magnetic and structural peaks were measured around T-N,T- yielding the transition temperature, the critical exponents, and proving that a lattice distortion accompanies the magnetic transition. In two EuTe/PbTe superlattices, a satellite structure of the magnetic peak was observed, demonstrating the existence of magnetic interlayer correlations, and the capability of x-ray resonant diffraction to detect such correlations in AFM systems, which to date was believed to be possible only by neutron diffraction.

The local Fe ferromagnetic (FM) moment at the grain boundaries of a ceramic sample of Ca2FeReO6 double perovskite was investigated by means of x-ray magnetic circular dichroism spectroscopy at the Fe L2,3 edges and compared to the overall bulk magnetization. We found that, at the grain boundaries, the Fe FM moments at H=5T are much smaller than expected and that the M×H curve is harder than in the bulk magnetization. These results suggest a larger degree of Fe∕Re antisite disorder at the grain boundaries of this sample, shedding light into the intriguing nonmetallic resistivity behavior despite the reported presence of free carriers.

Largely unquenched Re 5d orbital magnetic moments in half-metallic Ba2FeReO6 drive a symmetry lowering transition from a cubic paramagnet to a compressed tetragonal (c/a < 1) ferrimagnet below Tc ~ 305 K, with a giant linear magnetoelastic constant and the spins lying spontaneously along the unique tetragonal axis. The large orbital magnetization and degree of structural deformation indicate proximity to a metal-insulator transition. These results point to an incipient orbitally ordered state in the metallic ferrimagnetic phase.

The compound La0.66Ba0.40Mn0.61Fe0.33O3 shows anisotropic magnetic correlations with no long-range order. Specific heat measurements suggest these correlations represent the bulk. Orbital correlations of Mn(3+)e(g) electrons, surviving in an environment of largely disordered exchange interactions, are invoked to account for this magnetic state. These results argue in favor of a strain-field mechanism for orbital ordering in manganites.