When studying nominal magnetite nanoparticles, it is mandatory to obtain a precise structural characterization to get an accurate relationship with their physiochemical properties. The great deal of information accumulated to date on the characterization of nominal magnetite and maghemite NPs does not clarify if the synthesized materials are single o multiphase systems involving bulk-like stoichiometric oxides (Fe 3 O 4 , γ-Fe 2 O 3 , α-Fe 2 O 3 , ...), or single or multiphase entities formed by nonstoichiometric oxides. In this work we propose a new approach to determine the structure of Fe oxide NPs by using the Fe K-edge X-ray absorption near edge spectroscopy. We report here an X-ray absorption near edge spectroscopy study at the Fe K-and L 2,3 -edges, on nominal magnetite nanoparticles synthesized by different methods. In addition, X-ray magnetic circular dichroism was recorded at the Fe L 2,3 -edges, in selected samples. We have found that the experimental spectra are not well reproduced by any linear combination of the absorption spectra of Fe 3 O 4 and γ-Fe 2 O 3 bulk references, even taking into account other oxides as goethite or ferrihydrite. The analysis of the Fe K-edge XANES spectra shows that it is the size, and not the synthesis method, which determines the structure of the NPs. Our experimental results indicate that, irrespective of the synthesis method, the nominal magnetite NPs are, actually, a single phase non stoichiometric Fe 3−δ O 4 oxide. At the origin of this phase are the cation vacancies, which lead to the modification of the structural arrangements at the Fe sites with respect to those found in bulk-like iron oxides.

We present here direct experimental evidence of the magnetic polarization of Zn atoms in ZnO nanoparticles capped with different materials by means of x-ray magnetic circular dichroism ͑XMCD͒. Our results demonstrate that the magnetism in this material is intrinsic and relays in the ZnO conduction band. The analysis of both x-ray absorption spectroscopy and XMCD signals points out the formation of a well-defined interface between ZnO and the capping molecule in which the exotic magnetism arises at the hybridized band formed among Zn and the bonding atom of the molecule. The magnetic properties of these systems should critically depend on the details of this interface which may offer a new insight into the different observations for seemingly identical materials.

Mössbauer spectral measurements between 4.2 and 640 K have been carried out on TbFe 11 Ti and TbFe 11 TiH. The insertion of hydrogen into TbFe 11 Ti to form TbFe 11 TiH increases its ordering temperature, magnetization, magnetic hyperfine fields, and isomer shifts as a result of lattice expansion. Further, the insertion of hydrogen reinforces the basal magnetic anisotropy of the terbium sublattice and, as is shown by ac susceptibility measurements and thermomagnetic analysis, the spin reorientation observed in TbFe 11 Ti at 338 K disappears in TbFe 11 TiH. The Mössbauer spectra have been analysed with a model that considers both the easy magnetization direction and the distribution of titanium atoms in the near-neighbour environment of the three crystallographically distinct iron sites. The assignment and the temperature dependencies of the hyperfine fields and isomer shifts are in complete agreement with a Wigner-Seitz cell analysis of the three iron sites in RFe 11 Ti and RFe 11 TiH, where R is a rare-earth element. A complete analysis of the quadrupole interactions in both magnetic phases and in the paramagnetic phase of TbFe 11 Ti supports the Mössbauer spectral analysis, and indicates that in the basal magnetic phase the iron magnetic moments are oriented along the equivalent [100] and [010] directions of the unit cell.

No abstract

We present an x-ray magnetic circular dichroism ͑XMCD͒ study performed at the R L 2 edge in RAl 2 and RT 2 ͑T =Fe,Co͒ intermetallic compounds. By analyzing the modification of the XMCD spectra as both the rare earth and the alloyed element ͑Al, Fe, or Co͒ are changed in the compound, an extra contribution, XMCD T , has been identified and undoubtedly associated with the R͑5d͒-T͑3d͒ hybridization in Fe and Co compounds. The sign and intensity of this contribution are found to be related to the magnetism of the T neighboring atoms as well as to the strength of the T-R interaction. These findings open the possibility of experimentally quantifying both the R͑4f͒-R͑5d͒ and the T͑3d͒-R͑5d͒ hybridizations, which govern the R-T interaction.

X-ray diffraction, isothermal magnetization at 5 and 300 K, ac magnetic susceptibility measurements between 5 and 200 K, and iron-57 Mössbauer spectral measurements between 4.2 and 295 K have been carried out on ErFe 11 Ti and ErFe 11 TiH. Hydrogen uptake has been measured by gravimetric analysis and the insertion of hydrogen into ErFe 11 Ti increases its magnetization, magnetic hyperfine fields, and isomer shifts as a result of the associated lattice expansion. Peaks and steplike changes in both the real and imaginary components of the ac magnetic susceptibility are observed at ϳ50 and 40 K for ErFe 11 Ti and ErFe 11 TiH, respectively, and are assigned to spin-reorientation transitions resulting from the temperature dependence of the sixth-order Stevens crystal-field term of erbium. The Mössbauer spectra have been analyzed with a model which considers both these spin reorientations and the distribution of titanium atoms in the near-neighbor environment of the three crystallographically distinct iron sites. The assignment and the temperature dependencies of the hyperfine fields and isomer shifts are in complete agreement with the WignerSeitz cell analysis of the three iron sites in ErFe 11 Ti and ErFe 11 TiH. The changes in the hyperfine field and isomer shift with the number of titanium near neighbors of the three iron sites are in agreement with the values observed for related titanium-iron intermetallic compounds.

A new series of compounds with the -type structure (, Dy, Ho, Er and Lu) has been synthesized and studied by x-ray diffraction, ac susceptibility and magnetization versus temperature and field measurements. The maximum Curie temperature is for the Tb compound . Spin reorientation transitions have been observed for the compounds and . A first order magnetization process has been observed at low temperature in and . The importance of the metallic radius of M in the magnetic properties of (for the same rate of M substitution) is discussed.

No abstract