Structural, magnetic, and transport properties of Co nanoparticles within a Cu matrix

We have studied the structural, magnetic, and transport properties of Co nanoparticles embedded in a Cu matrix using x-ray absorption fine structure (XAFS) spectroscopy, macroscopic magnetization, and transport measurements. We observed by XAFS, on annealed samples containing 3, 10, 12, and 25 at. % Co, that there is a systematic contraction of the average coordination distance around Co atoms with the increasing Co content and annealing. The results are consistent with the growing of small fcc nanoparticles and correlated to the evolution in the superparamagnetic behavior, observed by magnetization measurements. We present a simple model to connect the diameter of Co nanoparticles to the average coordination distance and its mean-square fluctuation. Our analysis leads to cluster sizes ranging from 1 nm to more than 8 nm. We found that for a particle diameter of about 4.4 nm the sample containing 10 at. % Co presents a maximum in the giant magnetoresistance (GMR). Moreover, we observed that the disorder is significantly reduced around that size, due to the hardening of Co bonds, further favoring the GMR. Our analysis gives direct structural parameters and emphasizes their role in the transport and magnetic properties of the Co-Cu system.68

Local anisotropy in strained manganite thin films

We report on an angular resolved x-ray absorption spectroscopy study of the local atomic structure around the manganese ions in La0.7Sr0.3MnO3 thin films epitaxially grown on tensile and compressive substrates. Ab initio calculations provide strong support to the analysis of the experimental data and make possible the unambiguous derivation of a model of local distortion around the manganese atoms, without modification of the tilt angle Mn-O-Mn, among the octahedra. This distortion, tending to localize the charge carriers, is the driving parameter in the modifications of the magnetic and transport properties observed in thin films with respect to bulk systems. (C) 2003 American Institute of Physics.83173587358

Local structure in LaMnO3 across the Jahn-Teller transition

We report on an EXAFS study of the local environment of manganese atoms in LaMnO3 across the cell ordering transition temperature. During the EXAFS analysis the phase derivative method was used prior to conventional fits in order to prevent possible mistakes arising from the correlation between parameters in the fitting method. The combination of the two procedures assert the result that the local splitting in the Mn-O distance is maintained above the Jahn-Teller transition and support the order-disorder character predicted for this transition in LaMnO3.T11542843

Local atomic and electronic structure in LaMnO3 across the orbital ordering transition

The local atomic disorder and electronic structure in the environment of manganese atoms in LaMnO3 has been studied by x-ray absorption spectroscopy over a temperature range (300-870 K) covering the orbital ordering transition (similar to710 K). The Mn-O distance splitting into short and long bonds (1.95 and 2.15 Angstrom) is kept across the transition temperature, so that the MnO6 octahedra remain locally Jahn-Teller distorted. Discontinuities in the Mn local structure are identified in the extended x-ray fine structure spectra at this temperature, associated with a reduction of the disorder in the superexchange angle and to the removal of the anisotropy in the radial disorder within the coordination shell. Subtle changes in the electronic local structure also take place at the Mn site at the transition temperature. The near-edge spectra show a small drop of the Mn 4p hole count and a small enhancement in the pre-edge structures at the transition temperature. These features are associated with an increase of the covalence of the Mn-O bonds. Our results shed light on the local electronic and structural phenomena in a model of order-disorder transition, where the cooperative distortion is overcome by the thermal disorder.702

Local structure in strained manganite thin films

We report on a polarized X-ray absorption spectroscopy study, combining experimental measurements and ab initio calculations, of La0.7Sr0.3MnO3 films, epitaxially grown on tensile and compressive substrates. Measurements show significant modi. cations in the coordination shell around manganese atoms in the film plane for both substrates. We show that biaxial strain is locally accommodated in the coordination shell, by distortion of the MnO6 octahedron, without change in the tilt angle. The modi. cations of the near edge spectra were correlated to modi. cation in the average Mn-O bond distance and distortion of the MnO6 octahedra. This distortion tending to localize the charge carriers may account for the decrease of the Curie temperature observed in thin films with respect to bulk systems.T11558959

Incorporation of Ni into natural goethite: An investigation by X-ray absorption spectroscopy

Goethite (alpha-FeOOH) is abundant at the Earth's surface and has the unusual capacity to adsorb and fix ions from migrating solutions. Understanding the mechanisms by which foreign elements are incorporated into natural goethite has implications for environmental and mining problems. X-ray absorption spectroscopy (XAS) was used to obtain structural information on the local environment around Ni in natural Ni-containing goethite (1.8-4.1 mol% Ni) from Vermelho lateritic deposit of Serra dos Carajas (Brazil) and in synthetic analogues. The data were collected at the LNLS XAS beam line at the Ni and Fe K-edges, at room temperature, and at the Ni K-edge at 8 K. Nickel was found in essentially the same environment in all natural and synthetic samples, with negligible thermal disorder. The coordination polyhedron is a tetragonal dipyramid of oxygen atoms showing that Ni preserves its usual local symmetry. This finding is compatible with a model in which substitution of Ni for Fe is accompanied by a proton capture resulting in NiO2(OH)(4) octahedra. The polyhedral linkages are similar to that of pure alpha-FeOOH, consisting of four shared edges at about the same metal-metal distances, as in the pure mineral. The third and longest metal-metal distance is about 6% larger than the expected corner-sharing distance in the alpha-FeOOH structure, showing that incorporation of Ni locally distorts and opens the structure.884179587688

Phonon activity and intermediate glassy phase of YVO3

We show that in YVO3 additional hard phonons gradually become zone center infrared active below similar to210 K, verifying that a lattice phase transition takes place at about that temperature. Their gradual increment in intensity between similar to210 and similar to77 K is associated with a "glassy" behavior found in the temperature-dependent V K edge pseudoradial distribution. This translates into an increase in the Debye-Waller factors ascribed to the appearance of V local structural disorder below similar to150 K. Conflicts between various ordering mechanisms in YVO3 bring up similarities of the intermediate phase to known results in dielectric incommensurate systems, suggesting the formation of commensurate domains below 116 K, the onset temperature of G-type antiferromagnetism. We propose that similar to210 and similar to77 K be understood as the temperatures where the commensurate-incommensurate and incommensurate-commensurate "lock-in" phase transitions take place. We found support for this interpretation in the inverted lambda shapes of the measured heat capacity and in the overall temperature dependence of the hard phonons.69

CsCl-modified Ga2S3-La2S3 glasses: Structural approach by x-ray absorption spectroscopy

Gallium-lanthanum sulfide glasses are potential hosts for 1.3-mum optical fiber amplifiers and for fiber lasers in the near and middle infrared. In these glasses the addition of CsCl increases the thermal stability region making possible to draw optical fibers, without altering the optical properties of the glass. Ga2S3-La2S3 glasses modified by 10 to 40% CsCl have been studied by x-ray absorption spectroscopy, to investigate the structural role of CsCl, The chlorine environment is found similar to that in CsCl. The gallium-based network is composed from almost regular tetrahedra weakly connected by corners and is not altered by the addition of CsCl.o TEXTO COMPLETO DESTE ARTIGO, ESTARÁ DISPONÍVEL À PARTIR DE AGOSTO DE 2015.1651349135

Depth magnetization profile of a perpendicular exchange coupled system by soft-x-ray resonant magnetic reflectivity

The magnetic profile across the interface of a perpendicular exchange coupled [NiO=CoO]3/Pt-Co/Pt(111) system is investigated. The magneto-optic Kerr effect reveals a strong coupling between the antiferromagnetic (AFM) oxide and the ferromagnetic (FM) Pt-Co layer, by an increasing coercivity and a rotation of the easy magnetization axis of the FM layer along the AFM spins. Soft x-ray resonant magnetic reflectivity is used to probe the spatial distribution of the out-of-plane magnetization inside the oxide above its ordering temperature. It extends over 1 nm and exhibits a change of sign