Identifying the presence of magnetite in an ensemble of iron-oxide nanoparticles: a comparative neutron diffraction study between bulk and nanoscale.

Scientific interest in iron-oxides and in particular magnetite has been renewed due to the broad scope of their fascinating properties, which are finding applications in electronics and biomedicine. Specifically, iron oxide nanoparticles (IONPs) are gathering attraction in biomedicine. Their cores are usually constituted by a mixture of maghemite and magnetite phases. In view of this, to fine-tune the properties of an ensemble of IONPs towards their applications, it is essential to enhance mass fabrication processes towards the production of monodisperse IONPs with controlled size, shape, and stoichiometry. We exploit the vacancy sensitivity of the Verwey transition to detect the presence of magnetite. Here we provide direct evidence for the Verwey transition in an ensemble of IONPs through neutron diffraction. This transition is observed as a variation in the Fe magnetic moment at octahedral sites and, in turn, gives rise to a change of the net magnetic moment. Finally, we show this variation as the microscopic ingredient driving the characteristic kink that hallmarks the Verwey transition in thermal variation of magnetization.

Effects of A-site disorder in the properties of A2CoMnO6 (A = La, Tb).

We have studied the structural and physical properties of the La2-xTbxCoMnO6 series. The crystal and magnetic structures of these compounds were determined by x-ray and neutron diffraction techniques. All samples belong to the family of double perovskites with space group P21/n, but the Co/Mn ordering is not perfect, and antisite defects are formed. The concentration of these defects increases for intermediate compositions, indicating that La/Tb disorder influences the Co/Mn arrangement. A ferromagnetic ground state is established due to the strong Mn(4+)-O-Co(2+) superexchange interaction. For the intermediate compositions and at low temperature, the Co/Mn ordering is accompanied by the ordering of Tb(3+) moments in the ab-plane, indicating a mutual polarization between both sublattices. Macroscopic magnetic properties reveal that Curie temperature decreases as Tb content increases in correlation with the increase of the structural distortion. All samples show semiconducting behaviour, and overall the electrical resistivity increases with decreasing La-content. The dielectric constant (ε') has a value of around 12 at low temperatures for all samples, revealing the lack of permanent dipoles. The temperature dependence of ε' on warming exhibits a strong increase that depends heavily on the frequency of the electric field. This effect is ascribed to non-intrinsic effects such as contacts or internal barrier-layers.

Evolution of magnetoelectric properties of Sc-diluted TbMnO3.

The magnetoelectric properties of the TbMn(1-x)Sc(x)O3 series have been studied at low temperatures by means of heat capacity, magnetic measurements and impedance spectroscopy. TbMnO3 exhibits as expected three transitions upon lowering the temperature corresponding to the magnetic ordering of the two sublattices (Mn and Tb) and the ferroelectric transition. Ferroelectricity disappears with Sc dilution for x > 0.1 because the non-collinear magnetic arrangement is destroyed. The dilution of Mn with a non-magnetic ion is also detrimental to the magnetic ordering of both Mn and Tb sublattices. The system evolves to a magnetic glassy state for the intermediate compositions. Formal TbScO3 shows Sc-deficiency and long range magnetic ordering of Tb(3+) moments in the ab-plane brought by the direct interaction between Tb(3+) ions. This ordering is different from the one found in TbMnO3 due to the lack of magnetic coupling between Tb- and Mn-sublattices. A small substitution of Sc by Mn in TbScO3 destroys the Tb ordering giving rise to a magnetic glass behaviour. This effect is ascribed to the partial polarization of Tb sublattice by the paramagnetic Mn which competes with the direct Tb-Tb exchange.

Enhancement of ferromagnetic correlations on multiferroic TbMnO3 by replacing Mn with Co.

The structural, electronic and magnetic properties of TbMn(1-x)Co(x)O(3) (0.1 ≤ x ≤ 0.9) compounds are reported. The samples are isostructural to TbMnO(3) adopting the orthorhombic distorted perovskite structure (Pbnm), except for x = 0.4, 0.5 and 0.6, where an ordered double perovskite structure (P2(1)/n) is found. X-ray absorption spectra at the Mn and Co K edges show an incomplete charge transfer between Mn and Co atoms yielding a mixed valence state Mn(3+)/Mn(4+) and Co(3+)/Co(2+) for the whole series. Neutron powder diffraction measurements show the development of a ferromagnetic ground state for the intermediate compositions (0.3 ≤ x ≤ 0.6) indicating that the ferromagnetic superexchange Mn(4+)-O-Co(2+) interaction is the strongest among a wide set of competitive interactions. The ferromagnetic ordering is, however, not fully achieved and coexists with glassy magnetic properties. With increasing concentration of Co (x ≥ 0.7) the long range ordering vanishes and only a glassy magnetic behavior with slow dynamics is found. These properties could be related to the existence of magnetically inhomogeneous small clusters arising from competitive magnetic interactions.

Structural properties of Pb2MnW(1-x)Re(x)O6 double perovskites.

Pb2MnW(1-x)Re(x)O6 samples have been synthesized and their structure determined by powder x-ray diffraction. These samples undergo a first order structural phase transition between 413 and 445 K depending on the composition. Above this temperature, the samples are cubic. Below the transition temperature, solid solutions are found for x ≤ 0.2 and x ≥ 0.5. The W-rich samples adopt an orthorhombic cell whereas the Re-rich compounds are monoclinic. In the intermediate region, 0.2 < x < 0.5, both phases coexist. X-ray absorption spectra did not reveal significant changes in the local structure for Pb, Mn or Re atoms across the structural phase transition. All the atoms exhibit distorted environments in the whole series. In the case of Pb and W(Re) atoms, the local distortion remains in the high temperature phase. Samples with x ≤ 0.2 also show a sharp discontinuity in the dielectric permittivity at the phase transition temperature indicating the presence of a concomitant electrical ordering in the bulk grains. Such an anomaly in the dielectric constant is not observed for the x ≥ 0.5 samples, compatible with the lack of dipole ordering for this composition range. The different electrical behaviours also explain the differences in the entropy content for the two types of transition.

Origin of the pre-peak features in the oxygen K-edge x-ray absorption spectra of LaFeO3 and LaMnO3 studied by Ga substitution of the transition metal ion.

We report on experimental oxygen K-edge x-ray absorption near edge structure (XANES) spectra of the LaFe(1 - x)Ga(x)O(3) and LaMn(1 - x)Ga(x)O(3) series. Transition metal substitution by the 3d full shell Ga atom is mainly reflected in a systematic decrease of the pre-edge structures in the XANES spectra of the two series. This result shows that the associated states originate from the hybridization of oxygen 2p and unoccupied Fe (or Mn) 3d states. In order to gain insight into the states associated with the pre-edge spectral features, we have performed ab initio theoretical calculations based on multiple scattering theory. Simulations with variable cluster size and composition around the absorber oxygen in the LaFeO(3) and LaMnO(3) crystal structures were carried out. We find that the low-energy pre-peak is reproduced once the absorbing oxygen and the two nearest neighbour Fe (or Mn) ions are considered in the cluster. Conversely, higher energy pre-peaks only arise when the full oxygen coordination geometry around the two metal sites is taken into account, implying that their energy distance is a reflection of the strength of the oxygen ligand field. Substitutions of the two nearest neighbours by Ga atoms in the cluster of calculation lead to changes in the theoretical spectra that reasonably agree with the evolution of the pre-peaks in the experimental XANES spectra of both the series.

Order-disorder nature of the antiferroelectric transition in Pb(2)MnWO(6).

The nature of the para-antiferroelectric phase transition in Pb(2)MnWO(6) was investigated by x-ray absorption spectroscopy. Extended x-ray absorption spectra (EXAFS) and x-ray absorption near-edge structure (XANES) at the Pb L(3)-edge, W L(3)-edge and Mn K-edge were measured at temperatures above and below the transition temperature, T(c)∼445 K. The temperature dependence of the local geometrical and electronic structure around each of the three atoms was quantitatively determined. XANES spectra confirm the expected formal valence state as Pb(2+), W(6+) and Mn(2+) both above and below T(c). There are no significant changes of the local structure around the three atoms, which remains distorted in the whole temperature range. This indicates the existence of dynamic local distortions with a short-range order in the paraelectric phase. Below T(c), the local distortions (and the electrical dipoles) are static and their ordered arrangement gives rise to the orthorhombic crystallographic structure. Above T(c), these distortions are dynamically disordered, causing an average cubic crystallographic structure. Hence, the phase transition is of the order-disorder type.

The checkerboard pattern of Bi(0.63)Sr(0.37)MnO(3) determined using resonant x-ray scattering at the Mn K edge.

A study using resonant x-ray scattering at the Mn K edge has been carried out on a Bi(0.63)Sr(0.37)MnO(3) single crystal. This compound undergoes a metal-insulator phase transition to the so-called charge ordered (CO) state at about 530 K. Strong resonance signals were observed at room temperature as the energy was tuned through the Mn K edge for several superstructures of the CO phase. The energy, polarization and azimuth angle dependences agree with a checkerboard ordering in the ab plane of manganese atoms of two types, in terms of their different local geometrical structures. One of the sites is anisotropic-a tetragonal distorted oxygen octahedron-and the other is isotropic-a nearly undistorted one, as observed for Bi(0.5)Sr(0.5)MnO(3) and other half-doped manganites. This result indicates that the checkerboard pattern is strongly stable and extends to doping concentrations x<0.5. No superstructures corresponding to the doubling of the c axis were detected. Intermediate valence states lower than 3.5, according to the fractional charge segregation, were deduced for the two non-equivalent Mn atoms, i.e. Mn(3.30+) and Mn(3.44+).

[Strongyloides stercoralis hyperinfection in a patient with acquired immunodeficiency syndrome]. / Hiperinfección por Strongyloides stercoralis en un paciente con síndrome de inmunodeficiencia adquirida.

Strongyloides stercoralis hyperinfection is a rare but know complication in patients with intestinal parasitism. Immunosuppressive therapy and acquires immunodeficiency syndrome (AIDS) are major risk factors for dissemination. Diagnosis requires identification of larvae in body fluids or biopsy and the mortality rates can be as high as 80%. The enteric bacteria are carried by invasive larvae and this can result in septicemia, pneumonia, meningitis, and disseminated bacterial infection in many parts of the body. We present a case of Strongyloides stercoralis hyperinfection complicated by Escherichia coli bacteraemia in a 45-year-old man African with AIDS.

Hiperinfección por Strongyloides stercoralis en un paciente con síndrome de inmunodeficiencia adquirida / Strongyloides stercoralis hyperinfection in a patient with acquired immunodeficiency syndrome

El síndrome de hiperinfección por Strongyloides stercoralis, es una rara pero conocida complicación en pacientes con parasitación intestinal. La utilización de terapia inmunosupresora y el síndrome de inmunodeficiencia adquirida (SIDA) son los principales factores de riesgo para la diseminación. El diagnóstico se realiza por la visualización de larvas en muestras corporales o biopsias y la mortalidad alcanza cifras tan elevadas como el 80%. Las bacterias entéricas pueden ser transportadas por las larvas durante su migración y dar como resultado cuadros de septicemia, neumonía, meninigitis y otras infecciones bacterianas diseminadas. Presentamos un caso de síndrome de hiperinfección por Strongyloides stercoralis en un paciente africano de 45 años diagnosticado de sida

Strongyloides stercoralis hyperinfection is a rare but know complication in patients with intestinal parasitism. Immunosuppresive therapy and acquires immunodeficiency syndrome (AIDS) are major risk factors for dissemination. Diagnosis requires identification of larvae in bodyfluids or biopsy and the mortality rates can be as high as 80%. The enteric bacteria are carried by invasive larvae and this can result in septicemia, pneumonia, meningitis, and disseminated bacterial infection inmany parts of the body. We present a case of Strongyloides stercolaris hyperinfection complicated by Escherichia coli bacteriemia in a 45-year-old man African with AIDS

Electronic structure of Ag2Cu2O4. Evidence of oxidized silver and copper and internal charge delocalization.

The electronic structure of the recently isolated silver copper oxide Ag(2)Cu(2)O(4) is analyzed along with its precursor Ag(2)Cu(2)O(3) and similar binary oxides, Ag(2)O, AgO, CuO, and NaCuO(2), using X-ray photoemission (XPS) and X-ray absorption (XAS) measurements. The results for Ag(2)Cu(2)O(4) reveal an electronic distribution in which silver and copper share a delocalized valence scheme with both metals in formal oxidation states larger than the usual Ag(I) and Cu(II). Only one type of crystallographic silver or copper is found, but disorder-strain parameters are considerable and the possibilities of thermal disorder, atomic motion, oxygen contribution, mixed valence, and internal charge delocalization are considered. Classical coordination descriptions for oxidized silver are revisited in terms of this new internal charge delocalization framework found for the electronic structure.

Corynebacterium macginleyi como agente excepcional de endocarditis infecciosa: a propósito de un caso / Corynebacterium macginleyi as an exceptional cause of endocarditis: a case report

No disponible

No disponible

Corynebacterium macginleyi como agente excepcional de endocarditis infecciosa: a propósito de un caso / Corynebacterium macginleyi como agente excepcional de endocarditis infecciosa: a propósito de un caso

No disponible

No disponible

Cooperative Jahn-Teller phase transition in LaMnO3 studied by X-ray absorption spectroscopy.

The local structure of LaMnO3 across the Jahn-Teller (JT) transition at T(JT)=750 K was studied by means of x-ray absorption near edge structure and extended x-ray absorption fine structure at the Mn K-edge. Our results indicate a similar electronic local structure for Mn atoms above and below T(JT) and a dynamical tetragonal JT distortion of MnO6 octahedra above T(JT). The structural transition is originated by the ordering of tetragonally distorted octahedra. The entropy content of the transition is analyzed within the framework of the three-state Potts model with nearest neighbor antiferrodistortive coupling.

Diffraction anomalous fine structure of forbidden Bragg reflections: charge localization and structure of the octahedral site in magnetite.

Resonant X ray scattering has been used to investigate charge localization on the octahedral iron atoms in magnetite below and above the Verwey temperature. We have measured the DAFS spectra of the 002 and 006 "forbidden" Bragg reflections permitted by the anisotropy of the iron anomalous scattering factor. We performed ab initio calculations which are in fair agreement with the experiment in the near edge region and demonstrate the sensitivity of the DAFS spectra to tiny structural and electronic changes. No change is observed, in the energy and azimuthal dependences, when the sample is cooled down below the Verwey temperature. Charge ordering can be definitely excluded and different charge localisation schemes discarded. Ab initio simulations, performed by using the refined crystallographic structure proposed for the room temperature phase, do not show a good agreement with the experiment in the extended region of the DAFS spectrum. This point is being investigated.

Mn K-edge XANES spectra of manganites measured by Kbeta emission.

The electronic state of Mn atoms in mixed valence manganites has been studied by means of X-ray absorption spectroscopy at the Mn K-edge. Higher resolution than in conventional measurements has been achieved by measuring the Mn Kbeta fluorescence line. We have found a unique resonance at the edge in the XANES spectra of intermediate composition RE1-xCa(x)MnO3 samples. The features of these XANES spectra do not depend on small changes in the local structure around the Mn atom. However, the spectra of the intermediate composition samples can not be reproduced by a linear combination of REMnO3 and CaMnO3 spectra. Accordingly, the electronic state of Mn atoms in these compounds can not be considered as a mixture of Mn3+ and Mn4+ pure states.

Has charge ordering actually been observed in manganites by resonant X-ray scattering?

The comparison between X-ray resonant scattering experiments and X-ray absorption data shows that no real Mn3+/Mn4+ charge ordering occurs in manganites with Mn3+/Mn4+=1 ratio. However, these experiments demonstrate the presence of two different types of manganese atoms with a different local geometrical structure. A structural model, which accounts for both X-ray absorption and resonant scattering experiments, is proposed.

Local structure and oxidation state of Mn and Ni atoms in LaNi1-xMnxO3+delta perovskites.

X-ray absorption spectroscopy has been used to study LaNi1-x Mn(x)O3+delta perovskites (x = 0.1, 0.25, 0.5, 0.75, 0.9). We have analysed both, the oxidation state and the local structure around the transition metal atoms. XANES spectra of these samples suggest a mixed valence state for both transition metal atoms. The Mn K-edge is shifted to high energies as the Ni content is increased up to x=0.5. The opposite behavior is found instead, for the Ni K-edge. Samples with x=0.5 and x=0.25 present the Mn K-edge at the same energy but the absorption features are different for both samples. The local structure has also been studied by means of the EXAFS technique. The first coordination shell nicely reflects the structural change observed in this series from rhombohedral perovskites (x>0.5) to orthorhombic perovskites x<0.5). The changes in the local structure for Ni and Mn atoms agree with the variation of the oxidation state observed in the XANES spectra.