78 research outputs found
Change in the Magnetic Domain Alignment Process at the Onset of a Frustrated Magnetic State in Ferrimagnetic La2Ni(Ni1/3Sb2/3)O6 Double Perovskite
We have performed a combined study of magnetization hysteresis loops and time
dependence of the magnetization in a broad temperature range for the
ferrimagnetic La2Ni(Ni1/3Sb2/3)O6 double perovskite. This material has a
ferrimagnetic order transition at ~100 K and at lower temperatures (~ 20 K)
shows the signature of a frustrated state due to the presence of two competing
magnetic exchange interactions. The temperature dependence of the coercive
field shows an important upturn below the point where the frustrated state sets
in. The use of the magnetization vs. applied magnetic field hysteresis data,
together with the magnetization vs. time data provides a unique opportunity to
distinguish between different scenarios for the low temperature regime. From
our analysis, a strong domain wall pinning results the best scenario for the
low temperature regime. For temperatures larger than 20K the adequate scenario
seems to correspond to a weak domain wall pinning.Comment: 4 pages, 5 figures included. Manuscript submitted to IEEE
Transactions on Magnetics, proceedings of the LAW3M 2013 conferenc
Magnetic properties of the double perovskites LaPbMSbO6 (M = Mn, Co and Ni)
New double perovskites LaPbMSbO6, where M2+ = Mn2+, Co2+, and Ni2+, were
synthesized as polycrystals by an aqueous synthetic route at temperatures below
1000 oC. All samples are monoclinic, space group P21/n, as obtained from
Rietveld analysis of X-ray powder diffraction patterns. The distribution of M2+
and Sb5+ among the two octahedral sites have 3% of disorder for M2+ = Ni2+,
whereas for M2+ = Mn2+ and Co2+ less disorder is found. The three samples have
an antiferromagnetic transition, due to the antiferromagnetic coupling between
M2+ through super-superexchange paths M2+ - O2- - Sb5+ - O2- - M2+. Transition
temperatures are low: 8, 10 and 17 K for Mn2+, Co2+, and Ni2+ respectively, as
a consequence of the relatively long distances between the magnetic ions M2+.
Besides, for LaPbMnSbO6 a small transition at 45 K was found, with
ferrimagnetic characteristics, possibly as a consequence of a small disorder
between Mn2+ and Sb5+. This disorder would give additional and shorter
interaction paths: superexchange Mn2+ - O2- - Mn2+.Comment: 4 pages, 4 figures included. Manuscript submitted to IEEE
Transactions on Magnetics, proceedings of the LAW3M 2013 conferenc
Tailoring the ground state of the ferrimagnet La2Ni(Ni1/3Sb2/3)O6
We report on the magnetic and structural properties of La2Ni(Ni1/3Sb2/3)O6 in
polycrystal, single crystal and thin film samples. We found that this material
is a ferrimagnet (Tc ~ 100 K) which possesses a very distinctive and uncommon
feature in its virgin curve of the hysteresis loops. We observe that bellow 20
K it lies outside the hysteresis cycle, and this feature was found to be an
indication of a microscopically irreversible process possibly involving the
interplay of competing antiferromagnetic interactions that hinder the initial
movement of domain walls. This initial magnetic state is overcome by applying a
temperature dependent characteristic field. Above this field, an isothermal
magnetic demagnetization of the samples yield a ground state different from the
initial thermally demagnetized one.Comment: 21 pages, 8 figures, submitted to JMM
Perovskite oxides: Oxygen electrocatalysis and bulk structure
Perovskite type oxides were considered for use as oxygen reduction and generation electrocatalysts in alkaline electrolytes. Perovskite stability and electrocatalytic activity are studied along with possible relationships of the latter with the bulk solid state properties. A series of compounds of the type LaFe(x)Ni1(-x)O3 was used as a model system to gain information on the possible relationships between surface catalytic activity and bulk structure. Hydrogen peroxide decomposition rate constants were measured for these compounds. Ex situ Mossbauer effect spectroscopy (MES), and magnetic susceptibility measurements were used to study the solid state properties. X ray photoelectron spectroscopy (XPS) was used to examine the surface. MES has indicated the presence of a paramagnetic to magnetically ordered phase transition for values of x between 0.4 and 0.5. A correlation was found between the values of the MES isomer shift and the catalytic activity for peroxide decomposition. Thus, the catalytic activity can be correlated to the d-electron density for the transition metal cations
Magnetization reversal in mixed ferrite-chromite perovskites with non magnetic cation on the A-site
In this work, we have performed Monte Carlo simulations in a classical model
for RFeCrO with R=Y and Lu, comparing the numerical simulations
with experiments and mean field calculations. In the analyzed compounds, the
antisymmetric exchange or Dzyaloshinskii-Moriya (DM) interaction induced a weak
ferromagnetism due to a canting of the antiferromagnetically ordered spins.
This model is able to reproduce the magnetization reversal (MR) observed
experimentally in a field cooling process for intermediate values and the
dependence with of the critical temperatures. We also analyzed the
conditions for the existence of MR in terms of the strength of DM interactions
between Fe and Cr ions with the x values variations.Comment: 8 pages, 7 figure
Synthesis and structural, magnetic, electric, and thermoelectric characterization of layered Rh1âxIrxTe2 (0â€xâ€1)
Crystallographic analysis and thermoelectric studies of solid solutions Rh1âxIrxTe2 (0â€xâ€1) are reported. All compositions show layered structures belonging to the PÂŻ3m1 space group at room temperature. IrTe2 presents a first-order phase transition from the hexagonal to the triclinic lattice (PÂŻ1 space group), which is monitored by synchrotron radiation x-ray powder diffraction. In the cooling-down process the transition appears at 240 K while in the warming-up process it begins at 280 K, showing a remarkable hysteresis. All compositions show a strong metallic behavior with enhanced Pauli paramagnetism and two regimes in the electrical resistivity. These regimes are associated with electron-electron scattering (at low temperature ÏâŒT2) and electron-phonon coupling (higher temperatures ÏâŒT). The Seebeck coefficient shows hole-type carriers for all the compounds
Phase coexistence of multiple copper oxides on AgCu catalysts during ethylene epoxidation
Alloy catalysts under reaction conditions are complex entities. In oxidizing atmospheres, multiple phases can coexist on a catalyst s surface as a result of phase segregation and preferential oxidation. Such a scenario can result in unusual substoichiometric and metastable phases that could play important roles in catalytic processes. For instance, AgCu alloys known to exhibit enhanced epoxide selectivity in partial oxidation of ethylene form an oxide like surface structure under reaction conditions. Under these conditions, copper oxides are stable, while silver oxides are not. Consequently, copper segregates to the alloy s surface and forms an oxide overlayer. Little is known about the structure or function of such overlayers, and it is unknown whether they play an active role in the catalyst s enhanced selectivity. In order to develop a clearer picture of such catalysts, the current work utilizes several in situ spectroscopic and microscopic techniques to examine the copper oxide phases that form when AgCu is exposed to epoxidation conditions. It is found that several forms of oxidic Cu coexist simultaneously on the active catalyst s surface, namely, CuO, Cu2O, and some previously unreported form of oxidized Cu, referred to here as CuxOy. Online product analysis, performed during the in situ spectroscopic measurements, shows that increased epoxide selectivity is correlated with the presence of mixed copper oxidation states and the presence of the CuxOy species. These results support previous theoretical predictions that oxidic copper overlayers on silver play an active role in epoxidation. These results furthermore emphasize the need for in situ spectromicroscopic methods to understand the complexity of alloy catalyst
Reflectividad infrarroja de SrRuOâ
En este trabajo presentamos los espectros de reflectividad infrarroja a temperatura ambiente y a 80K piara la perovskita distorsionada SrRuOâ. Con las relaciones de Krammers-Kronig calculamos las funciones Ăłpticas y reconstruimos el espectro de reflectividad con un modelo basado en la generalizaciĂłn de la relaciĂłn de Lyddane- Sachs-Teller y el modelo de Drude. Con estos datos calculamos el nĂșmero de portadores, la movilidad efectiva y la conductividad Ăłptica, comparando nuestros resultados con los conocidos para diferentes compuestos.Centro de QuĂmica InorgĂĄnic
The role of the copper oxidation state in the electrocatalytic reduction of CO2 into valuable hydrocarbons
Redox-active copper catalysts with accurately prepared oxidation states (Cu0, Cu+ and Cu2+) and high selectivity to C2 hydrocarbon formation, from electrocatalytic cathodic reduction of CO2, were fabricated and characterized. The electrochemically prepared copper-redox electro-cathodes yield higher activity for the production of hydrocarbons at lower oxidation state. By combining advanced X-ray spectroscopy and in situ micro-reactors it was possible to unambiguously reveal the variation in the complex electronic structure that the catalysts undergo at different stages (i.e. during fabrication and electrocatalytic reactions). It was found that the surface, sub-surface and bulk properties of the electrochemically prepared catalysts are dominated by the formation of copper carbonates on the surface of cupric-like oxides, which prompts catalyst deactivation by restraining effective charge transport. Furthermore, the formation of reduced or partially-reduced copper catalysts yields the key dissociative proton-consuming reactive adsorption of CO2 to produce CO, allowing the subsequent hydrogenation into C2 and C1 products by dimerization and protonation. These results yield valuable information on the variations in the electronic structure that redox-active copper catalysts undergo in the course of the electrochemical reaction, which, under extreme conditions are mediated by thermodynamics but, critically, kinetics dominate near the oxide/metal phase transitions
Raman and infrared spectroscopy of Sr2BâČUO6 (BâČ = Ni; Co) double perovskites
Temperature dependent normal modes and lattice thermal expansion of Sr 2BâČUO6 (BâČ = Ni, Co) double perovskites were investigated by Raman/infrared spectroscopies and synchrotron X-ray diffraction, respectively. Monoclinic crystal structures with space group P21/n were confirmed for both compounds, with no clear structural phase transition between 10 and 400 K. As predicted for this structure, the first-order Raman and infrared spectra show a plethora of active modes. In addition, the Raman spectra reveal an enhancement of the integrated area of an oxygen stretching mode, which is also observed in higher-order Raman modes, and an anomalous softening of âŒ1 cm-1 upon cooling below T* ⌠300 K. In contrast, the infrared spectra show conventional temperature dependence. The band profile phonon anomalies are possibly related to an unspecified electronic property of Sr2BâČUO6 (BâČ = Ni, Co).Centro de QuĂmica InorgĂĄnic
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