Search

The densities of metastable and resonant atom were measured in atmospheric pressure Ar-He surface-wave plasmas. Measurements were performed using an absorption spectroscopy method taking into account the Voigt profiles of the plasma lines. The density values of the argon 3P2, 3P0 (metastable atoms) and 3P1 (resonant atoms) levels measured in pure argon discharges are in good agreement with those reported in the literature. A drastic decrease of metastable and resonant densities is observed when introducing helium in amounts as low as 2%. The influence of electron density and gas temperature on the population mechanisms (direct electron excitation from the ground state and dissociative recombination) of metastable and resonant atoms is discussed using a simplified theoretical model. [ABSTRACT FROM AUTHOR]

An experimental characterization of a helium surface-wave discharge (SWD) at atmospheric pressure using spectroscopic techniques is presented. Values of plasma parameters (densities and temperatures) were obtained from these techniques and the degree of thermodynamic equilibrium was shown to be that of an ionizing plasma. The experimental values of the plasma parameters were compared to those obtained from a theoretical model developed for the same He discharge type and to those found in the literature for other noble gases (neon and argon) SWDs also generated at atmospheric pressure. These comparisons enable to infer the main cause of departure from local thermodynamic equilibrium to processes involving He molecular ions, namely, associative ionization and dissociative recombination. [ABSTRACT FROM AUTHOR]

The great tendency of Bi–Sr manganites to present charge order (CO) at very high temperatures and partial hybridization of the 6s2 lone pair with O p-orbitals makes these materials interesting as potential candidates for obtaining high dielectric constants with very small frequency dependence at room temperature. We have studied the dielectric properties of Bi1-xSrxMnO3 (x=0.40,0.50) perovskites that present CO well above room temperature (TCO∼525 K). Thick (∼4 mm) samples exhibit, at room and higher temperatures, high values (above 106) of the dielectric constant along a huge range of frequencies in which [variant_greek_epsilon]r′ is nearly unvarying. The dependence on the thickness of the dielectric constant, its evolution with temperature, and the Cole–Cole diagrams evidence the influence of extrinsic effects. To properly compare the results in the two compounds, we have performed a detailed structural study using neutron and synchrotron diffraction. CO is more extended in x=0.40. The results in the two compositions allowed us to correlate CO and phase separation with the huge values of [variant_greek_epsilon]r′ found in these systems. [ABSTRACT FROM AUTHOR]

Presents a study that developed formulas for the initial magnetization curves and hysteresis loops based on the exponential-law model and valid for a general specimen shape which includes the slab and cylinder. General space-dependent supercurrent densities for different cases and magnetization stages; Expressions derived from the exponential-law model for different parameters; Discussion on the shielding effect of supercurrent.

We present a comparative study of the magnetic transitions in metallic Pr0.50Sr0.50CoO3 (PSCO) perovskites prepared in polycrystalline and thin film forms. As the bulk system, the strained epitaxial PSCO (010) film grown on LAO (100) is metallic in all the temperature range, with a ferromagnetic transition at 225K, close to Tc ~ 235K in the ceramic PSCO specimen. Unlike the bulk system, the PSCO film does not show the second magnetic transition on cooling. In the ceramic sample, the second magnetic transition is coupled to an orthorhombic-to-monoclinic symmetry change. There is a contraction of the average bond distance in the monoclinic phase below Ta, but the bond length is not modified across the transition. The orthorhombic to monoclinic structural transition stabilizes four short Pr-O2 bonds to basal oxygens in CoO6 octahedra. A strong hybridization of Pr 4f and O 2p orbitals in these bonds can be activated at Ta and probably assist the magnetostructural transition. [ABSTRACT FROM AUTHOR]

We present a study on the magnetic and electronic properties of Bi0.75Ca0.25MnO3, investigated by means of magnetometry, diffraction, and muon spin relaxation techniques. The anisotropic changes of cell parameters across the structural transition in Bi0.75Ca0.25MnO3 (275 K) differ markedly from the lattice anomalies not only in Bi0.75Sr0.25MnO3 (600 K) but also in Bi0.50Ca0.50MnO3 (325 K). We have confirmed the absence of a ferromagnetic component under zero field in Bi0.75Ca0.25MnO3. The application of magnetic fields produces a continuous progressive polarization of Mn moments. The ground state of Bi0.75Ca0.25MnO3 is characterized by a high degree of spin disorder and frustrated interactions. A well-developed long-range magnetic order is not observed. Our results indicate a ground state in Bi0.75Ca0.25MnO3 distinct to Bi0.75Sr0.25MnO3. [ABSTRACT FROM AUTHOR]

We present a study on the magnetic and electronic properties of Bi0.75Ca0.25MnO3, investigated by means of magnetometry, diffraction, and muon spin relaxation techniques. The anisotropic changes of cell parameters across the structural transition in Bi0.75Ca0.25MnO3 (275 K) differ markedly from the lattice anomalies not only in Bi0.75Sr0.25MnO3 (600 K) but also in Bi0.50Ca0.50MnO3 (325 K). We have confirmed the absence of a ferromagnetic component under zero field in Bi0.75Ca0.25MnO3. The application of magnetic fields produces a continuous progressive polarization of Mn moments. The ground state of Bi0.75Ca0.25MnO3 is characterized by a high degree of spin disorder and frustrated interactions. A well-developed long-range magnetic order is not observed. Our results indicate a ground state in Bi0.75Ca0.25MnO3 distinct to Bi0.75Sr0.25MnO3. [ABSTRACT FROM AUTHOR]

We study the effect of the substitution of Mn by d0 nonmagnetic ions in Pr0.50Ca0.50Mn1-xTixO3 (x=0.01, 0.03, and 0.05) manganites. Synchrotron and neutron powder diffractions have allowed us to characterize the dependence of the coexisting phases, the evolution of their relative fractions, and magnetic structures with the Ti substitution level. In consistence with the Ti4+ valence state (d0), the proportion of microdomains exhibiting pseudo-CE-type magnetic order increases with the Ti content at expenses of the CE-type regions in the material. The magnetic disorder greatly increases with Ti contents. The absence of ferromagnetism is confirmed in zero field. The apparition of magnetization steps has been further characterized by neutron powder diffraction under magnetic field. [ABSTRACT FROM AUTHOR]

The magnetization avalanches under field have been investigated at low temperature in Pr0.50Ca0.50Mn1-xNixO3 (x=0.05). In zero field, a 5% of Ni at the Mn site provokes the coexistence of three types of magnetic/structural mesoscopic domains at low temperature: ferromagnetic metallic (majority phase), pseudo-CE and CE-type (minority phase). The ultrasharp steps observed in the low-temperature isothermal magnetization and resistivity curves were investigated by neutron powder diffraction in magnetic fields up to 5 T. At 3 T an abrupt growth of ferromagnetic microclusters occurs at the expenses of the two types of antiferromagnetic orbital-ordered domains (CE and pseudo-CE regions). [ABSTRACT FROM AUTHOR]

This work presents an investigation of the central role of oxygen on the physical properties of highly ordered layered cobaltites. It includes the study of the structural and magnetic properties of PrBaCo2O5+δ compounds with high oxygen content (0.5⩽δ⩽0.90), with special emphasis on PrBaCo2O5.75. For δ=0.75 the ordering of oxygen vacancies gives rise to a tetragonal sP4/mmmd structure with a≈c≈2ap. On cooling, a ferromagnetic spin alignment precedes the apparition of antiferromagnetic order scanted G-typed. The compounds are ferromagnetic for δ⩾0.80. Measurements on PrBaCo2O5.75 support that, above the magnetic ordering temperature, Co3+ ions in octahedral environment are in HS state. [ABSTRACT FROM AUTHOR]

The magnetic properties of low-doped Bi1-xSrxMnO3 manganites (x<0.50) are investigated. Although a progressive dilution by Sr doping of the ferromagnetic ground state of pure BiMnO3 was previously proposed, it is found that compounds with x=0.25, 0.33, and 0.5 present an antiferromagnetic ground state. However, it is confirmed that a strong ferromagnetic component is easily induced under field in the compounds with x=0.25 and 0.33, both presenting the same type of charge and polaron ordering. [ABSTRACT FROM AUTHOR]

The magnetization avalanches under field have been investigated at low temperature in Pr0.50Ca0.50Mn1-xNixO3 (x=0.05). In zero field, a 5% of Ni at the Mn site provokes the coexistence of three types of magnetic/structural mesoscopic domains at low temperature: ferromagnetic metallic (majority phase), pseudo-CE and CE-type (minority phase). The ultrasharp steps observed in the low-temperature isothermal magnetization and resistivity curves were investigated by neutron powder diffraction in magnetic fields up to 5 T. At 3 T an abrupt growth of ferromagnetic microclusters occurs at the expenses of the two types of antiferromagnetic orbital-ordered domains (CE and pseudo-CE regions). [ABSTRACT FROM AUTHOR]

With the aim of determining and discriminating the influence of both steric and band-filling effects on the strength of the magnetic interactions of double perovskites, we have studied the structural and magnetic properties of substituted LxSr2-xFeMoO6 (L=Ca, La, and Nd) compounds. Our results show that the raising of Curie temperature in electron-doped samples (La and Nd substitutions) is mainly due to band-filling effects, being the influence of structural distortions (steric effects) is of less relevance. This fact contrasts with the well-known properties of manganites, where a strong correlation between structural properties and magnetic interactions is present. We argue that this disagreement relies on the different symmetries of the electronic orbitals involved in the magnetic interactions in both cases. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The relaxation of the initial permeability has been measured in polycrystalline Sr hexaferrites with the initial composition of W phase (SrO·9Fe[SUB2]O[SUB3]). The samples have been prepared by means of standard ceramic techniques at different temperatures in the 1250 °C

The effects of magnetic interactions on the hard magnetic properties of SmCo[SUB5] ball milled with NiO [antiferromagnetic (AFM) at room temperature] or with CoO [paramagnetic (PM) at room temperature] have been studied. The FM-AFM system exhibits improved magnetic properties (coercivity and squareness) for all compositions. The effects of magnetic interactions on the magnetic properties are analyzed in terms of classical DM plots. The plots show that in both systems magnetizing-like FM-FM exchange interactions are predominant for fields μ[SUB0]H < μ[SUB0]H[SUBC], while long-range dipolar interactions prevail for μ[SUB0]H > μ[SUB0]H[SUBC]. The different types of magnetic interactions are found to depend on the degree of SmCo[SUB5] dispersion in the AFM (NiO) or PM (CoO) matrices. Moreover, the role of the AFM appears to be to enhance both the dipolar and exchange-like interactions, although the exchange effects appear to be responsible for the improvement of the magnetic properties. [ABSTRACT FROM AUTHOR]

Compares disaccommodation and ferromagnetic resonance measurements in polycrystalline yttrium-iron-garnet samples. Details on the experiment; Results of the study; Conclusions.

Focuses on a study which evaluated the frequency dependence of the alternating current susceptibility in polycrystalline samples of YBaCuO superconductor. Experimental details; Results and discussion; Conclusions.

Investigates the time effects associated with domain-wall stabilization processes for yttrium iron garnet (YIG) polycrystalline samples sintered in different atmospheres. Effect of oxygen partial pressure on relaxation in garnet structures; Isochronal relaxation spectra obtained for YIG samples sintered under several partial oxygen pressures; Kinds of cations sites in the garnet structure.

We study the effect of the substitution of Mn by d0 nonmagnetic ions in Pr0.50Ca0.50Mn1-xTixO3 (x=0.01, 0.03, and 0.05) manganites. Synchrotron and neutron powder diffractions have allowed us to characterize the dependence of the coexisting phases, the evolution of their relative fractions, and magnetic structures with the Ti substitution level. In consistence with the Ti4+ valence state (d0), the proportion of microdomains exhibiting pseudo-CE-type magnetic order increases with the Ti content at expenses of the CE-type regions in the material. The magnetic disorder greatly increases with Ti contents. The absence of ferromagnetism is confirmed in zero field. The apparition of magnetization steps has been further characterized by neutron powder diffraction under magnetic field. [ABSTRACT FROM AUTHOR]