Magnetic hysteresis mechanism in Lu0.9Sr0.1Cr0.5Fe0.5O3studied by Monte Carlo simulations.

In this work we report magnetic properties of the orthorhombic perovskite Lu0.9Sr0.1Cr0.5Fe0.5O3synthesized by a wet chemical method. As in LuCr0.5Fe0.5O3the compound with Sr shows the magnetization reversal phenomenon, but the magnetic order and the compensation temperature occur at higher temperatures. Interestingly, in M vs H curves a hysteresis loop is observed when Cr4+and Cr3+ions coexist as a consequence of the aliovalent substitution of Lu3+by Sr2+in the B sites of the perovskite. To explain this behavior, we performed numerical simulations with a magnetic model for Lu1-xSrxCr0.5Fe0.5O3perovskites withx= 0 andx= 0.1. We found that the ferromagnetic coupling of Fe3+and Cr4+through superexchange interactions (according the empiric Goodenough-Kanamori-Anderson rules) increases the magnetization at high fields and that the presence of ferromagnetic clusters explains the hysteretic behavior found in simulations.

Aging effect on vanadium oxide hybrid nanotubes.

Vanadium oxides present a rich magnetic phase diagram depending on the oxidation state of the V ions. In particular the vanadium oxide nanotubes (VO x NTs) present several promising perspectives for different technological applications for which it is essential to know the oxidation state of V ions, as well as to evaluate the stability with the aging time of the tubes. In this work we present a systematic study of the time evolution of the magnetic properties of VO x NTs. For this complete characterization, we used electron spin resonance (ESR) and dc-susceptibility techniques, which were supplemented with TEM microscopy and XANES. We observed that for aging in normal environmental conditions of pressure, temperature and humidity, the V4+ ions oxidize to V5+ . Although the multiwall tubular structure is maintained, this oxidation process produces a marked change in the magnetic properties. We conclude that the aging of the samples affects the V4+ /V5+ relationship in the VO x NTs, which may contribute to explain the significant dispersion of data reported in the bibliography.

Ternary erbium chromium sulfides: structural relationships and magnetic properties.

Single crystals of four erbium-chromium sulfides have been grown by chemical vapor transport using iodine as the transporting agent. Single-crystal X-ray diffraction reveals that in Er(3)CrS(6) octahedral sites are occupied exclusively by Cr(3+) cations, leading to one-dimensional CrS(4)(5-) chains of edge-sharing octahedra, while in Er(2)CrS(4), Er(3+), and Cr(2+) cations occupy the available octahedral sites in an ordered manner. By contrast, in Er(6)Cr(2)S(11) and Er(4)CrS(7), Er(3+) and Cr(2+) ions are disordered over the octahedral sites. In Er(2)CrS(4), Er(6)Cr(2)S(11), and Er(4)CrS(7), the network of octahedra generates an anionic framework constructed from M(2)S(5) slabs of varying thickness, linked by one-dimensional octahedral chains. This suggests that these three phases belong to a series in which the anionic framework may be described by the general formula [M(2n+1)S(4n+3)](x-), with charge balancing provided by Er(3+) cations located in sites of high-coordination number within one-dimensional channels defined by the framework. Er(4)CrS(7), Er(6)Cr(2)S(11), and Er(2)CrS(4) may thus be considered as the n = 1, 2, and infinity members of this series. While Er(4)CrS(7) is paramagnetic, successive magnetic transitions associated with ordering of the chromium and erbium sub-lattices are observed on cooling Er(3)CrS(6) (T(C)(Cr) = 30 K; T(C)(Er) = 11 K) and Er(2)CrS(4) (T(N)(Cr) = 42 K, T(N)(Er) = 10 K) whereas Er(6)Cr(2)S(11) exhibits ordering of the chromium sub-lattice only (T(N) = 11.4 K).

Preparation and characterization of polyaniline-containing Na-AlMCM-41 as composite material with semiconductor behavior.

Composite material formed from a mesoporous aluminosilicate, Na-AlMCM-41, with conducting polyaniline (PANI) has been synthesized by an in situ polymerization technique. Studies of aniline adsorption over mesoporous Na-AlMCM-41 synthesized in our laboratory allowed us to find the modes in which aniline interacts with the active sites of Na-AlMCM-41. In order to obtain the best reaction conditions to polymerize aniline onto Na-AlMCM-41, aniline was first polymerized to produce pure PANI. Hence, the oxidative in situ polymerization was carried out by two procedures, differing in the polymerization time and in static or stirring conditions. Studies of infrared spectroscopy and UV-vis spectroscopy indicated that higher polymerization time and static conditions allowed us to obtain mainly polyaniline in emeraldine form on the host. The N(2) isotherm of the polyaniline/Na-AlMCM-41 composite (PANI/MCM) indicated that the shape was similar to that of MCM, but the shift to saturation transition to lower partial pressure shows that the channels are occupied by PANI and they are now narrowed. The thermal properties of PANI, Na-AlMCM-41, and composite were investigated by TGA analyses and we found that the polymer shows higher thermal stability when it is forming the composite. Scanning electron microscopy indicated that PANI is not on the outer surface of the host. Conductivity studies show that PANI/Na-AlMCM-41 exhibits semiconductor behavior at room temperature and its conductivity was 7.0 x 10(-5) S/cm, smaller than that of pure polyaniline. PANI/Na-AlMCM-41 conductivity shows an increase as temperature increases. Magnetic measurements at room temperature confirmed that the composite has paramagnetic behavior; at lower temperatures the composite became diamagnetic.