Negative magnetic relaxation in superconductors

It was observed that the trapped magnetic moment of HTS tablets or annuli increases in time (negative relaxation) if they are not completely magnetized by a pulsed magnetic field. It is shown, in the framework of the Bean critical-state model, that the radial temperature gradient appearing in tablets or annuli during a pulsed field magnetization can explain the negative magnetic relaxation in the superconductor

Pr 0.5 Ca 0.5 MnO 3 thin films deposited on LiNbO 3 substrates

Thin films of Pr 0.5 Ca 0.5 MnO 3 have been deposited on z-cut LiNbO 3 by pulsed laser ablation. The X-ray diffraction measurements showed that the films have grown highly oriented on LiNbO 3 , with a pseudocubic (111) preferred growth direction. The thicknesses of the films, measured by low angle X-ray reflectivity, are between 13 and 140 nm. Their electrical resistivity present a semiconducting-like behaviour with an anomaly around 240 K, that corresponds to the charge ordering transition. The temperature of the transition (T_CO) was estimated from ln(r) vs. (1/T) plots. The charge ordering temperature was found to be dependent on the strain induced by the lattice mismatch on the films.Fundação para a Ciência e a Tecnologia (FCT

Field-induced segregation of ferromagnetic nano-domains in Pr0.5_{0.5}Sr0.5_{0.5}MnO3_3, detected by 55^{55}Mn NMR

The antiferromagnetic manganite Pr$_{0.5}$Sr$_{0.5}$MnO$_3$ was investigated at low temperature by means of magnetometry and $^{55}$Mn NMR. A field-induced transition to a ferromagnetic state is detected by magnetization measurements at a threshold field of a few tesla. NMR shows that the ferromagnetic phase develops from zero field by the nucleation of microscopic ferromagnetic domains, consisting of an inhomogeneous mixture of tilted and fully aligned parts. At the threshold the NMR spectrum changes discontinuously into that of a homogeneous, fully aligned, ferromagnetic state, suggesting a percolative origin for the ferromagnetic transition.Comment: Latex 2.09 language. 4 pages, 3 figures, 23 references. Submitted to physical Review

Phase and frequency control in the vibrating wire magnetic susceptometer

The recently developed vibrating wire susceptometer (VWS) is an instrument inherently suitable to perform measurements with high sensitivity in a wide temperature range (up to 1200 K). In the present work we describe the principle of the electronic control of phase and frequency of the VWS that allows accurate determination of both the real and imaginary part of the susceptibility. In particular, the high quality factor, intrinsic to the mechanical system of the VWS, allows in principle to detect very small energy losses due to magnetic dissipative processes in the specimen

Comparison of different synthetic approaches for the fabrication of a bio-inspired 1D-coordination polymer: From solution chemistry to mechanochemistry

Here we compare the effectiveness of three different synthetic approaches that we have applied to get a new coordination polymer (1) deriving from trans-ferulic acid (H2FA) and CoCl2⋅6H2O. The solvothermal reaction between the two reagents conducted in DMF for 6 days leads to X-ray-quality single crystals of 1, whose structural analysis revealed to correspond to the 1D-polymer [Co2(FA)Cl2(DMF)3]n. The yield of the reaction ranged from 24% to 52%, depending on the experimental conditions. The 1D-chains contain ferulate dianions bridging two different Co2+ cations, one octahedral and one tetrahedral. The same reaction conducted under reflux using H2FA or its ammonium salt (NH4)2FA leads to 1 but only in trace amounts (yield ≤ 11%). Complete conversion of the reagents to 1 was instead observed by grinding (NH4)2FA and CoCl2⋅6H2O by means of a planetary ball mills, already after 1 h of reaction. 1 turned out to be reactive towards moisture, with formation of the inorganic crystalline compound [CoCl2(H2O)4]⋅2H2O and a not yet defined amorphous phase that we tentatively formulate as [CoFA(H2O)x]n. Finally, the thermal behavior and magnetic properties of 1 are also reported

A Vibrating Wire Susceptometer with a special electronic control for fast measurements at high temperatures

The measurement of the temperature behaviour of initial magnetic susceptibility is a powerful method for the thermomagnetic analysis of ferromagnetic materials. However, its application to nanostructured materials with technical relevance, particularly in the case of metastable systems, is made difficult by several conflicting conditions: the necessity to employ low magnetic fields, the required high sensitivity and the need for rapid scans in the high-temperature range. The vibrating wire susceptometer, an instrument belonging to the class of alternating gradient force magnetometers, has, in theory, the right characteristics to make such measurements. However, management of the instrument when carrying out rapid scans is intrinsically complex and requires a special electronic controller described here in detail. A combination of two phase-locked loop blocks is needed to provide the correct phase shift to ensure the locking of the resonance frequency while the instrument is working. A new measurement procedure that keeps the oscillation amplitude constant has also been implemented and it has proved to be very useful for rapid overview of the sample magnetic properties. The limitations of the controller performance due to the presence of noise are discussed. Extensive test measurements were carried out and analysed

Metodo di preparazione del composto magnetocalorico AlFe2B2

È descritto un metodo di preparazione del composto magnetocalorico AlFe2B2, del tipo che prevede una reazione di combustione di una miscela di reagenti comprendenti polveri elementali di ferro, alluminio e boro, o pre-alligate ferroalluminio, ferro-boro, detta combustione essendo innescata per mezzo di una esposizione di detta miscela a microonde e detto composto magnetocalorico AlFe2B2 essendo poi separato dai sottoprodotti di detta reazione di combustione, caratterizzato dal fatto che dette microonde, sono utilizzate prevalentemente in campo elettrico (E mode) o in campo magnetico (H mode), in modo tale che dette microonde, in campo prevalentemente elettrico, tendano a surriscaldare le polveri di alluminio e boro, mentre dette microonde, in campo prevalentemente magnetico, tendano a surriscaldare le polveri di ferro. È parte integrante della presente invenzione anche una procedura di separazione del composto magnetocalorico AlFe2B2 dai sottoprodotti di detta reazione di combustione

Effect of Vanadium on the RE and Fe Sublattice Anisotropies in some REFe12-xVx (RE= Y, Er, Tb) Tetragonal Compounds

The compounds REFe12−xVx(RE=Y,Er,Tb) having the tetragonal crystal structure of ThMn12 exist in the composition range 1.5≤x≤4. The dependence of the magnetic properties on the vanadium content were investigated in order to evidence and study any influence of vanadium on the magnetic anisotropy of both iron and RE sublattices and, as a consequence, on the magnetic phase diagram of these systems. The influence of vanadium was found to be remarkable on Tc, Ms, and Fe anisotropy, which decrease in all the compounds with increasing V content. However the largest effect was observed on the RE anisotropy and particularly in TbFeV, where the anisotropy of the Tb sublattice is turned from planar to axial by increasing the vanadium concentration, thus giving rise to strong effects on the macroscopic magnetic behavior and on the magnetic phase diagram of these systems. The large variation of the spin reorientation transition temperature Tsr, observed by varying V content on both ErFeV and TbFeV, can justify the spread in the literature data which could be attributed to small not considered stoichiometry differences with respect to the nominal composition