Luis Fernández Barquín

Self-propagating high-temperature synthesis of SrFe12O19 from reactions of strontium superoxide, iron metal and iron oxide powders

Abstracts are not published in this journal

Self-propagating high-temperature synthesis of barium-chromium ferrites BaFe12-xCrxO19 (0 <= x <= 6.0)

Pure and chromium substituted barium ferrites BaFe12-xCrxO19 (0 less than or equal to x less than or equal to 6.0) have been synthesised in air by self-propagating high-temperature synthesis (SHS): a combustion process involving a reaction of barium peroxide, iron oxide, chromium oxide and iron metal powder Two series of SMS samples were produced: series 1-zero field SHS, and series 2-SHS in a magnetic field of 1.1 T,both followed by sintering at 1200 degrees C for 2 h. X-ray data showed that hexagonal ferrites were produced, and systematic changes in lattice parameters were seen as a function of the Cr content. Scanning electron microscopy indicated crystallites of order 1 mu m. Energy dispersive x-ray analysis (EDAX) showed that the samples were homogeneous with the; expected Ba:Fe:Cr ratios. Mossbauer, x-ray and magnetic hysteresis data showed a progressive change in the sublattice occupancy and magnetization with Cr content. Some differences in magnetic: parameters were observed between series 1 and 2, implying that the use of a magnetic field during SHS can influence product microstructure. In particular the coercive forces in the Ct doped ferrites showed maxima at x = 1.5 and x = 1.0 for series 1 and 2 respectively, with the series 2 coercivities being consistently 40-50% smaller than their series 1 counterparts. This indicates that applied held SNS provides an alternative route to reducing coercivity in hexagonal ferrites in lieu of conventional approaches such as cationic doping.

The effect of large magnetic fields on solid state combustion reactions: novel microstructure, lattice contraction and reduced coercivity in barium hexaferrite

Combustion reactions of BaO2, Fe2O3 and Fe performed in large applied fields have revealed unexpected modifications in the microstructure of the multiphase combustion product, and in the lattice parameters and coercivity of monophase BaFe12O19 obtained by post-production grinding and sintering.

Convenient, low energy routes to hexagonal ferrites MFe12O19(M=Sr, Ba) from SHS reactions of iron, iron oxide and MO2 in air

Thermal initiation of a mix of metal superoxide (MO2 , M=Sr, Ba), iron and iron powder in air induces a self propagating reaction with velocity 0.5 mm s–1 and the formation of predominantly MFe12O19 . Heating the mixture to 1150 °C for 2 h produces pure crystalline MFe12O19 . The ferrites were characterised by X-ray powder diffraction (Rietveld analysis), FTIR, VSM magnetism, EDAX/SEM, electron probe analysis and Mossbauer spectroscopy. The ferrites showed good purity, coercivity, remanence and hysteresis loops compared to commercial samples. Preparation of ferrite solid solutions MxM′yFe12O19 (M=Sr, Ba, Pb) was investigated.

Structural and magnetoresistive properties of mechanically alloyed Fe-Co-Ag

The structural, magnetic and magnetoresistive properties of a wide range of mechanically alloyed Fe-Co-Ag granular materials were investigated by means of x-ray diffraction, Mossbauer spectroscopy, differential scanning calorimetry, DC magnetometry and magneto-resistance measurements. Binary Fe-Co alloys, milled using Syalon containers, showed a mixed fcc-bcc structure for Fe compositions between 20 and 25 at.%. The ternary alloys were found to comprise dispersions of Fe-Co grains in an fcc silver matrix, with the structure and magnetic properties of the Fe-Co grains mirroring those of the corresponding binary Fe-Co alloys. For materials with Fe:Co ratios in the region of the mixed fcc-bcc phases, additional components were noted in their Mossbauer spectra (a secondary sextet and a paramagnetic doublet, accounting for up to 63% of the spectral area), indicating the intermixing of a significant number of small Fe-Co clusters or fine grains into the Ag matrix. Alloys in this region were also found to have the largest magnetoresistive responses, with the maximum being a 5.8% effect in (Fe0.15Co0.85)30Ag70, as measured at 10 K in a field of 3 T. This indicates the dominant significance of fine grain intermixing in these alloys.

Unravelling the onset of the exchange bias effect in Ni(core)@NiO(shell) nanoparticles embedded in a mesoporous carbon matrix

Ni(core)@NiO(shell) nanoparticles (NPs) were synthesized through the pyrolysis of an inorganic precursor taking place within the pores of an active carbon matrix at different temperatures between 673 and 1173 K, and a subsequent oxidation in air. For the lowest temperature (673 K), the smallest average size of the NPs (9 nm) and the largest percentage of NiO (82%) are observed. Upon increasing the temperature up to 1173 K, an average diameter of 23 nm is observed while the NiO percentage decreases below 20%. We found that each NP consists of a Ni core surrounded by a structurally disordered NiO shell with a constant thickness of ∼2 nm, regardless of the core size. The spins inside the NiO shell freeze into a spin glass (SG)-like state below Tf ∼ 40 K. The magnetic exchange coupling between the Ni core and the NiO shell spins gives rise to the occurrence of the exchange bias (EB) effect, whose temperature dependence follows a universal exponential trend in all samples. The SG nature of the shell spins yields a vanishing EB above Tf. This is far below the Neel temperature of bulk antiferromagnetic NiO (TN ∼ 523 K) that usually determines the onset of the EB effect in Ni/NiO interfaces.

Magnetic, Structural, and Particle Size Analysis of Single- and Multi-Core Magnetic Nanoparticles

We have measured and analyzed three different commercial magnetic nanoparticle systems, both multi-core and single-core in nature, with the particle (core) size ranging from 20 to 100 nm. Complementary analysis methods and same characterization techniques were carried out in different labs and the results are compared with each other. The presented results primarily focus on determining the particle size-both the hydrodynamic size and the individual magnetic core size-as well as magnetic and structural properties. The used analysis methods include transmission electron microscopy, static and dynamic magnetization measurements, and Mössbauer spectroscopy. We show that particle (hydrodynamic and core) size parameters can be determined from different analysis techniques and the individual analysis results agree reasonably well. However, in order to compare size parameters precisely determined from different methods and models, it is crucial to establish standardized analysis methods and models to extract reliable parameters from the data.

1-Ethyl-2,3-dimethylimidazolium paramagnetic ionic liquids with 3D magnetic ordering in its solid state: synthesis, structure and magneto-structural correlations

Two novel paramagnetic ionic liquids, comprised of a 1-ethyl-2,3-dimethylimidazolium (Edimim) cation and a tetrahaloferrate(III) (FeX4) (X = Cl and Br) anion were synthetized and characterized by thermal, structural, Raman spectroscopy and magnetic studies. The crystal structures, determined by synchrotron X-ray powder diffraction and single crystal X-ray diffraction at 100 K for Edimim[FeCl4] and Edimim[FeBr4] respectively, are characterized by layers of cations (in non-planar configuration) and anions stacked upon one another in a three-dimensional (3D) manner with several non-covalent interactions: halide–halide, hydrogen bond and anion–π. Magnetization measurements show the presence of three-dimensional antiferromagnetic ordering below the Neel temperature (TN) with the existence of a noticeable magneto-crystalline anisotropy in the bromide compound. The corresponding magneto-structural correlations evidence that the 3D magnetic ordering mainly takes place via Fe–X⋯X–Fe (X = Cl and Br) interactions, displaying a higher superexchange magnetic interaction between the planes. Comparison with the Emim[FeX4] (X = Cl and Br) phases (Emim: 1-ethyl-3-methylimidazolium) reveals that the methylation at the C(2) position onto the imidazolium cation ring causes an increase of the melting point and a decrease of the TN. In contrast, the comparative study with Dimim[FeX4] (X = Cl and Br) compounds (Dimim: 1,3-dimethylimidazolium) shows a lower TN in the chloride compound, Edimim[FeCl4], whereas it is higher for the bromide, Edimim[FeBr4]. This fact is attributed to the spin delocalization of iron atoms in [FeBr4]− and discards the hypothesis that a bigger imidazolium ion size causes a weaker magnetic coupling in paramagnetic ionic liquids based on tetrahaloferrate anions and imidazolium cations with 3D magnetic ordering in its solid state.

Moment canting and structural anisotropy in amorphous alloys: experiments using synchrotron Mössbauer radiation

Abstract Moment canting, a phenomenon where the atomic moments in a soft magnet do not completely align with an applied field of substantial magnitude, has been measured in field annealed (FA) and stress relieved (SR) ribbons of the amorphous alloy Fe78Si9B13. Measurements were made using a synchrotron Mossbauer (SM) source, taking advantage of the fully polarised state of the source to simplify the analysis. Mean in-plane canting angles of order 12° were found for both sets of ribbons in an applied field of 1.1 T, in keeping with earlier results in smaller fields. The spectra were found to be dependent on the direction of the applied field in the ribbon plane, indicating structural anisotropy in all the samples, consistent with the presence of residual casting strains.

Preparation of Fe–Zr–B amorphous alloys by chemical reduction

Abstract Amorphous Fe–Zr–B alloys are of interest because of the intriguing magnetic properties they display, from paramagnetism to ferromagnetism to re-entrant spin glass phases, with, in some cases, all three states in one sample. The standard method of preparing such alloys is melt quenching. Presented in this paper is the first data on amorphous Fe–Zr–B alloys prepared by a chemical route: the reduction of an aqueous solution of iron sulphate and zirconium sulphate with an aqueous solution of sodium borohydride. Full characterisation data obtained by X-ray diffraction, Mossbauer spectroscopy, differential scanning calorimetry, FT–IR spectroscopy, EDAX and scanning electron microscopy, electron microprobe analysis and X-ray photoelectron spectroscopy are presented, and the amorphicity of the product is verified.