H.R. Bertorello

Exchange constants and transfer integrals of spinel ferrites

Abstract We have studied Fe3O4, CoFe2O4, NiFe2O4 and CuFe2O4 considering Neels two-sublattice collinear model and the superexchange theory. We have found that, for this theory to be valid, the interaction between ions in tetrahedral sites should be very weak, and we have proposed a model to find the constants JAB and JBB with the assumption that JAA = 0. Using this model, the calcuted values of these constants follow the expected tendencies for the four systems under study, as well as the ion-to-ion constants of Andersons theory which we obtained through the assessment of the transfer integrals.

Localized canting effect in Zn-substituted Ni ferrites

Abstract A model of three sublattices (A, B and B′) to describe the magnetic behavior of mixed Ni–Zn ferrites is presented. It is proposed that only sublattice B′ is affected by canting, due to the substitution of Zn2+ ions in sublattice A. The cases where: (i) only Fe3+ ions are found in B′, and (ii) B′ is occupied by Fe3+ and Ni2+ ions are considered. The two-sublattice model with canting in sublattice B is presented for comparison with cases (i) and (ii). The canting angle θL is calculated from energetic considerations, assuming that the Jij ion-to-ion interaction energies are not modified by the substitution of Zn2+ ions in sublattice A. The number of Bohr magnetons per molecule, nB, and the Curie temperature, Tc, are predicted as a function of Zn content, z. The experimental values of nB as well as the results for Tc are satisfactorily reproduced by the model described in (i). Results on Zn-substituted Li ferrite are also mentioned. It is found that additional considerations on the interactions are needed for this system, as lithium acts as a non-magnetic dilution in the octahedral sublattice.

Magnetic properties of Ba hexaferrite and Fe compounds produced by milling and annealing in air

The structure and magnetic properties of different magnetic compounds are studied. The compounds are produced by first milling 80% Ba hexaferrite and 20% Fe for 40 h (giving severely deformed hexaferrite plus Fe), and for 60 h (giving magnetite plus Ba carbonate). These two precursors are annealed at 850, 900 and 1000°C in air and characterized by their structure and magnetic properties. When the precursor consists of deformed hexaferrite and Fe, the thermal treatment promotes the elimination of defects in the hexaferrite and the oxidation of Fe to hematite and some magnetite. In the other case, that is, when the precursor is magnetite and Ba carbonate the annealing produces the re-transformation to the original hexaferrite and the excess of magnetite transforms to hematite. In this situation the annealing at 1000°C produces a compound with a coercivity of 4.4 kOe and a maximum magnetization at 15 kOe of over 51 emu/g, values which are comparable to the ones in the literature for this material considering that the fraction of hexaferrite in the samples is 80%.

Synthesis and characterization of a biospecific adsorbent containing bovine serum albumin as a ligand and its use for bilirubin retention

Epoxy-activated gels from cross-linked polybutadiene-hydroxyethylmethacrylate (PB-HEMA) copolymer and epichlorohydrin (ECH) were prepared and characterized. Albumin was covalently bonded to the matrix and used as support of affinity chromatography in bilirubin (BR) retention experiments. PB-HEMA-ECH with different amounts of immobilized albumin (between 5.20 and 6.80 mg/g dry gel) were obtained. Bilirubin retention of 3.10 mg/g of these beads was observed at 5 degrees C.

Model of interactions in nanometric particles of barium hexaferrite

Abstract Magnetic interactions in nanostructured Ba hexaferrite systems with partially coupled particles are studied. In this kind of systems, there are regions with lower effective magnetic anisotropy K eff where nucleation of magnetization reversal occurs. The Preisach distribution functions were measured for samples with different irreversible susceptibility behavior. Several peaks in the Preisach distributions were identified and were associated with different particle clusters. The coercive fields of these clusters were associated with K eff . A model was developed linking K eff with the exchange-coupled volume ( β ) between two neighboring particles and the number of particles in clusters. The behavior that followed the best physical interpretation led to conclude that magnetization reversal starts in the exchange-coupled volume β between two neighboring particles and then propagates to the rest of the particle.

Magnetic and structural study of mechanochemical reactions in the Al-Fe3O4 system

The solid state reaction between Al and Fe3O4 (magnetite) using mechanochemical activation of powder mixtures under Ar atmosphere is studied. The phase evolution during the reaction is analyzed by X-ray diffraction (XRD), vibrating sample magnetometry (VSM), differential thermal analysis (DTA) and scanning electron microscopy (SEM). At 37 minutes of high-energy ball-milling the disappearance of reactive phases and the production of α-Fe, FeAl2O4 and α-Al2O3 is observed, together with significant changes in the magnetic behavior of the system. The composition and properties of samples heated up to 1200°C are also investigated. The behavior of the saturation magnetization Ms is interpreted on the basis of the formation of a variable composition spinel phase Fe [Alx Fe2−x] O4 with 0 ≤ x ≤ 2 and a canting effect due to the presence of Al3+ ions in the spinel structure.

Magnetic interactions and Preisach distributions of nanostructured barium hexaferrite

Abstract Barium hexaferrite (phase M) samples with different nanostructures were studied. Sample M1 is composed of nanocrystals of BaFe12O19 produced after milling the elemental oxides (Fe2O3 and BaCO3) and heating in air atmosphere. Two more samples were produced by milling the same oxides and a 20% excess of α-Fe. The resulting powder (composed of phase M and a ∼20% hematite) was heat-treated in different conditions, resulting in samples MF1 (with a partially sintered structure) and MF8 (with almost completely sintered structure). In order to have an insight into the interactions in each sample, Preisach distributions were obtained using first-order reversion curves (FORCs) measurements. The Preisach distribution corresponding to M1 is a Gaussian-shaped one, with a maximum around 4.1 kOe . The distribution of MF1 has a narrow and high peak at 5.3 kOe , a number of overlapping small peaks down to 2.6 kOe and a distinct and low-intensity peak at 2 kOe . MF8 has a Preisach distribution with a succession of equally spaced distinct peaks from 4.2 to 1.5 kOe . The found Preisach distributions suggest that the interactions occur among nanocrystals inside conglomerates with different number of particles.

An approach to the magnetic disaccommodation in Ti-substituted Ba–W hexaferrites

The relaxation of the initial permeability has been measured in polycrystalline Ti-doped Ba–W hexaferrites with nominal composition BaTixFe18−xO27, with x ranging among x=0 and 0.3. The samples have been sintered at different temperatures in the 1250 °C<T<1350 °C range in a reducing CO2 atmosphere and rapidly quenched to promote the presence of crystal vacancies. X-ray diffraction spectra reveal the presence of hexaferrite as main phase. In the temperature range between 80 and 500 K, the time decay of the initial permeability after sample demagnetization has been plotted by means of isochronal curves. The isochronal disaccommodation spectra show an increase in the amplitude of the different disaccommodation peaks in substituted samples regarding pure Ba–W hexaferrites. These peaks are centered at 180, 300, and 380 K and they are related to the presence of both ferrous cations and lattice vacancies. The increase of the disaccommodation maxima in Ti-substituted hexaferrites is ascribed to the additional amo...

Surface modification of polyvinyl chloride with biodegradable monomers

Graft copolymerization of sucrose acrylate (SA) onto 55 μm films of poly(vinyl chloride) (PVC) was investigated with benzophenone or 2,2′-azoisobutironitrile as the initiator. The grafting reactions were carried out in a closed cell containing the films to be treated with a solution of the sensitizer and the monomer. In this system, ultraviolet light through a quartz window initiated the reaction by exciting the initiator used. The grafting took place on a PVC surface layer, from the solution. The homopolymer formed during copolymerization was removed by washing. The percentage of grafting was determined by gravimetric measurements and by dye adsorption (crystal violet), and the characterization of PVC(g)SA was carried out by the use of infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and thermogravimetric analysis. The weathering behavior of PVC(g)SA was studied with microorganisms in a growing medium, and the degradation was monitored by the weight loss of the films, FTIR, and SEM.

Magnetic viscosity in a nanocrystalline two phase composite with enhanced remanence

Abstract The intrinsic values of the magnetic viscosity and the fluctuation field in a two phase composite Nd 2 Fe 14 B+0.18 vol. αFe were investigated. The composite studied, with equiaxed and isotropically oriented grains had mean grain sizes favoring a strong exchange coupling between the phases. The relations between the intrinsic and the experimentally measured magnitudes are reviewed and the approximations discussed. For applied fields close to the coercivity, the time dependence of the magnetization is not well described by a simple logarithmic law. The intrinsic fluctuation field is found to increase when the reverse internal field increases in all the ranges investigated and two different regimes are identified. These different regimes are discussed considering the distributions of critical fields for nucleation of inverse domains in the hard phase, at hard–soft and hard–hard grain boundaries, respectively.