P.G. Bercoff

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.

Relative paleointensity of the geomagnetic field over the last 21,000 years BP from sediment cores, Lake El Trébol (Patagonia, Argentina)

Four cores from Lake El Trébol (Patagonia, Argentina) have been used to estimate regional geomagnetic paleointensity. The rock magnetic studies indicate that the magnetic mineralogy of the clay-rich sediments is dominated by pseudo-single domain magnetite in a range of grain sizes and concentration which are suitable for paleointensity studies. The remanent magnetisation at 20 mT (NRM20 mT) has been normalised using the an-hysteric remanent magnetisation at 20 mT (ARM20 mT), the saturation of the isothermal remanent magnetisation at 20 mT (SIRM20 mT) and the low field magnetic susceptibility (k). Coherence function analysis indicates that the normalised records are free of environmental influences. Our paleointensity (NRM20 mT/ARM20 mT) versus age curve shows good agreement with published record from another lake in Argentina and with records from other parts of the world, suggesting that, in suitable sediments, paleointensity of the geomagnetic field can give a globally coherent, dominantly dipolar signal.

Mössbauer analysis of Nd-Co M-type strontium hexaferrite powders with different iron content

The structural analysis of strontium hexaferrites SrFexO19 (for x = 12, 11 and 10) and substituted samples Sr0.7Nd0.3Fe12 − yCo0.3O19 (for y = 0.3,1.3 and 2.3) prepared through the citrate precursor method is shown. Nd and Co substitution modifies saturation magnetization (MS) and increases coercivity (Hc) in samples heat-treated at 1100°C for two hours. Mössbauer analyses show different iron occupancy and the influence of the Fe3 +  content is particularly emphasized. Hematite segregation is observed for some compositions. Samples with low Fe3 +  content show the best magnetic properties with no secondary phase segregation.

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%.

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...

Magnetization mechanisms in ordered arrays of polycrystalline Fe100−xCox nanowires

Magnetization reversal processes and coercivity mechanisms in polycrystalline Fe100−xCox nanowire arrays, resulting from an AC electrodeposition process, are investigated. The array coercivity is described on the basis of polarization reversal mechanisms operating in individual wires, under the effect of inter-wire dipolar interactions described by a mean field approximation. For individual wires, a reversal mechanism involving the nucleation and further expansion of domain-wall like spin configuration is considered. The wires have a mean grain size larger than both the nanowire diameter and the exchange length, so localized and non-cooperative nucleation modes are considered. As the Co content increases, the alloy saturation polarization gradually decreases, but the coercive field and the relative remanence of the arrays increase, indicating that they are not controlled by the shape anisotropy in all the composition range. The coercive field dependence on the angle between the applied field and the wire ...