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First order reversal curves (FORCs) distributions are a powerful tool for investigating hysteresis and interactions in magnetic systems and have been widely applied. La–Co substitution in barium hexaferrites has also been extensively studied. The most effective substitution to improve the magnetic properties (coercive field and energy product) is given by x = y = 0.2 in the formula Ba 1 - x La x Fe 12 - y Co y O 19 . In this work, this stoichiometry is initially used to obtain a state where more than one phase is present. The magnetic behavior as a function of temperature was studied in order to have an insight into the magnetic interactions that originate a decrease in the magnetic performance of Ba hexaferrite magnets. The sample was structurally characterized by X-ray diffraction (XRD) and magnetically studied in a SQUID magnetometer. FORC distributions were used to study the dependence of the magnetic interactions with the temperature. FORC diagrams performed on the sample at different temperatures exhibit similar characteristics, such as the spread in the h c – h u plane and a spread out of the h c -axes. These features are interpreted in terms of exchange-interacting particles and dipolar interactions, respectively. As the temperature decreases, stronger interactions are noticed among hard and soft phases.

Structural and magnetic properties of melt-spun Nd4.5Fe72Co3Cr2Al1B17.5 ribbons were studied by means of differential scanning calorimetry, Mossbauer effect spectroscopy, X-ray diffraction and first-order reversal curve distributions. The presence of a solid solution (Fe, Co) in ribbons annealed at 6851C for 10 min was detected from Mossbauer spectra. Correlations between the observed structural changes at higher annealing temperatures and modifications in the interaction fields of precipitated phases are discussed. r 2007 Elsevier B.V. All rights reserved.

Fine particles of hematite (mean size 55 nm) were produced by ball milling a mixture of hematite and pure Fe and annealing at 1000 °C. X-ray diffraction (XRD) and Mossbauer spectroscopy show that only α-Fe 2 O 3 is present in the final product, with lattice and Mossbauer parameters that correspond to bulk hematite. ZFC and FC magnetization measurements were performed from 5 to 300 K, at different applied fields. Two magnetic regimes were observed: one at low temperatures (⩽100 K) that we ascribe to the magnetic moments in the outer shell of the particles that couple to the magnetic moments in the core, and another at higher temperature that corresponds to the Morin transition, finding that the Morin temperature is T M =246 K. The memory effect is clearly observed in magnetic measurements that start from different remanence states and explained as dependent on the ordering of the magnetic moments within the particles.

A medium pore zeolitic material with MFI structure modified with Co cations was synthesized. The magnetization M and magnetic susceptibility χ were measured as a function of T after ZFC and as a function of the applied magnetic field for T between 4.2 and 30 K. No blocking temperature was detected and a pure superparamagnetic behavior was obtained. M vs. T and χ vs. T were adjusted by Brillouin and Langevin functions obtaining the same results in both cases. Curve fitting gave an effective moment of ∼ 90 μ B for the cases under study. The results are analyzed in terms of the possible cluster structure that may appear.

The magnetization reversal of partially exchange-coupled particles is studied in detail. The starting point is the observation of a complicated phenomenology in the irreversible susceptibility and FORC distribution functions of Ba hexaferrite samples obtained by means of different sintering conditions. Several peaks in the first-order reversal curve (FORC) distribution functions were identified and associated with clusters with different number of particles. The switching fields of these clusters were related to an effective anisotropy constant K eff that depends on the number of particles in the cluster. K eff is linked to the exchange-coupled volume between two neighboring particles and as a weighted mean between the anisotropy constants of the coupled and uncoupled volumes. By using the modified Brown's equation α ex = 0.322 is obtained. In order to interpret these results, the switching field of a two-particle system with partial exchange coupling is studied. It is assumed that the spins reorientation across the contact plane between the particles is like a Bloch wall. The energy of the system is written in terms of the fraction of volume affected by exchange coupling and the switching fields for both particles are calculated. At small interaction volume fraction each particle inverts its magnetization independently from the other. As the fraction of exchange-coupled volume increases, cooperative effects appear and the two particles invert their magnetization in a cooperative way. The proposed model allows to interpret for the first time the empirical factor α ex in terms of physical arguments and also explain the details observed in the FORC distribution function.

A study of the experimental behavior of the reversible (Mrev) and the irreversible (Mirr) DCD magnetization components in enhanced-remanence SmCo films was carried out. The evolution of the DCD Mrev and Mirr components were determined at room temperature by measuring sets of first-order reversal curves from different points on the initial magnetization curve and the demagnetization curve. From these data, Mrev(Mirr)Hi curves were built, Hi being the internal field of the sample. Recent studies show that the behavior of the Mrev(Mirr) curves at a constant internal field can be used as an indicator of the mechanism of reversal magnetization. The non-monotonic behavior showed by these curves at low fields in our material, seems to be associated with an incoherent mechanism of magnetization reversal in the nanograins of SmCo.

The relaxation of the initial permeability has been measured in polycrystalline Sr hexaferrites with the initial composition of X-phase (2SrO·15Fe 2 O 3 ) in the temperature range between 80 and 500 K. The time decay of the initial permeability after sample demagnetization is represented by means of isochronal disaccommodation curves which show the presence of different disaccommodation processes whose maxima lie at 380, 300 and 160 K (resp. A, B and D peaks). This behaviour is explained regarding the spectra corresponding to barium ferrites in order to ascribe the different relaxation processes found for ionic transitions in the cationic sites within the hexagonal structure.

Magnetic aftereffect phenomena in yttrium iron garnets and their relationship to the dynamics of domain walls are analyzed by means of the magnetic disaccommodation (DA) technique. The samples have been prepared using a standard ceramic technique in CO 2 atmosphere in order to enhance the formation of ferrous ions in the lattice. The results reveal the presence of the well-known process II around 130 K and a process at higher temperatures (187 K). Furthermore, accommodation (or negative DA) processes have been detected at temperatures just below process II. The features of these phenomena are strongly related to the measurement frequency and, hence, they are more noticeable as this frequency is increased. Such behavior is associated with the resonance of the domain wall motion at frequencies around the measurement frequency. The characteristic parameters that define this resonance depends upon time due to the presence of an induced anisotropy relaxation process that occurs after demagnetization.

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.