C. de Julián

Coercivity of Fe‐SiO2 nanocomposite materials prepared by ball milling

Samples containing Fe nanoparticles dispersed in SiO2 were prepared by high energy ball milling. We have studied the variation with the milling time of both the saturation magnetization and the coercive force of these samples. The rapid increase of coercivity with the decrease of temperature observed in the low temperature range suggested the presence of superparamagnetic particles in the samples. Nevertheless, our experimental data were in poor agreement with the well‐known T1/2 law describing the coercive force of superparamagnetic particles. Coercivities as high as 540 and 850 Oe were obtained for samples with x=0.3 at room temperature and at 1.7 K, respectively. From the analysis of the temperature dependence of the saturation magnetization the spin wave stiffness constant was obtained. This quantity evidenced the enhancement of the thermal demagnetization associated with the reduction in size.

THE EFFECT OF THE DISTRIBUTION OF VACANCIES ON THE MAGNETIC PROPERTIES OF GAMMA -FE2O3 PARTICLES

Samples of γ-Fe 2 O 3 have been prepared from uniform α-Fe 2 O 3 particles obtained by homogeneous hydrolysis in solution and directly by spray pyrolysis methods. The crystalline structure of these samples was analyzed by using x-ray and electron diffraction. It was concluded that, depending on the preparation method and parameters, different degrees of order in the distribution of vacancies were present. The study of the magnetic properties of these samples included the measurement of both intrinsic (saturation magnetization) and extrinsic properties (coercive force and magnetization behavior in the approach to saturation region). It was found that all the above-mentioned quantities depended on the degree of order in the distribution of vacancies.

Mössbauer analysis of the phase distribution present in nanoparticulate Fe/SiO2 samples

Results are presented on the phase distribution and magnetic properties of nanoparticulate Fe/SiO2 samples prepared by high-energy ball milling. From our data, we suggest that the elevated coercivities measured in the samples are linked to the coupling of the core of the particles (α-Fe) with a highly disordered phase present in their surface region and whose behavior exhibits some similarities with a spin-glass phase.

Interactions and magnetic viscosity: Nonmonotonic time variation of the magnetization during relaxation at constant demagnetizing field

We present magnetic relaxation data of PrNdFeB magnets. These data were obtained by measuring the time evolution of the magnetization under constant applied demagnetizing fields. The results corresponding to fields far from the range of the coercive force evidenced a nonmonotonic time variation of the magnetization. We propose that the occurrence of magnetic interactions underlies the observed anomalous behavior. This idea is checked through a micromagnetic simulation of the time evolution of the magnetization of a low field reversed nucleus which is exchange and magnetostically coupled to the main hard phase.

Grazing-incidence small-angle X-ray scattering and X-ray diffraction from magnetic clusters obtained by Co + Ni sequential ion implantation in silica

We present a structural investigation on Co-Ni alloy nanoclusters obtained by sequential ion implantation in silica slides. The study is based on small angle X-ray scattering for obtaining the cluster size distribution and volume fraction and on X-ray diffraction for nanocrystalline structures, both performed in grazing incidence mode to enhance the cluster signal. A systematic comparison with cluster size distribution obtained from transmission electron microscopy points out the potential of the technique for investigating these composite glasses. The nanocrystal structure turns out to depend on the relative Co/Ni dose: the ccp structure is preferred at low Ni content, while the hcp structure predominates when increasing the Co content.

MAGNETIC HARDENING OF MELT-SPUN 2:14:1-BASED MATERIALS BY HIGH HEATING RATE AND SHORT TIME CRYSTALLIZATION TREATMENTS

Systematic studies have been carried out about the effects of the thermal treatment parameters on melt-spun materials quenched at different cooling rates and based on the 2:14:1 hard magnetic phase. Samples of nominal compositions Dy 3 Nd 10.2 Fe 79.6 B 6 Si 1.2 and Pr 3 Nd 10.2 Fe 79.6 B 6 Si 1.2 were annealed at temperatures above that of crystallization of the amorphous phases present upon quenching, for times ranging from 1 to 30 min and by using different heating rates up to the annealing temperature. It is concluded that best hysteretic properties can be achieved in samples quenched at intermediate cooling rates by means of short-time thermal treatments performed by using high heating rates up to the treatment temperature. Low heating rates and long time anneals led to the deterioration of the hard magnetic behavior, due to the segregation of soft crystalline phases.

Magnetic viscosity of uniform /spl gamma/-Fe/sub 2/O/sub 3/ particles with different degrees of cationic ordering

Results are presented about the time dependence of the magnetization in /spl gamma/-Fe/sub 2/O/sub 3/ particles obtained from /spl alpha/-Fe/sub 2/O/sub 3/ through a reduction-oxidation process. The precursor particles were prepared by homogeneous hydrolysis in solution. This preparation method led to a narrow distribution of particle dimensions which allowed us to consider the particles as being monodisperse. The viscosity study included the evaluation of the activation volume of pressed powder samples formed by spindle-like /spl gamma/-Fe/sub 2/O/sub 3/ particles (aspect ratio 6) with different degree of order in their cationic distribution, at temperatures from 100 to 480 K. >

Experimental Comparison and Analysis of Coercivity Models

Results corresponding to the temperature dependence of the coercive force of Ba hexaferrite samples are discussed in the framework of two different coercivity models based, respectively, on considerations about the coupling between the magnetization and the microstructure (micromagnetic model) and on the conditions of propagation of a reversed magnetization nucleus (global model). We conclude that, in the case of our samples, upon modifying the global model to account for the temperature evolution of the shape of the reversed nucleus, equivalent information about the local anisotropy at the site from which magnetization reversal proceeds can be obtained from both models.

On the relationships between the temperature dependence of the magnetization and the average grain size in nanostructured samples

We report on the low temperature thermal demagnetization behavior observed in samples of the Vitrovac 7600TM commercial amorphous alloy which were submitted to different crystallization treatments, all of them resulting in the induction of nanostructures. Our experimental data were analyzed in terms of the predictions of different models of ferromagnetism allowing us to conclude that our samples behaved as weak ferromagnets with nanostructure dependent spin wave spectra and a significant contribution to the demagnetization processes from single particle excitations.

Phase segregation and interactions in Dy-substituted melt spun Nd-Fe-B alloys

We have analyzed in this work the effects of the partial and total substitution of Nd by Dy in melt spun Nd/sub 15/Fe/sub 76/B/sub 9/ alloys prepared by crystallization of amorphous precursors. Depending on the crystallization parameters, and particularly on the heating rate employed in order to reach the annealing temperature, samples with either one or two reversal steps in the demagnetization curve may be produced. The temperature of spin reorientation (SRT), measured by two different methods, was shown to be independent of the composition, close to 110 K, for samples with a Dy content between 20% and 80% of the total rare earth. This led us to suggest that two different hard phases with low Dy and Nd content, respectively, are present in the samples. The existence of either one or two reversal steps, depending on the heating rate employed, strongly supports the existence of exchange-driven collective demagnetization processes.