F. Cebollada

Interactions and hysteresis behaviour of Fe/SiO2 nanocomposites

Abstract We present a study of interactions in mechanically alloyed Fe/SiO2 nanocomposites. The ΔM plots and the switching field distributions reveal that increasing Fe concentrations lead to higher dipolar fields and larger dipolarly coupled regions. We propose a switching mechanism based on avalanche phenomena triggered by the reversal of individual particles, irrespective of the Fe concentration being above or below percolation.

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.

Modeling the influence of intergranular phases on the hysteretic behavior of hard magnetic polycrystals

We have modeled, in the framework of a micromagnetic approximation, the field evolution of the configuration of magnetic moments induced by the occurrence of interfacial exchange coupling at grain boundary regions. The models describe hard ferromagneticpolycrystals where secondary intergranular phases are present. Depending on the paramagnetic or soft ferromagnetic nature of the secondary phases, on the exchange to anisotropy ratio of the main phase and on the intensity of the exchange coupling at the interfaces the reversal, from a remanence state, of the magnetization of a linear chain of magnetic moments perpendicular to the interface takes place for demagnetizing fields which vary from the anisotropy field of the hard phase (grains decoupled by a thick paramagnetic layer) down to tenths of that field (coupling through a soft layer). The dependence of these phenomena on the thickness of the intergranular phases has been also examined.

Magnetic viscosity and coercivity analysis in mechanically alloyed and melt-spun NdDyFeB magnets

Abstract In this work we study the temperature dependence of coercivity and magnetic viscosity in mechanically alloyed magnets of composition (Nd1−xDyx)16Fe76B8 (0.1

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.

Coercivity of crystallized melt-spun Nd15−xDyxFe76B9 (x = 3, 6, 9, 12, 15)

Abstract The effect on hysteretic properties of crystallization treatments in melt-spun materials of composition Nd 15− x Dy x Fe 76 B 9 ( x = 3, 6, 9, 12, 15) has been studied. We have determined by means of differential scanning calorimetry the crystallization temperature of the alloys and studied the effect of diverse crystallization treatments on their first magnetization and demagnetization curves.

MAGNETIC INTERACTIONS IN FE-BA HEXAFERRITE NANOCOMPOSITE MATERIALS

Results are presented on the hysteretic behavior of Fe-rich, composite Ba hexaferrite–Fe samples prepared by ball milling. The most remarkable feature of these samples was the observation of loops which were displaced in the negative sense of the field axis. Similar to this, the field evolution of the isothermal and the demagnetization remanences evidenced the achievement upon the application of 8 kOe fields of nonequivalent values of both quantities. Our results are discussed considering the different magnetic hardness of the two phases forming the sample and the occurrence of interphase (dipolar) interactions.

Magnetization reversal models and the temperature dependence of the coercive force in melt spun PrNdFeB magnets

In this work we have studied the temperature dependence of several hysteresis parameters measured in PrNdFeB samples that were obtained from melt spun amorphous precursors crystallized by employing different heating rates. Analysis of the spin reorientation temperature by means of thermal evolution of the remanence and of the demagnetization differential susceptibility revealed the presence in all samples of at least two hard 2:14:1 phases (Nd rich and Pr rich, respectively). The demagnetization curves of most samples exhibited a single magnetization reversal process thus indicating that the hard phases were coupled. Despite the multiphase character of the samples, it was possible to carry out the analysis of the temperature dependence reversal process through micromagnetic and global models, and by accepting a linear variation of the intrinsic properties with the degree of substitution of Nd by Pr. Best fits for both models were obtained when using parameters corresponding to Nd-rich 2:14:1 phases, which...

Study of the surface roughness of Co-based amorphous alloys by STM

Abstract Since the STM gives the vertical dimension of surfaces, it allows us a quantitative evaluation of the surface roughness by analyzing the STM images. This kind of evaluation is of importance in Co-based alloys because the surface roughness is related to magnetic properties in low and medium magnetostriction. This paper presents a study of the surface roughness of Co-based alloys obtaining the fractal dimensionality, the amplitude of the superficial features as well as the mean size of those features and its relation to magnetic properties.

Correlation between high field magnetization measurements and STM imaging of the atomic structure in amorphous Co100-xPx

Abstract Atomic resolution images of the surface of amorphous films of composition Co 100- x P x (10 ≤ x ≤ 20) have been obtained by means of a scanning tunneling microscope. Evaluation of close neighbour distances was carried out and local five-fold symmetry sites and structures resembling dislocations were observed.