M. Zazo

Influence of sintering atmosphere on the magnetic after-effect in strontium ferrites

Copyright (c) 1996 Elsevier Science B.V. All rights reserved. The relaxation of the initial permeability after sample demagnetization has been measured in polycrystalline strontium ferrite samples with nominal composition SrO.6Fe 2 O 3 (M-phase), sintered under different oxygen pressures, and has been represented by isochronal curves. Very different behaviours are observed. With lower oxygen pressures the isochronal spectra are similar to the curves obtained for barium ferrites, with relaxation peaks at room temperature. In higher oxygen partial pressures, the relaxation peaks are found at lower temperatures.

Giant magnetoimpedance effect enhancement by circuit matching

GMI response of amorphous wires can be greatly increased by working at resonant conditions in a LC cell. This feature of the circuit can be exploited to increase the sensibility of current GMI-based sensors while selecting the working frequency.

Effect of slowly relaxing impurities on ferrimagnetic resonance linewidths of single crystal nickel ferrites

Ferrimagnetic resonance (FMR) linewidths have been measured at X-band frequencies (11 GHz) from 77 K to 400 K in single crystal nickel ferrites with composition Ni/sub x/Fe/sub 3-x/O/sub 4/ with x=0.9, 1.0, 1.2. Measurements have been carried out by means of an automatic technique based on a modified short-circuit SMA transmission line. Linewidths ranged from 40 Oe to 80 Oe in all the temperature range. The dependence of the FMR linewidth with temperature leads to a contribution of the valence-exchange mechanism for x 1. The collection of data obtained by the automatic system allows the authors to achieve the values for the relaxation times and activation energies of these mechanisms. >

Study of the conductivity of a metallic tube by analysing the damped fall of a magnet

The fall of a magnet through a hollow conducting tube is described. Although this experiment is well known, a detailed treatment by means of a circuit analysis allows us to relate the conductivity of the tube to the characteristic parameters of the experiment.

Effect of sintering atmosphere on magnetic aftereffect in polycrystalline yttrium iron garnet samples

Time effects associated with domain‐wall stabilization processes are investigated at temperatures ranging from 80 to 420 K, for yttrium iron garnet polycrystalline samples sintered in different atmospheres. A very different behavior for samples sintered in high and low oxygen partial pressures is found. Above 0.1 atm the isochronal relaxation spectra show a peak around 180 K that could be associated with an electron diffusion controlled domain‐wall relaxation. The activation energy for the process is close to 0.4 eV. Below this pressure, the spectra show both positive and negative peaks as a result of the competition between disaccommodation and accommodation processes. These peaks are closely temperature spaced in the neighborhood of 130 K, and the relaxation mechanism responsible for this behavior is not yet clear.

Magnetic levitation by induced eddy currents in non-magnetic conductors and conductivity measurements

We report a study on magnetic levitation by induced ac currents in non-magnetic conductors at low frequencies. Our discussion, based on Faradays induction law, allows us to distinguish the two components of the current responsible for levitation and heating, respectively. The experimental evaluation of the levitation force in a copper ring revealed the accuracy of our analysis, clearly illustrating its asymptotic behaviour versus frequency, and validating it for the qualitative analysis of magnetic levitation and heating in conductors of different shapes such as tubes and discs, composed of collections of conductive loops. The analysis of the results allows precise values of its electrical conductivity to be found. With the help of a simulation technique, this work also reveals the progressive deformation undergone by magnetic induction lines due to magnetic screening when frequency increases.

STUDY OF THE FREQUENCY DEPENDENCE OF THE FERROMAGNETIC RESONANCE LINEWIDTHS OF NICKEL FERRITES FROM 8-60 GHZ

In this communication we present in a systematic way experimental data on ferromagnetic resonance (FMR) linewidths of polycrystalline nickel ferrites (NixFe3−xO4 with 0.8<×<1.5) from 8 to 12.4 GHz and from 26.5 to 60 GHz. Data from 26.5 to 60 GHz were taken at room temperature while measurements at X band have been carried out from 77 to 400 K. Neither in X band nor from 26.5 to 60 GHz was a clear frequency dependence found. Classical contributions to the FMR linewidth according to Sparks’ outline are analyzed in the whole range of frequencies from 8 to 60 GHz. Contributions of the porosity, anisotropy, slowly and rapidly relaxing impurities, valence exchange and eddy current mechanisms are considered.

Comparison between disaccommodation and ferromagnetic resonance measurements in polycrystalline nickel ferrites

FerroMagnetic Resonance (FMR) measurements of polycrystalline nickel ferrites at 11 GHz from 77 to 400 K have been carried out. The different contributions to the FMR linewidth have been studied by a computational technique based on the parameters previously obtained for single-crystal nickel ferrite. A new qualitative comparison between the FMR and disac-commodation measurements by means of the induced anisotropy formalism is presented.

A study of the magnetic after-effect in W-hexagonal compounds

The relaxation of the initial permeability is measured in barium ferrite samples with W-hexagonal structure. In the temperature range between 80 and 420 K three clear relaxation processes are found; these features are influenced by sintering temperature and atmosphere. Peak C (370 K) seems to arise from long range cation diffusion processes via vacancies, with activation energies around 1.1 eV. Peak B (300 K) can be attributed to local rearrangements of ferrous cations in octahedral position via lattice vacancies and can be fitted by the superposition of two Debye processes with activation energies close to 0.80 and 0.84 eV. Peak A exhibits the characteristics of a single Debye process with an activation energy on the order of 0.48 eV and its origin is not yet clear. >

Magnetic aftereffect and electrical conductivity in double perovskite Ba2FeMoO6

In this work a study of the magnetic aftereffect and electrical conductivity in Ba2FeMoO6 is presented. Aftereffect measurements have been performed by recording the time evolution of magnetic permeability just after the sample demagnetization, while the electrical conductivity has been obtained by means of the four-probe technique working under ac excitation. The isochronal relaxation spectrum was constructed by evaluating at each temperature the relative variation of permeability between the end of demagnetization and a set of indexed time windows according to the fastness of the magnetic relaxation. The magnetic aftereffect spectra show two well-defined peaks related to the magnetic phase transition and the orientational processes that can be fitted by activation energies close to 0.65 eV. Electrical conductivity presents metallic behavior in the ferromagnetic region and semiconducting response in the paramagnetic zone with activation energy of 0.03 eV.