C. de Francisco

Automatic disaccommodation measuring system

Magnetic after effect measurement constitutes an advantageous procedure for the investigation of point defects giving considerable information about the crystal lattice dynamics. Unfortunately, most standard measuring methods involve tedious impedance bridges which are not suitable for systematic research. Thus a new automatic measuring system, based on the use of an inductance-capacitance resistor LCR meter, has been developed with special emphasis on soft magnetic materials. The resolution capability of our system is pointed out by measuring the disaccommodation of a ferrite sample.

Data processing for magnetic disaccommodation spectroscopy

The authors address the problems of analyzing experimental data obtained from magnetic disaccommodation measurements in order to extract information about the nature of the underlying phenomena. Some data processing methods commonly used in this type of investigation are reviewed, and novel techniques aimed at the optimization of the signal-to-noise ratio, the separation of overlapping modes and the determination of the relaxation parameters are discussed. The analysis strategies are divided into two groups: correlation techniques, characterized by different weighting functions and transform techniques, based on the use of the Fourier and Laplace transforms. As an example application, the relaxation process III in a vacancy-doped magnetite sample is analyzed. >

Analysis of Magnetic After-Effect Spectra in Titanium-Doped Magnetite

The magnetic after-effect (MAE) spectra of titanomagnetites, Fe 3-x-Δ Ti x O 4 , with constant vacancy content, Δ 10 -4 , and systematically varied Ti 4+ -substitutions, 0.01 ≤ x ≤ 0.4, are thoroughly analyzed in the temperature range 4 K < T< 460 K. These spectra are characteristically modified in the presence of already small Ti 4+ -substitutions (x ≥ 0.01): (i) the order-dependent, low-temperature (T < 125 K) electronic relaxations are severely affected; (ii) the ionic, vacancy mediated process III 1,2 (300 K) splits up into two satellites III * (260 K) and II 1,2 (around 400 K); (iii) near 200 K, additionally process IV 1,2 develops which is deduced to be of interstitial type. In terms of plausible models, the mechanisms and respective interrelations of these various processes are clearly revealed and their activation parameters numerically determined.

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.

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. >

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.

Thermal expansion of Ti-substituted barium hexaferrite

Thermal expansion measurements in the range of 20–500 °C were carried out on both poly- and single crystalline samples of the hexagonal magnetoplumbite ferrite with composition BaTiFe11O19. The continuous scanning of the thermal expansion reveals the existence of a λ-type anomaly near the Curie temperature. In contrast to the usual observed anisotropic behavior for magnetoplumbite hexaferrites, the averaged thermal expansion coefficient for the a direction is larger than for the c direction, i.e., 12.8×10−6 and 9.5×10−6 °C−1, respectively.

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...

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.

MAGNETIC DISACCOMMODATION IN NORMAL SPINEL STRUCTURES

Magnetic disaccommodation is investigated for ferrous ferrites with normal spinel structure. This study shows the presence of a new relaxation process in the temperature region IV (≃285 K) that has not been detected in inverse spinels. A numerical analysis of the isochronal spectra reveals that it is composed of two simple Debye processes with activation energies close to 0.75 eV and 0.80 eV, and preexponential factors 3×10−13 s. This relaxation is attributed to the formation of an induced anisotropy due to jumps of B-Fe ions into vacancies (as process III), but in presence of a divalent cation occupying an A-site.