J. M. Muñoz

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 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.

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.

About vacancy and Fe2+ contents in LiZnTi ferrites

A thermogravimetric method is applied to the evaluation of the cationic vacancy δ and Fe2+ contents in LiZnTi ferrites with nominal composition Li1+0.5(1−x+y)Zn2+xTi4+yFe3+2.5−0.5(x+3y)−zMn3+zO4, where some iron excess is provided by the contamination taking place in the milling stages of the ceramic procedure used to synthesize the ferrite. Experimental results for samples with x=0.25, y=0.83 and z=0.05 are shown. The single phased samples, with a cell parameter of 8.36 A, were analysed in an automatic thermobalance in the form of 5×20 mm rods and sintered in O2, air and CO2, suspended in a platinum wire. Changes in δ due to atmosphere changes are much more significant than those in [Fe2+]. When sintering in O2, an increase of the iron defect in the nominal composition practically does not affect [Fe2+], but reduces δ drastically, and this reduction can be correlated to changes in the ferrite magnetic properties. To our knowledge, this is an unexpected and significant result, since most discussions in the literature on iron excess ferrites usually attribute the changes in magnetic properties to the influence of [Fe2+], ignoring δ.

Stretched-exponential approach in relaxing systems

The stretched-exponential or Kohlrausch-Williams-Watts function has been widely used in the description of a large variety of relaxation phenomena in condensed matter physics. In this work, we consider this approach as a fundamental law and extend its applicability range to thermally activated relaxation processes. A non-iterative and robust technique is developed in order to obtain the characteristic parameters of this type of phenomena. The method is based on a set of closed-form expressions for the amplitude, activation energy, pre-exponential factor and stretched-exponent that can be directly computed from isothermal curves. This strategy is applied to describe the magnetic disaccommodation spectra observed around room temperature in a magnetite sample.

Comparison between disaccommodation and ferromagnetic resonance measurements in polycrystalline yttrium–iron–garnet samples

Polycrystalline yttrium–iron–garnet (YIG) samples were sintered in air at temperatures ranging from 1350 to 1450 °C. Ferromagnetic resonance measurements have been accomplished at 11 GHz from 77 to 350 K. The different contributions to the linewidth have been analyzed. These results are compared with previous measurements of magnetic disaccommodation in the same samples. A qualitative relationship between both kind of measurements for polycrystalline YIG samples by means of an induced anisotropy formalism is provided.

Experimental study on magnetic relaxation in YBa2Cu3O7 by means of a simple inductive technique

The time dependence of the field-cooled susceptibility in polycrystalline samples of YBa2Cu3O7 has been studied in the temperature range 77–95 K. Field-cooled and zero-field-cooled susceptibility measurements and its variation with the ac magnetic field are also reported. Measurements have been carried out by means of an inductive technique. Our fully automatic system allows us to measure the first data points of field-cooled susceptibility versus time from the beginning of the magnetic relaxation just after removing the field.

Ferromagnetic resonance in double perovskite Ba2FeMoO6

In this work the study of the ferromagnetic resonance and of the electrical conductivity in the perovskite sample, Ba2FeMoO6, is presented. Double perovskite has been manufactured by conventional ceramic method sintering in an inert gas atmosphere. Electrical conductivity, of about 1000–1400 S/m, presents metallic behavior in the ferromagnetic region and semiconducting response in the paramagnetic zone. FMR linewidths have been measured at 8.9 GHz from 200 to 350 K. A decrease in the linewidth with increasing temperature has been obtained. This behavior has been attributed to the presence of valence exchange mechanism, which takes place in samples with Fe 2+ cations. Electrical conductivity measurements corroborate this fact: the activation energy obtained in the paramagnetic zone, 0.03 eV, is close to the magnitude of Fe 3+ –Fe 2+ hopping. r 2002 Elsevier Science B.V. All rights reserved.