J.M. Munoz

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. >

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. >

Proposal for a standard problem for Preisach based models

Preisach based models have been usually considered as very convenient tools to simulate both the static and the transient behavior of the irreversible magnetization. In the present work, a very simple physical system consisting of a square array of aligned magnetic particles has been considered in order to be characterized by means of Preisach parameters. A uniform magnetic field is applied in the direction of the easy axis of the particles so that the system has only one dimension. Results reveal that, despite the simplicity of this system, it cannot be successfully defined by means of Preisach parameters. The distribution of interaction fields in the system does not follow a simple Gaussian function. A distribution composed of three balancing peaks is found instead.

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.

On the origin of magnetic AC susceptibility non-SRT anomalies in intermetallic compounds

The anomaly detected in the magnetic AC susceptibility of many intermetallic compounds between 100 and 300 K, and in particular in Nd/sub 2/Fe/sub 14/B at 220 K, has been induced in a controlled manner by thermal annealing. The anomaly has been interpreted in terms of thermal activated processes of defects imposing their dynamical behavior on the domain walls coupled to them, thus solving the controversy on its origin. >

Effect of Sn addition on the magnetic aftereffects of yttrium iron garnets

The magnetic aftereffect (MAE) phenomena in yttrium iron garnets (YIG) with Sn substitutions is presented. These studies provide very useful information about lattice symmetry and domain wall dynamics. This is specially interesting for a material as YIG, whose properties change significantly through the addition of a wide variety of dopants. The analysis of MAE is usually carried out by means of magnetic disaccommodation techniques, i.e., a time variation of domain wall mobility after a magnetic shock.

Magnetic aftereffects in Si-doped YIG

Magnetic aftereffect processes in yttrium iron garnet with silicon substitutions are analyzed by means of magnetic disaccommodation techniques. We have recorded the time evolution of the initial magnetic permeability after sample demagnetization for polycrystalline specimens with nominal composition Y/sub 3/Fe/sub 5-x/Si/sub x/O/sub 12/ (0/spl les/x/spl les/1) in the temperature range from 80 K to 500 K. We have found that silicon substitutions give rise to new relaxation and frequency dependent accommodation processes at high and low temperatures, respectively. The origins of both types of processes are finally discussed.

Magnetic aftereffects in hexagonal ferrites with BaO/spl middot/9Fe/sub 2/O/sub 3/ stoichiometric initial composition

We have measured the relaxation of the initial magnetic permeability after sample demagnetization, in the temperature range 80 K < T < 420 K, in polycrystalline barium hexaferrites with a Ba:Fe ratio in the initial composition of 1:18 corresponding to stoichiometric W phase. The materials were sintered under different temperature and oxygen pressure conditions. The results represented by isochronal curves show the existence of up to four relaxation processes. Additional thermogravimetric and X-ray diffraction studies reveal the successive formation of M, X, and W hexagonal phases with the increase of the sintering temperature. In this framework, we ascribe the relaxation processes to the progressive increase of ferrous cations in the different sites within the hexagonal lattice.