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The analysis of the induced current distribution in conducting wires subjected to a harmonic axial voltage is important in designing many electrical devices such as transformers and transmission lines. The azimuthal magnetic field induces axial electric currents and therefore the impedance of the wire depends on the excitation frequency. The current density is increasingly confined to a thin layer at the boundary of the wire as the frequency increases. To minimize this effect at higher frequencies it is necessary to enhance the surface-to-volume ratio by using thin high-conductivity wires. The study of induction phenomena in conducting samples subjected to a harmonic longitudinal magnetic field has attracted less attention. The time-varying magnetic flux induces eddy currents, which flow perpendicularly to the axis of the sample. We study the electromagnetic field in samples of simple geometry, making the usual approximations in good conductors. The validity of our calculations extends to several GHz and allows us to propose a method for determining the electrical conductivity by measuring the phase angle of the complex mutual inductance between a primary coil, responsible for the external magnetic field, and a secondary winding around the sample. [DOI: 10.1119/1.3160663]

We present a simple and precise method to measure the azimuthal electrical conductivity in nonferromagnetic tubes and rods at low frequencies. The results are readily obtained by analyzing the eddy currents in the sample when it is subjected to an axial harmonic magnetic field. The information about the eddy currents can be obtained from a measurement of the phase of the induced voltage in a short probe coil located inside the tube or around the rod when the sample is axially excited by a long primary coil. At low frequencies, the eddy currents are distributed over the full depth of the sample and any irregularities can be detected when the probe coil is moved along the sample axis. [DOI: 10.1119/1.3184154]

We discuss a technique for measuring the electrical conductivity in metallic tubes based on Faraday's induction law. A detailed study of magnetic screening in the low frequency approximation leads to expressions that relate the screening coefficient to the geometry, magnetic permeability, and conductivity of the metallic tubes. We discuss experimental results for different tubes.

Dipolar interactions on magnetic nanowire arrays have been investigated by various techniques. One of the most powerful techniques is the ferromagnetic resonance spectroscopy, because the resonance field depends directly on the anisotropy field strength and its frequency dependence. In order to evaluate the influence of magnetostatic dipolar interactions among ferromagnetic nanowire arrays, several densely packed hexagonal arrays of NiFe nanowires have been prepared by electrochemical deposition filling self-ordered nanopores of alumina membranes with different pore sizes but keeping the same interpore distance. Nanowires' diameter was changed from 90 to 160 nm, while the lattice parameter was fixed to 300 nm, which was achieved by carefully reducing the pore diameter by means of Atomic Layer Deposition of conformal Al2O3 layers on the nanoporous alumina templates. Field and frequency dependence of ferromagnetic resonance have been studied in order to obtain the dispersion diagram which gives information about anisotropy, damping factor, and gyromagnetic ratio. The relationship between resonance frequency and magnetic field can be explained by the roles played by the shape anisotropy and dipolar interactions among the ferromagnetic nanowires. [ABSTRACT FROM AUTHOR]

We report the effect of temperature on the ferromagnetic resonance (FMR) spectra of nanostructured line arrays. Different temperature dependences are observed for permalloy an nickel based samples. The qualitative features of the temperature dependence of the resonance field and linewidth can be described by the usual expression of slow relaxing linewidth mechanism and Bloch equation.

Research into the magnetic aftereffect and electrical conductivity of Ba2FeMoO6 is discussed. It was established that the magnetic aftereffect spectra indicate two clearly defined peaks connected with the magnetic phase transition and the orientational processes that can be fitted by activation energies close to 0.65 eV.

An interaction model is derived to analyze the magnetic relaxation of polycrystalline samples of BiPbSRCaCuO high-Tc superconductors. It is shown that effective energy increases with increasing temperature and decreasing magnetic field. This is attributed to the interactions among the vortices in the sample. Comparison of the model's results with those from the Anderson-Kim model also showed good agreement.

An increase in the frequency of polycrystalline YBaCuO samples decreases the width of diamagnetic transition and shifts the intergrain losses to higher temperatures. The Anderson flux creep theory helps study this shift, which indicates a lowering of the magnetic flux in grain boundaries. Cooling of the sample in a dc magnetic field causes a decrease in activation energy values.

Magnetic disaccommodation spectra (77-420 K) for YIG polycrystalline samples sintered during 8 hours in air atmosphere at different sintering temperatures ranging from 1350 [degrees] C to 1450 [degrees] C are presented. Samples sintered at temperatures from 1350 [degrees] C to 1415 [degrees] C present a relaxation process around 125 K, its amplitude decreases with increasing sintering temperature and its activation energy is close to 0.34 eV. For samples sintered from 1415 [degrees] C to 1450 [degrees] C the relaxation shifts to higher temperatures and the activation energy is dose to 0.4 eV. No accommodation processes are observed. This behavior is understood in terms of induced anisotropy effects of electronic origin similar to those observed in spinel ferrites.

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 which 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 activation energy of the order of 0.48 eV and its origin is not yet clear.

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 NiXFe3-xO4 with x = 0.9, 1.0, 1.2. Measurements have been carried out by means of an automatic technique based on a modified short-circuited 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 and a contribution of the Ni2+ ion acting as a slowly relaxing impurity for x > 1. The collection of data obtained by the automatic system allows us to achieve the values for the relaxation times and activation energies of these mechanisms.

The influence of the sintering atmosphere on the magnetic properties of yttrium iron garnets was investigated. The results showed that samples sintered under high and low oxygen pressure regimes exhibited markedly different behavior. At pressures above 0.1 atm, an isochronal relaxation peak which could possibly be related to domain wall relaxation was observed at around 180 K. At lower pressure regimes, the accomodation and disaccomodation processes produced both positive and negative peaks.

The ferromagnetic resonance (FMR) and the electrical conductivity in the perovskite sample, Sr(sub 2)FeMoO(sub 6), was analyzed. The FMR linewidth analysis has not only indicated the presence of Fe(super 2+) cation but has also shown data on the microstructure of the sintered polycrystalline sample finding the value of the porosity.

In this work a study of the magnetic aftereffect and electrical conductivity in Ba[sub 2]FeMoO[sub 6] 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. &! copy; 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Compares disaccommodation and ferromagnetic resonance measurements in polycrystalline yttrium-iron-garnet samples. Details on the experiment; Results of the study; Conclusions.

Presents a study which examined the frequency dependence of the ferromagnetic resonance linewidths of nickel ferrites. Experimental details; Results and discussion; Conclusions.