A. G. Flores

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.

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.

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.

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.

Ferromagnetic resonance in low interacting permalloy nanowire arrays

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 ...

Internal magnetoimpedance of amorphous wires

In this paper, the internal impedance, that is, the impedance without consideration of any external induction or parasite capacity, provides clearly more valuable information about the physics inside the amorphous wire although for applications the total impedance is easier to measure.

The dilution of silica in a NiZnCo spinel ferrite matrix

Silica may dissolve up to about 0.05 wt% in an iron excess NiZnCo spinel matrix, affecting noticeably the magnetic properties at low and high values of the applied field. The initial permeability is reduced by about 50% with the addition of only 0.1 % SiO2, while the coercivity almost triples, without noticeable changes in porosity or grain distribution. The vacancy and Fe2+ content of the sintered material also show a considerable change. The energy associated to the transformation Fe3+ → Fe2+ for the samples without SiO2 was found to be 0.41–0.42 eV, while for the samples with SiO2 additions it decreased to 0.26 eV. The changes in magnetic properties are thought to be caused, more than because of the mentioned changes in ion distribution, by the stresses and defects created in the lattice due to the very small size of the Si4+ ion, which substitutes Fe3+ ions in tetrahedral sites. The transition Fe3+ → Fe2+ seems to take place exclusively in the octahedral sites.

DEPENDENCE OF THE FERRIMAGNETIC RESONANCE LINEWIDTH ON SINTERING CONDITIONS IN MANGANESE FERRITE

Ferrimagnetic resonance linewidth of manganese ferrites, Mn1.0Fe2.0O4 has been measured at 8.9 GHz from 77 to 320 K. Single crystal was prepared by floating zone technique and their composition confirmed by inductively coupled plasma spectrometry. Polycrystalline samples were sintered under different conditions of temperature, time, and atmosphere. Eddy current and valence exchange contributions according to Sparks’ theory and a new procedure for evaluating superficial and porosity mechanisms are taken into account to analyze the ferrimagnetic resonance linewidth. Different contributions are present depending on sintering conditions: valence exchange mechanism appears only in samples sintered under low temperatures and high pressures; conductivity decreases for samples sintered at high pressures; porosity is highly raised up when sintering time decreases. Resonance fields, obtained from experimental data, are used to calculate anisotropy fields for polycrystalline samples.

Anisotropy Field in Ni Nanostripe Arrays

Anisotropy fields of nanostripe arrays can be investigated by various techniques. One of these techniques is the ferromagnetic resonance spectroscopy due to the fact that the resonance field depends directly on the anisotropy field strength and its angular spread. In order to understand the dynamic fenomena in these samples, nanostripe arrays of Ni have been prepared by interference lithography. The film thickness is 45 nm for a lattice period of 2.7 μm and the stripe width equals 1500 and 750 nm. Ferromagnetic resonance measurements have been carried out at a frequency of 9.75 GHz as a funcion of H. The resonance field of the absorption peak increases upon changing the angle from parallel (0°) to perpendicular (90° ) to the film plane. This behaviour can be explained by the relation between resonance field, frequency and anisotropy field determined by demagnetizing factors and magnetization angle with respect to the sample easy axis direction after energy minimization.

Susceptibility Characterization of Phase Transition in Ti0.2Fe2.8O4 Ferrite

Titanomagnetite Ti 0.2 Fe 2.8 O 4 has been prepared by floating zone technique and its single crystal spinel structure confirmed by the inductively coupled plasma spectrometry method. Its magnetic behavior and its temperature dependence have been determined by VSM magnetometry. Particular attention has been paid to the measurements around the order temperature. This type of magnetic study has been performed in other ferrites such as nickel ferrite, but there is no previous result for this single crystal ferrite-like material. An analysis of the results using modified Arrot plots and the Kouvel-Fisher method has allowed us to evaluate the Curie temperature and the critical exponents for the spontaneous magnetization, β, and the zero field susceptibility, y. Critical exponents obtained suggest that the short-range interactions dominate in the critical region.