The long-range dipole-dipole interaction in an array of ferromagnetic microwires is studied through magnetic hysteresis measurements and Monte Carlo simulation. The experimental study has been performed on glass-coated amorphous Fe 77.5 Si 7.5 B 15 microwire with diameter of 5 m and lengths from 5 to 60 mm. Hysteresis loops performed at room temperature for an array of N microwires (Nϭ2, 3, 4, and 5͒ exhibit jumps and plateaux on the demagnetization, each step correspondent to the magnetization reversal of an individual wire. A model has been constructed taking into account the fact that the magnetization reversal is nucleated at the ends of each wire, under the influence of a dipolar field due to all other wires. Measurements for two wires allowed us to conclude that the dipolar field ͑or constant coupling͒ is independent of distance, at least for an array of a few wires. With the exception of three wires, where frustration seems to be present, the predicted reversal fields of our model are in good agreement with measurements. In order to study the role played by the number of wires on the demagnetization process, we calculate hysteresis loops for a large number of wires through the Monte Carlo method.

This study reports on the pioneering use of the layer-by-layer (LbL) technique to produce multilayered (1 to 50 bilayers) bifunctional nanocomposite films consisting of negatively charged citrate-coated maghemite nanoparticle (cit-MAG) hosted in positively charged conducting polyaniline (doped-PANI). The aim is to use the LbL assembly to fabricate thin nanocomposite films displaying superparamagnetic and conductivity properties and with fine control of the end properties as a function of the preparation condition. Multilayered cit-MAG/PANI bifunctional nanocomposite films were systematically investigated in order to access information regarding the nanofilm structure, electrical conductivity, and magnetic properties. Using the isothermal adsorption of each individual electrolyte (cit-MAG dispersion and doped-PANI solution) onto solid substrates (silicon and glass) the average time for deposition of a single layer (cit-MAG or doped-PANI) was fixed in 3 min. Independent evaluation using UV−vis spectroscopy and atomic force microscopy indicated a linear correlation between the nominal number of adsorbed cit-MAG/PANI bilayers and the material content (film thickness), even for the smallest number of adsorbed bilayers. Values of electrical conductivity (film thickness) found for the 10-bilayered cit-MAG/PANI nanocomposite films were in the range of 10−2−10−4 Scm−1 (25−63 nm) for γ-Fe2O3 concentration within the employed magnetic fluid suspension in the range of 10−4−10−3 g L−1. Values of the blocking temperature obtained from ZFC/FC curves recorded for the nanofilm produced using the highest γ-Fe2O3 concentrated suspension (2 × 10−3 g L−1) monotonically increase from 30 to 40 K as the number of cit-MAG/PANI bilayers increases from 5 to 50 bilayers. Therefore, we found that the end properties can be easily and precisely modulated by varying the concentration of the magnetic fluid used for film deposition and/or controlling the nominal number of cit-MAG/PANI bilayers in the nanocomposite.

We present an experimental study of the reproducibility of the different switching processes occurring in rings. Using superconducting quantum interference device and magnetoresistance measurements, we can measure hysteresis loops of arrays of rings and single structures at varying temperatures and thereby separate the influence of thermal excitations and defects ͑extrinsic and intrinsic͒. We find that the temperature dependence of the switching fields and their distributions can be correlated with the different physical processes occurring during different transitions. Measurements of the angular dependence of the switching fields of a single ring allow us to distinguish the contributions of extrinsic and intrinsic defects to the switching field distributions and, counterintuitively, it is established that transitions involving nucleation processes are less prone to defects and thermal excitations than processes involving domain-wall or vortex core depinning and propagation.

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Giant magneto impedance (GMI) has been measured in Fe77.5Si13.5B9 wires and Co67Fe3Cr3Si15B12 ribbons. The samples were annealed in order to induce specific anisotropies. An hysteretic behavior of the GMI is observed in both cases. Results show that this hysteretic behavior is related to irreversible changes in the magnetization processes of the samples.

We study the controlled introduction of defects in GaMnAs by irradiating the samples with energetic ion beams, which modify the magnetic properties of the DMS. Our study focuses on the low-carrier-density regime, starting with as-grown GaMnAs films and decreasing even further the number of carriers, through a sequence of irradiation doses. We did a systematic study of magnetization as a function of temperature and of the irradiation ion dose. We also performed insitu room temperature resistivity measurements as a function of the ion dose. We observe that both magnetic and transport properties of the samples can be experimentally manipulated by controlling the ion-beam parameters. For highly irradiated samples, the magnetic measurements indicate the formation of magnetic clusters together with a transition to an insulating state. The experimental data are compared with mean-field calculations for magnetization. The independent control of disorder and carrier density in the calculations allows further insight on the individual role of this two factors in the ion-beam-induced modification of GaMnAs.

In this study we describe the fabrication and characterization of nanocomposites consisting of layer-by-layer assembled polyaniline, sulfonated polystyrene, and maghemite nanoparticle layers. In order to assemble the starting components via electrostatic interaction, stable magnetic fluid containing maghemite nanoparticles (d approximately = 7 nm) with either positive or negative surface charges was used as source of nanoparticles for the layer-by-layer assembly. The structure, morphology, electrical and magnetic properties of such nanocomposite films were investigated by UV-Vis spectroscopy, atomic force microscopy, electrical, and magnetic measurements. The amount of PANI, PSS and maghemite nanoparticles within the nanocomposite films increased almost linearly with the number of deposited layers. Atomic force microscopy image of typical polyaniline/maghemite nanocomposites reveal nanoparticles adsorbed all over the film surface. The as-produced nanocomposite exhibits electrical conductivity and superparamagnetism behavior at room temperature, the latter confirmed by the absence of magnetic hysteresis.

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