The giant magneto‐impedance effect (GMI) is studied as a function of the structural modification induced in an Fe73.5Si13.5B9Cu1Nb3amorphous alloy wire by annealing. The values of GMI are correlated to those structural changes and with the corresponding variation of the magnetic properties and intrinsic resistivity. Excellent soft magnetic properties, associated with low resistivity values, make this nanostructured material as one of the most promising for future applications of the GMI effect. The tailoring of the structure which can be induced by adequate thermal treatments easily allows one to obtain excellent combinations of circumferential permeability μφ and resistivity ρ during different devitrification stages, in order to produce materials for specific aims. Maximum GMI ratios of 200% are found after annealing the wires in the range 550–600 °C, where an optimum compromise between μφ and ρ is found. A simple model is developed correlating the fundamental physical properties of the soft magnetic wires with the measured values of both components of the impedance, allowing the prediction of experimental GMI ratios and an easy visualization of the phenomenon.

In this work the effect of a SiO2 coating on the magnetic properties of Fe3O4 nanoparticles obtained by the sol-gel method is analyzed. Two sets of samples were prepared: Fe3O4 nanoparticles and Fe3O4@SiO2 core-shell composites. The samples display the characteristic spinel structure associated with the magnetite Fe3O4 phase, with the majority of grain sizes around 5-10 nm. At room temperature the nanoparticles show the characteristic superparamagnetic behavior with mean blocking temperatures around 160 and 120 K for Fe3O4 and Fe3O4@SiO2, respectively. The main effect of the SiO2 coating is reflected in the temperature dependence of the high field magnetization (μ(0)H = 6 T), i.e. deviations from the Bloch law at low temperatures (T < 20 K). Such deviations, enhanced by the introduction of the SiO2 coating, are associated with the occurrence of surface spin disordered effects. The induction heating effects (magnetic hyperthermia) are analyzed under the application of an AC magnetic field. Maximum specific absorption rate (SAR) values around 1.5 W g(-1) were achieved for the Fe3O4 nanoparticles. A significant decrease (around 26%) is found in the SAR values of the SiO2 coated nanocomposite. The different heating response is analyzed in terms of the decrease of the effective nanoparticle magnetization in the Fe3O4@SiO2 core-shell composites at room temperature.

The effects of the partial substitution of Fe by Co in an FeSiBCuNb alloy on the crystallization process and on the magnetic properties have been analyzed. The crystallization of the amorphous melt-spun alloys, gives rise to an ultrafine structure composed by a residual amorphous matrix and crystals (tens of nm in size). Saturation magnetization and Curie temperature are analyzed in the nanocrystalline state in terms of the estimated compositional dependence of both structural phases. Coercivity behavior is interpreted within the framework of the random anisotropy model, where the structure and magnetic nature of the precipitated crystalline phase plays the dominant role. While a soft magnetic behavior is ascribed to the magnetic coupling between the $\alpha -\mathrm{FeSiCo}$ crystals for low Co concentrations, the precipitation of Co rich crystals in the Co base samples gives rise to a drastic magnetic hardening. The main interest of these new soft magnetic FeCoSiBCuNb alloys lies in applications at elevated temperatures, due to the improvement of the soft magnetic response with respect to the conventional FeSi based nanocrystalline alloys.