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In this work, a sensitive tool to detect magnetic nanoparticles (Fe3O4) based on a non-linear Giant Magnetoimpedance (GMI) effect is presented. The GMI sensor is designed with four nearly zero magnetostrictive ribbons connected in series and was analysed as a function of a constant external magnetic field and exciting frequency. The influence of the magnetic nanoparticles deposited on the ribbon surface was characterized using the first (fundamental) and second (non-linear) harmonics of the magnetoinductive voltage. The results show a clear enhancement of the sensor response in the high magnetic field region (H=1.5 kA/m) as a consequence of the stray field generated by the magnetic nanoparticles on the GMI ribbons' surface. The highest sensitivity ratios are obtained for the non-linear component in comparison with the fundamental response. The results open a new research strategy in magnetic nanoparticle detection. [ABSTRACT FROM AUTHOR]

NiFe2O4 and NiFe2O4-SiO2 nanoparticles were synthesized by a sol-gel method using citric acid as fuel, giving rise its combustion to the crystallization of the spinel phase. Different synthesis conditions were analyzed with the aim of obtaining stoichiometric NiFe2O4 nanoparticles. The spinel structure in the calcined nanoparticles (400 °C, 2 h) was evaluated by x-ray diffraction. Their nanometer size (mean diameters around 10-15 nm) was confirmed through electron microscopy (field emission scanning electron microscopy and transmission electron microscopy). Rietveld refinement indicates the existence of a small percentage of NiO and Fe3O4 phases and a certain degree of structural disorder. The main effect of the silica coating is to enhance the disorder effects and prevent the crystalline growth after post-annealing treatments. Due to the small particle size, the nanoparticles display characteristic superparamagnetic behaviour and surface effects associated to a spin-glass like state: i.e., reduction in the saturation magnetization values and splitting of the zero field cooled (ZFC)-field cooled (FC) high field magnetization curves. The fitting of the field dependence of the ZFC-FC irreversibility temperatures to the Almeida-Thouless equation confirms the spin-glass nature of the detected magnetic phenomena. Exchange bias effects (shifts in the FC hysteresis loops) detected below the estimated freezing temperature support the spin-glass nature of the spin disorder effects. [ABSTRACT FROM AUTHOR]

The so-called metamagnetic shape memory alloys transform from a ferromagnetic austenite into a weak magnetic martensitic phase, thus the application of a magnetic field, stabilizing the high magnetization phase, can induce the reverse martensitic transformation. Moreover, the martensitic transformation itself becomes arrested as its temperature range is lowered by the application of high enough magnetic fields. In this work the effect of the magnetic field on a Ni-Mn-In-Co metamagnetic shape memory has been studied by SQUID magnetometry. The arrest of the transformation produced by the field results in metastable states, whose evolution when the field is removed or reduced, follows logarithmic time dependence. The observed behavior is interpreted in terms of the magnetic contribution to the total entropy change associated with the magnetostructural transformation. [ABSTRACT FROM AUTHOR]

Ni–Mn–Ga ferromagnetic shape memory wires (Ni2.10Mn0.98Ga0.92, mean diameter 170 μm) are obtained by the rotating water bath melt spinning technique. The compositional heterogeneity linked to its dendritelike structure gives rise to a complex and broad martensitic transformation (MT). The reduced value of magnetization in the as-spun sample is ascribed to Mn–Mn antiferromagnetic interactions at structural defects as atomic disorder, vacancies, and antiphase boundaries structures. Moreover, the observed low temperature magnetic relaxation process is characterized by a splitting in the zero-field-cooled/field-cooled magnetization curves and the frequency shift in the ac magnetic susceptibility. The results are interpreted in terms of the coexistence of a reduced magnetization state and nearly noninteracting magnetic clusters. A high temperature treatment optimizes both the MT and the magnetic characteristics (i.e., decrease in the hysteresis of the MT and magnetization recovery, respectively). [ABSTRACT FROM AUTHOR]

Deals with a study which examined the giant magneto-impedance effect as a function of the structural modification induced in an Fe[sub73.5]Si[sub13.5]B[sub9]CuNb[sub3] amorphous alloy wire by annealing. Experimental details; Magnetic and structural characterization; Magneto-impedance effect.

The effect of tensile stress,σ, on hysteresis loops and magnetization profiles of Fe[SUB77.5]B[SUB15]Si[SUB7.5] amorphous wires has been measured. Measurements performed by a short movable coil permit one to obtain the dependence of remanent magnetization, μ[SUB0]M[SUBr], on the position of this coil, L. Increasing applied stress in tensile stress is applied, the initially rectangular local hysteresis loop measured for L=4 cm (at l=8 cm) becomes inclined, but then recovers its rectangular shape for larger stress. The mu;[SUB0]M[SUBr](σ) dependence shows an increase with σ for all L. The stress dependence of both μ[SUB0]M[SUBr] and H* can be understood in terms of a competition between magnetostatic and magnetoelastic energy. [ABSTRACT FROM AUTHOR]

Analyzes the changes in magnetic properties of an iron-rich amorphous wire when submitted to bending stresses. Calculation of bending stresses; Effects of the bending stresses on the wire radius of curvature; Discussion on the occurrence of the bistable behavior with increasing bending stresses.