Search

Magnetic nanowire arrays grown in anodized alumina templates offer a fertile ground to investigate the interplay among the exchange interaction, dipolar interaction, and magnetocrystalline anisotropy. In this study, we have successfully fabricated Co nanowire arrays with gradient diameter while maintaining the nanowire separation. Consequently, we are able to obtain nanowire arrays with dominant exchange interaction at one end with small nanowire dimeter and dominant dipolar interaction at the other with large nanowire diameter. These samples show very different magnetic properties from those observed in magnetic nanowire arrays with uniformly diameter. The easy magnetization axis, coercivity, saturation field, and reversal processes are all controlled collectively by both ends of the nanowires, which cannot be viewed as the combination of magnetic nanowire arrays with thin and thick diameters.

The circular permeability, μ = μ ′−j μ ″, of two very soft ferromagnetic wires (Co 68.2 Fe 4.3 Si 12.5 B 15 amorphous wire S1 and Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 nanocrystalline wire S2) has been determined from the measurements of the impedance, Z = R +j ωL , as a function of frequency ( f =0.1−100 kHz) and DC bias field ( H =0−89 kA/m). The axial field-dependent DC circular permeability, μ DC ( H ), of two samples exhibit the anomalous enhancement phenomenon as we observed in the similar samples before. The change of the low- f eddy-current anomaly factor, η , with H has been studied. With increasing H from 0 to 3600 A/m, η increase from 1.4 to 8.5 and from 2.8 to 4.2 for samples S1 and S2, respectively, which is contrary to the theoretical expectation that η ( H ) approaches the value of η =1 with the sample being saturated. This anomalous behavior of η ( H ) is analyzed in terms of the micro-domain structure resulted from magneto-static interaction between the magnetically hard inclusions and the soft matrix.

Giant magneto-impedance, GMI, effect of Fe 73.5− x Cr x Cu 1 Nb 3 Si 13.5 B 9 ( x =0 and 10) amorphous and nanocrystalline soft wires has been measured at the frequency range from 0.005 up to 5 MHz as a function of the varying DC axial field up to ±140 Oe. Thermal treatments (in conventional furnace and by current annealing, with and without applied stress) lead to partial devitrification and induced anisotropies; hence they exhibit enhanced soft magnetic behavior and modify the GMI response. The results obtained in the current-annealed samples, improved in comparison with those for samples annealed in furnace, are ascribed to the homogeneous structure as well as to the induced helical anisotropy. The influence of the sample length (1–8 cm long) on GMI as well as that of the induced anisotropies is analyzed considering the particular c -domain structure and magnetization processes of the wire.

The circular susceptibility χ of amorphous and nanocrystalline Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 wires was measured under an axially applied field H up to 89 kA/m. The χ ( H ) of the amorphous sample may be well described by the core–shell model of positive magnetostrictive amorphous wires; however, after nanocrystallization, the χ ( H ) can be a few to few tens times greater than the ideal circular susceptibility χ id = M s / H deduced from the assumption of a zero anisotropy. This phenomenon is discussed in terms of a negative local anisotropy field H k ∗ resulting from the existence of magnetic inclusions, but its actual mechanism has to be further studied.

The longitudinal magnetic field dependence of low-frequency circular susceptibility, χ ( H ), is measured for (Fe 1− x Cr x ) 77.5 Si 7.5 B 15 ( x =0,0.05,0.1, and 0.15) amorphous wires of radius r 0 =0.07 mm. In order to explain the experimental results, on the basis of the widely accepted core–shell model, we consider that there are a constant axial anisotropy K c in the core and a radius-dependent radial anisotropy K s ∝ r 8 / r 0 8 in the shell, both K c and K s being functions of stress-energy constant λ s σ 0 . A formula for χ ( H ) is derived from this model. The experimental data fits indicate that the magnetic structure can be well described by the improved core–shell model with reasonable values of λ s .

The circular susceptibility of nearly zero magnetostrictive amorphous wires with saturation magnetization M s under an axially applied magnetic field H can be one order of magnitude greater than M s / H , the theoretical circular susceptibility deduced from the assumption of zero anisotropies. This phenomenon should be seriously considered in recent studies on magneto-impedance, and it can also become a powerful probe to the micromagnetic structure and local anisotropies in ferromagnetic wires if its further understanding is reached.

After annealing at 475–525°C and premagnetizing at a positive field of 60 kA/m, the quasi-saturated hysteresis loops of Fe 65.9 Cr 11.6 Si 7.5 B 15 amorphous wires made by the in-rotating-water quenching technique are shifted in the negative field direction. The shift can be as large as 10 2 A/m, which is in contrast to the slight shift of 10 0 A/m in CoFeSiB-based ribbons found before. Although this phenomenon can be attributed to the existence of aligned hard magnetic crystallites, its detailed mechanism remains unclear.

The effects of the partial substitution of Fe by Mn on the crystallization process and on the magnetic properties in a melt spun Fe 68.5 Mn 5 Si 13.5 B 9 CuNb 3 alloy have been analyzed. The crystallization process and structure after devitrification of the amorphous state were analyzed by Differential Scanning Calorimetry (DSC), Thermogravimetry analysis (TGA) and X-ray diffractometry. A fine nanocrystalline structure, composed of BCC-Fe rich nanograins surrounded by the residual amorphous phase, is obtained after thermal treatments at 550–600 °C. The temperature dependence of the magnetic properties (i.e. magnetic permeability) is analyzed taking into account the main changes in both magnetic phases upon annealing.

An amorphous ferromagnetic wire with a highly positive saturation magnetostriction constant, made of Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 , was simultaneously submitted to both, an AC current flowing through it and a torsional stress, in order to induce a helical magnetic anisotropy that modifies the magnetic domain structure and therefore the magnetic response of the sample. We study the torsion-impedance effect in an Fe-rich amorphous wire when it is submitted to different applied torsional stresses in both senses, up to 0.8 π rad/cm, as a function of the drive current frequency, in the range of 10 Hz–2 MHz. The experimental results are explained on the basis of the core-shell magnetic structure model combined with the skin effect.