G. Infante

Local magnetization profile and geometry magnetization effects in microwires as determined by magneto-optical Kerr effect

The local magnetization profile along the length in magnetostrictive Fe-based magnetic microwires has been determined by magneto-optical Kerr effect. The study has been performed in microwires with different geometrical dimensions (i.e., diameter and length). The profiles of remanent magnetization and coercivity remain constant at the middle part for all microwires, whereas significant reduction of net magnetization accompanied by significant change of coercivity is observed when approaching their ends. This local region extends just few tens of micrometer for thin (around 1 μm diameter) wires and up to several hundreds of micrometer for thick (around 10 μm diameter) wires. That predicts that critical length to observe bistability goes from 50 μm to nearly 1 mm as diameter increases from 1 to 10 μm. Results are further interpreted considering the local distribution of magnetic charges at the ends which, arising to reduce stray fields, lead in some cases to inverted loops.

Nonlinear magnetoimpedance and parametric excitation of standing spin waves in a glass-covered microwire

The giant magnetoimpedance of an 8.5 μm glass-covered amorphous microwire was investigated in the frequency range of 10 MHz–3.5 GHz. It was found that when the exciting microwave current exceeds some threshold value, a periodic fine structure appears in the frequency dependence of the complex impedance. The appearance of this nonlinear phenomenon is interpreted to be a consequence of the parametric excitation of standing spin waves.

Locally induced domain wall damping in a thin magnetic wire

The damping mechanisms affecting the motion of a single domain wall were studied in a thin bistable magnetic wire. It was found that the overall damping is frequency and temperature dependent through the locally induced anisotropy via structural relaxation. This phenomenon can increase the overall damping by one order of magnitude and enables an effective tailoring of the domain wall dynamics according to required application.

Magnetic Microwires With Field-Induced Helical Anisotropy for Coil-Less Fluxgate

We present a new method for production of magnetic microwire with helical anisotropy. Coil-less fluxgate sensors are generally composed of a bimetallic wire excited by an alternating current; in order for the wire to work in coil-less fluxgate mode, the magnetic layer of the wire needs to have helical anisotropy. So far, we have achieved such anisotropy by mechanically twisting the wire. However, this method has some disadvantages for practical applications, mainly regarding the sensor stability. We propose a method that provides helical anisotropy by applying a helical field during the electrodeposition: this is achieved by the superposition of a longitudinal field generated by a Helmholtz coil and a circumferential field produced by a direct current flowing through the core of the wire during electrodeposition .

Bi-Metallic Magnetic Wire With Insulating Layer as Core for Orthogonal Fluxgate

In this paper, we examine the problems related to orthogonal fluxgates realized using magnetic microwires as core. Starting from a description of orthogonal fluxgates evolution, we give a theoretical analysis of the problems involving the full saturation of the wire, necessary condition to obtain proper working conditions. Bi-metallic wires (magnetic layer on copper wire, carrying the excitation current) have been proposed to achieve full saturation using lower current. In this paper, we present a further improvement: we realized microwires with insulation layer between the copper wire and the magnetic layer. The current flows only into the copper, regardless of the working frequency. Using insulation layer, we achieve 20 mA saturation current at 10 kHz, which is 3 times smaller than for similar wires without insulation layer.

M-H loop tracer based on digital signal processing for low frequency characterization of extremely thin magnetic wires

A high-sensitivity ac hysteresis loop tracer has been developed to measure the low frequency hysteresis loop of soft magnetic materials. It has been applied successfully to characterize straight pieces of amorphous glass-covered microwires with metallic nucleus down to 1.5 microm thick. Based on the electromagnetic induction law, the proposed design is extremely simple and exploits the capabilities of commercially available data acquisition cards together with digital signal processing in order to achieve high-sensitivity without the need of expensive analog equipment.

Double large Barkhausen jump in soft/soft composite microwires

The magnetic properties of double layer microwires consisting of a soft FeSiBP amorphous core, an intermediate non-magnetic glass spacer and a softer FeNi outer shell have been investigated. As in the case of other magnetostatically coupled two-phase systems, the hysteresis loops are characterized by two well-defined Barkhausen jumps corresponding each to the magnetization reversal of the individual phases, separated by a plateau. The strong dipolar interaction that leads to the appearance of the plateau is investigated in terms of the microwire geometry. It is shown that this source of coupling is capable of increasing up to one order of magnitude the switching field of the Fe-rich core. Thus, magnetic bistability can be effectively controlled in these kinds of composite wires.

Domain Wall Dynamics in Thin Magnetic Wires Under the Influence of Transversal Magnetic Field

We have studied the domain wall dynamics in thin amorphous glass-coating microwires under the influence of both axial and transversal magnetic fields. Two different regions of domain wall dynamics have been found according to the amplitude of the applied magnetic field rotated out of the wires axis. At low amplitude, the effect of applied field rotation is reduced to the reduction of its axial component and the domain wall velocity decreases. For high amplitudes of magnetic field, the maximum velocity was found when applied field forms angle -20° with the wires axis. The results are explained in terms of different domain wall structure, being transversal for low amplitudes of magnetic field and vortex for high amplitude of applied magnetic fields.

Circular Magnetoelastic Anisotropy Induced in the Nucleus of an FeSiB-CoNi Soft-Hard Bi-Phase Microwire

Multilayer amorphous microwires have been prepared coating a precursor FeSiB amorphous nucleus with a crystalline CoNi outer layer. The FeSiB as-cast amorphous wire is characterised by an unique bistable behavior where magnetization reverses from one remanent state to another through a single large Barkhausen event at a field value known as switching field, Hsw. In multilayer microwires, the presence of a second magnetic phase, a magnetically harder CoNi layer, substantially modifies the values of Hsw via two coupling mechanisms: magnetoelastic and magnetostatic interactions between the two phases. In this paper, a systematic study on the effects of the geometrical characteristic of the outer phase on the Hsw behavior is presented. M-H analysis and fluctuation studies by induction technique were performed on a family of samples having different CoNi thicknesses.

Ferromagnetic resonance study of FeCoMoB microwires during devitrification process

Magnetic properties of FeCoMoB glass-coated microwires with high positive magnetostriction have been investigated during the process of devitrification in the temperature range: 0-600 °C by ferromagnetic resonance (FMR) studies. The FeCoMoB microwire shows natural ferromagnetic resonance that reflects a complex anisotropy distribution. FMR spectrum for as cast sample shows up to four resonance maxima when ranging frequency from 10 MHz up to 11.3 GHz. After annealing, the anisotropy distribution becomes more regular and the number of FMR peaks decreases. The anisotropy and stress amplitude has been estimated from the FMR spectra, showing a strong decrease with annealing temperature and being low and constant for the nanocrystalline state. In addition, Gilbert damping decreases with annealing temperature, too. The low Gilbert damping (∼0.01) for the nanocrystalline state makes the nanocrystalline FeCoMoB microwire an ideal material for applications in which fast magnetization processes are required.