M. Knobel

Giant magnetoimpedance: concepts and recent progress

The giant magnetoimpedance effect (GMI) consists in drastic changes of the complex impedance of soft magnetic materials upon the application of an external magnetic field. The GMI effect is strongly dependent on the frequency of the applied current and the magnetic anisotropies present in the material, among other factors, which spawn a number of interesting new magnetic phenomena. In this context, one can roughly separate the research on GMI into approximately three aspects: (i) theory; (ii) applications and (iii) as a tool to investigate other magnetic parameters. In this work, an updated review of all these aspects is given.

Giant magneto-impedance effect in nanostructured magnetic wires

The giant magneto‐impedance effect (GMI) is studied as a function of the structural modification induced in an Fe73.5Si13.5B9Cu1Nb3 amorphous 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 wi...

Giant magnetoimpedance effect in soft magnetic wires for sensor applications

Abstract The giant magneto-impedance effect (GMI) consists of the large relative change of the impedance (up to around 300%) observed in magnetically very soft ribbon and wire alloys under the application of dc magnetic fields (units of kA m 1 ). The phenomenology of the GMI effect is firstly described including a discussion about its origin which mainly lies in the classical skin-effect. An alternative approach to GMI phenomena considering equivalent circuits is also introduced. The main requirements to detect GMI is to count on a sample with very large circular susceptibility and reduced resistivity provided the frequency of the ac current flowing along the sample (necessary to evaluate the impedance) is high enough (roughly above 0.1 MHz for most samples here considered). The dependence on dc magnetic field, mechanical stresses and particularly on thermal treatments resulting in the induced magnetic anisotropies or in the devitrification of amorphous samples into a nanocrystalline structure are reviewed. First results on GMI in glass-coated amorphous microwires are also reported. The use of the GMI as a tool for studying the inner circular magnetization process or for evaluating the magnetostriction is introduced. Finally, a description on various aspects regarding the development of magnetic field, current, proximity and stress sensor applications is presented.

Chemical Synthesis and Structural Characterization of Highly Disordered Ni Colloidal Nanoparticles

This work focuses on synthetic methods to produce monodisperse Ni colloidal nanoparticles (NPs), in the 4-16 nm size range, and their structural characterization. Narrow size distribution nanoparticles were obtained by high-temperature reduction of a nickel salt and the production of tunable sizes of the Ni NPs was improved compared to other methods previously described. The as-synthesized nanoparticles exhibited spherical shape and highly disordered structure, as it could be assigned by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Annealing at high temperature in organic solvent resulted in an increase of nanoparticle atomic ordering; in this case, the XRD pattern showed an fcc-like structure. Complementary data obtained by X-ray absorption spectroscopy confirmed the complex structure of these nanoparticles. Temperature dependence of the magnetic susceptibility of these highly disordered Ni NPs showed the magnetic behavior cannot be described by the conventional superparamagnetic theory, claiming the importance of the internal structure in the magnetic behavior of such nanomaterials.

PHOSPHATE COATING ON THE SURFACE OF CARBONYL IRON POWDER AND ITS EFFECT IN MAGNETORHEOLOGICAL SUSPENSIONS

Two types of carbonyl iron powders, (CIPs, BASF AG), the HS and HS-I (I = insulated, due a coating with phosphate), and two kinds of silica, one hydrophobic (Cab-O-Sil® TS610) and other hydrophilic (Cab-O-Sil® M5), were used to evaluate the influence of the surface treatment of the magnetic particle and the kind of fumed silica on the formulation of some magnetorheological suspensions (MRS). Oscillatory measurements at no field showed an evident difference between the silicas, but not a specific interaction with the phosphate coating on HSI. On the other hand, steady flow experiments also without magnetic field showed that the kind of silica and its specific interactions with the coating on iron powder drove the rheological behavior of the MRS on all region of the shear rate. Under magnetic field, the flow curves differences will be due to the iron particles and its magnetic properties, mainly on the region of higher shear rate.

Influence of Joule heating on magnetostriction and giant magnetoimpedance effect in a glass covered CoFeSiB microwire

The influence of annealing parameters (time ta, current Ia, and applied stress σa) on magnetic properties of Joule heated amorphous Co68.15Fe4.35Si12.5B15 glass covered microwire (13 μm) was investigated. Annealing under applied stress induces additional anisotropy which is proportional to σa and can be removed by subsequent heating without stress. The magnetoimpedance, measured on the sample with the lowest anisotropy field (HK≈120 Am−1), shows sharp maxima at H=±HK. For driving currents higher than 0.2 mA nonlinear behavior is observed, and the magnitude of giant magnetoimpedance significantly decreases. The maximum relative change of impedance (60%), observed for the highest frequency, 900 kHz, compares well with the values reported on conventional wires.

Evaluation of the linear magnetostriction in amorphous wires using the giant magneto-impedance effect

Abstract The stress dependence of the magneto-impedance effect is explored to estimate the magnetostriction constant and its stress derivative on a nearly non-magnetostrictive Co 68.1 Fe 4.4 Si 12.5 B 15 amorphous wire. Comparing the results with those obtained with the small-angle magnetization rotation method, good accuracy is obtained.

Frequency dependence of the magnetoimpedance in amorphous CoP electrodeposited layers

Magnetic properties and changes of impedance upon external field (MI) are studied in amorphous CoP magnetic layers obtained by galvanostatic electrodeposition over cylindrical Cu substrates. The magnetic layer thickness is controlled by deposition time and varies between 3 and 7 μm. Due to the columnar growth of Co, thicker layers have stronger perpendicular radial anisotropy. The field and frequency dependence of the impedance is measured in the kHz/MHz range. Although it is generally accepted that a radial anisotropy should be unfavorable to the MI effect, an increase of the MI ratio with the thickness of the magnetic layer, and thus with anisotropy, is observed. Results are explained in terms of a model considering the current distribution along the sample thickness with two well-defined regions having different transport and magnetic properties.

Magnetic and structural properties of fcc/hcp bi-crystalline multilayer Co nanowire arrays prepared by controlled electroplating

We report on the structural and magnetic properties of crystalline bi-phase Co nanowires, electrodeposited into the pores of anodized alumina membranes, as a function of their length. Co nanowires present two different coexistent crystalline structures (fcc and hcp) that can be controlled by the time of pulsed electrodeposition. The fcc crystalline phase grows at the early stage and is present at the bottom of all the nanowires, strongly influencing their magnetic behavior. Both structural and magnetic characterizations indicate that the length of the fcc phase is constant at around 260–270 nm. X-ray diffraction measurements revealed a strong preferential orientation (texture) in the (1 0–1 0) direction for the hcp phase, which increases the nanowire length as well as crystalline grain size, degree of orientation, and volume fraction of oriented material. The first-order reversal curve (FORC) method was used to infer both qualitatively and quantitatively the complex magnetization reversal of the nanowires...

Stress dependence of the giant magneto-impedance effect in amorphous wires

The recently discovered giant magneto-impedance (GMI) effect has been measured as a function of circular driving-field frequency and applied tensile stress on two near-zero-magnetostriction amorphous wires. The effect of different orientations of the induced magnetoelastic anisotropy has been verified, for the first time, by using wires with opposite magnetostriction constant, lambda s, signs (Fe4.9Co71.8Nb0.8Si7.5B15, lambda S=1.5*10-7, and Co68.1Fe4.4Si12.5B15, lambda S=-4*10-8). GMI ratios up to 300% were found in the magnetically softer (lower-magnetostriction) wire. The frequency dependence of GMI has been found to be strongly influenced by the magnetoelastic anisotropy induced in the amorphous wires. Results are interpreted in terms of changes in the magnetic penetration depth by modifications in the circumferential permeability originated by the action of external agents as field and mechanical stresses. AMI is therefore found to be largely determined by the magnetic domain configuration and relative contributions of both domain wall motions and magnetic moment rotations to the overall magnetization process.