Giant Magnetoimpedance Measurements Research Articles

Optimization of the high-frequency magnetoimpedance response in melt-extracted Co-rich microwires through novel multiple-step Joule heating

The optimization of the high frequency giant magnetoimpedance (GMI) effect and its magnetic field sensitivity in melt-extracted Co 69.25 Fe 4.25 Si 13 B 12.5 Nb 1 amorphous microwires was systematically studied through a multi-step Joule annealing (MSA) technique. The surface morphology, microstructure, surface magnetic property, and radio frequency GMI response of the Co-rich microwires were explored using techniques like microscopy, magneto-optical Kerr effect (MOKE) magnetometry, and the magnetic field dependence of the wire's radio frequency impedance (GMI). The multi-step Joule annealing protocol begins with an initial dc current amplitude ( i dc ) of 20 mA that was stepped up by 20 mA every 10 min to a maximum amplitude of 300 mA. Radio frequency GMI measurements at 20 MHz demonstrated a remarkable improvement of the GMI ratio and field sensitivity to 760% (1.75 times of that of the as-prepared wire) and 925%/Oe (more than 17.92 times of that of the as-prepared wire), respectively, after the MSA protocol with a maximum current amplitude of 100 mA. Microscopy and MOKE suggest that the MSA technique can enhance the microstructure and surface magnetic domain structure of the Co-rich magnetic microwire, which would give rise to an improved GMI ratio. The high GMI sensitivity at small magnetic fields renders these MSA-treated Co-rich microwires highly promising materials for biomedical devices that sense and monitor small, biological magnetic fields.

Route of magnetoimpedance and domain walls dynamics optimization in Co-based microwires

The influence of post-processing (annealing and stress-annealing) on the magnetic softness, Giant magnetoimpedance (GMI) effect and domain wall dynamics of Fe3.6Co69.2Ni1B12.5Si11Mo1.5C1.2 glass-coated microwires is studied.As-prepared Co-rich glass-coated microwire presents linear hysteresis loops and rather higher magnetoimpedance ratio with double-peak magnetic field dependence, typical for materials with transverse magnetic anisotropy.Considerable magnetic hardening and transformation of linear hysteresis loop with low coercivity (Hc ≈ 4 A/m) into rectangular with Hc ≈ 90 A/m upon annealing without stress is observed. However, we observed remarkable MI effect improvement at certain annealing conditions.Stress-annealing of studied microwire allows considerable magnetoimpedance ratio and domain wall velocity increasing. Additionally, remanent magnetization growth and coercivity decrease are generally observed upon stress annealing. Frequency dependence of maximum GMI ratio for as-prepared and annealed samples is evaluated. The obtained frequency dependence of the maximum GMI ratio allows determination of the optimal GMI measurement conditions for each sample. The observed stress-induced anisotropy and related changes in magnetic properties are discussed considering the internal stresses relaxation and “back-stresses”.

Giant Magneto-Impedance (GMI) Effect in Single-Layer Soft Magnetic Film Under Stress.

The stress-induced magnetic anisotropy can significantly affect giant magneto-impedance (GMI) effect of the soft magnetic film. This paper is devoted to the GMI effect of the single layer soft magnetic film implied without and with a stress. By simulating a physical model with MATLAB and COMSOL software, the impedance expression of the single layer soft magnetic film and the relation between external magnetic field and magnetic permeability are deduced. We observed that, without a stress, the sensitive region increased firstly and then decreased with the increasing of the excitation current frequency from 1 MHz to 200 MHz. While the film was subjected to the stress in the direction of the current with one end stressed, the stress on the film was gradually reduced from stressed end to free end. Also, the impedance change rate of the film changed when the stress was added, which is similar to the effect of adding a bias magnetic field on the film. More importantly, the addition of stress σ can induce the bias of the GMI measurement range and improve its sensitivity near zero magnetic fields. This may provide a new way for designing a GMI sensor with higher sensitivity and adjustable measurement range.

Magnetoimpedance effect in FINEMET/Ni80Fe20 composite ribbons

Giant magnetoimpedance (GMI) always attracts much attention owing to its importance in promising applications of industrial devices. Although GMI effect of ribbons have been studied for years, the composite ribbons coated with various films are still attractive due to its high sensitivity with respect to the applied magnetic field. Mostly, the magnetic properties were commonly inspected by hysteresis loops, which may not sensitive enough for Fe-based composite ribbons. Herein, by using both hysteresis loops and the GMI measurement, the magnetic properties of the FINEMT ribbons coated with different thickness of Ni80Fe20 film on free side, wheel side and twin sides have been thoroughly analyzed. It is revealed that the shift of anisotropy direction from preferable transverse to partially longitudinal as the film thickness increased, which affected the interaction between ribbon and Ni80Fe20 coating. Moreover, the asymmetric GMI response could be obtained on the ribbon coated with Ni80Fe20 film on single side, whereas the symmetric GMI response was observed for twin-side coating, which indicated the dipole interaction in the composite ribbons. We found that the ribbon with 260 nm Ni80Fe20 film showed the optimized transverse anisotropy, leading to the smallest Hc and the optimal soft magnetic properties. Hence, GMI measurement can be a sensitive alternative tool to investigate dipole interaction, which can be employed to manufacture novel linear GMI sensors and to properly regulate the operation field.

Giant magnetoimpedance intrinsic impedance and voltage sensitivity of rapidly solidified Co66Fe2Cr4Si13B15 amorphous wire for highly sensitive sensors applications

We report a systematic study of the influence of wire length, L, dependence of giant magneto-impedance (GMI) sensitivity of Co66Fe2Cr4Si13B15 soft magnetic amorphous wire of diameter ~100 µm developed by in-water quenching technique. The magnetization behaviour (hysteresis loops) of the wire with different length (L = 1, 2, 3, 5, 8 and 10 cm) has been evaluated by fuxmetric induction method. It was observed that the behaviour of the hysteresis loops change drastically with the wire length, being attributed to the existence of a critical length, L C, found to be around 3 cm. GMI measurements have been taken using automated GMI measurement system and the GMI sensitivities in terms of intrinsic impedance sensitivity (S Ω/Am −1) and voltage sensitivity (S V/Am −1) of the wire have been evaluated under optimal bias field and excitation current. It was found that the maximum (S Ω/Am −1) max ≈ 0.63 Ω/kAm−1/cm and (S V/Am −1) max ≈ 3.10 V/kAm−1/cm were achieved at a critical length L C ~ 3 cm of the wire for an AC current of 5 mA and a frequency of 5 MHz. These findings provide crucial insights for optimization of the geometrical dimensions of magnetic sensing elements and important practical guidance for designing high sensitive GMI sensors. The relevant combinations of magnetic material parameters and operating conditions that optimize the sensitivity are highlighted.

Magnetoimpedance-Based Probe of Various Concentrations of Corrosive Chemicals

We report on the possibility of exploiting the giant magnetoimpedance (GMI) effect of a soft ferromagnetic amorphous ribbon as a magnetic label-free probe of corrosive strengths of chemicals. Scanning electron microscopy, dc magnetization, and GMI measurements reveal that the presence of nitric acid $({\rm HNO}_{3})$ significantly alters the surface morphology, the magnetic permeability, and consequently the GMI response of a ${\rm Co}_{65}{\rm Fe}_{4}{\rm Ni}_{2}{\rm Si}_{15}{\rm B}_{14}$ amorphous ribbon. With an increase in the exposure time, the GMI ratio sharply decreases in the first few minutes and then remains almost unchanged. The relative change in the GMI ratio depends strongly on the corrosive strength of the chemical used (i.e., the concentration of ${\rm HNO}_{3}$ ). These findings are of practical importance in developing GMI-based detectors of corrosive chemicals.

Stress-induced magnetic hysteresis in amorphous microwires probed by microwave giant magnetoimpedance measurements

We report the results of a detailed study of the effects of tensile and torsional stresses on the giant magnetoimpedance (GMI) characteristics of vanishing-magnetostrictive Co-rich microwires at microwave frequency. A complex stress-induced hysteresis behaviour is identified in the GMI response in the presence of tensile and torsional stresses. It is also revealed that there exists a competition between these two kinds of stresses on the critical field via the interactions with the intrinsic anisotropy. An “enhanced core-shell” model is proposed here to resolve the physical origin of the low-field hysteresis and the dependence of induced anisotropy field on the applied tensile and/or torsional stress. Our results are of both technical importance to the design of non-contact stress sensors exploiting the GMI of microwires and fundamental significance to the understanding of the microwave GMI characteristics of soft magnetic microwires in the presence of external stresses.

Magnetization Process and GMI Effect in As-Cast Nanocrystalline Microwires

Results on the axial and circumferential magnetization processes in as-cast nanocrystalline glass-coated microwires are reported for the first time. Glass removal induces a peculiar change in the shape of the axial hysteresis loop. The initially rectangular axial hysteresis loop becomes almost anhysteretic after glass removal as a consequence of a change in the mechanism of axial magnetization, which has been explained by taking into account the changes induced by stress relief in the exchange length. Therefore, the nanosized crystalline grains can be coupled or uncoupled, depending on the presence of the glass coating. The change in the shape of the hysteresis loop is also observed on the circumferential direction. Results of giant magneto-impedance measurements support the explanation given, as the magneto-impedance response at 50 MHz increases about 9 times after glass removal due to both stress relief and to the phase consisting of coupled crystalline grains, which becomes the phase that preponderantly determines the magnetic behavior of the microwire after glass removal.

Equivalent Magnetic Noise Limit of Low-Cost GMI Magnetometer

We present a noise analysis of a giant magnetoimpedance (GMI) sensor using a peak detector at the optimal magnetic field working bias point of a sensor wire, by considering internal noise sources (intrinsic GMI device associated noise sources and conditioning electronic noise sources). An expression is obtained for the theoretical expected noise for known electronic design parameters and physical characteristics of the GMI wire. The most significant contributions to noise in a GMI measurement, using two basic oscillators (either a simple discrete RC oscillator or quartz oscillator) along with a peak detector, are presented. We discuss the expected extrinsic equivalent magnetic noise limit.

Non-linear magnetoimpedance in amorphous ribbons: Large asymmetries and angular dependence

The non-linear characteristics of transversal magnetization processes become apparent when large current amplitudes are used in giant magnetoimpedance measurements. Precise measurements reveal an unusual behavior, with the presence of asymmetries and amplitude jumps in the harmonic components of the voltage induced in the sample. In particular, the second harmonic presents a great sensitivity to sample orientation inside the applied magnetic field, which can be useful for a range of sensing applications.

High-frequency giant magnetoimpedance measurement and complex permeability behavior of soft magnetic co-based ribbons

We present a method for extracting the magnetic parameters of a soft magnetic Co-based alloy (Co/sub 68.25/Fe/sub 4.5/Si/sub 12.25/B/sub 15/) in the high-frequency regime (100 MHz-2.5 GHz). The method uses the magnetic sample (as-cast ribbon) as the conductor of a terminated microstrip transmission line. It uses the complex propagation constant of the line, obtained from open/short circuit input impedance measurements, to extract the per-unit-length series impedance. It then uses a theoretical model, based on the distributed parameters and the skin effect, to extract the bulk transverse relative permeability. The model considers the geometrical configuration of the magnetic conductor (ribbon) when determining the series impedance as well as the complex transverse relative permeability (/spl mu//sub tr/=/spl mu/'/sub tr/-j /spl mu/''/sub tr/). We report on the effect of applying and varying an external dc magnetic field.

Giant Magneto-Impedance Measurements at High Frequency in Planar Samples

Impedance measurements as a function of the applied magnetic field are performed at high frequency using an experimental set-up specially adapted for planar samples, which is based on a microstrip transmission line. Microwave measurement techniques and a consistent data reduction procedure allow obtaining very detailed and accurate impedance measurements. The results for two different samples, an amorphous ribbon and a trilayer thin film, reveal the importance of the ferromagnetic resonance contribution at high frequency. Besides, fine details of the magneto-impedance behavior at low field confirm that it is mainly governed by magnetization processes. At high frequency, when the ferromagnetic resonance contribution at low field weakens, an impedance peak re-appears, that seems to be related with the usual peak that exists at low frequency.

Study of the magnetic anisotropy induced in CoFeSiB amorphous ribbons by solidification in a magnetic field using giant magnetoimpedance measurements

Magnetoimpedance effect was used as a tool to investigate the effectiveness of the method to induce magnetic anisotropy by applying a DC magnetic field during the melt spinning process in amorphous magnetic ribbons. Amorphous ribbons of composition Co 69Fe 4Si 15B 12 were melt spun with and without applying the DC field in order to compare the results. From the different magnetoimpedance response to the external field in both kinds of ribbons, the induced magnetic anisotropy can be studied. Magnetoimpedance demonstrates to be a fine instrument to detect small changes of magnetic anisotropy and it is a good method to be employed in very narrow ribbons where other techniques are ineffective or unfeasible.

Magnetic and magnetotransport properties of Fe-based glass-covered microwires

Magnetic and magnetotransport properties of glass-covered amorphous microwires of nominal composition Fe89B1Si3C3Mn4 have been studied. Samples of two families with the same composition but different metallic core diameters were annealed in a furnace or by Joule heating. The electrical resistance was measured as a function of time during annealing to study the effects on the samples' microstructures (evolution from the amorphous to the crystalline phase). Static hysteresis loops were measured on all samples by means of a vibrating sample magnetometer. Giant magneto-impedance measurements (GMI) performed on a coaxial line using a vectorial network analyser at frequencies up to 6 GHz showed that furnace-annealed samples display larger anisotropies with respect to Joule-heated ones; however, Joule heating is more effective in improving the magneto-impedance response of the alloy. The maximum GMI ratio is ≈ 175% at 4 GHz. The influence of the different metallic core/total wire diameter on the induced anisotropies has been studied for the two families of samples through hysteresis loops and high-frequency GMI.

Thermal treatment effects on high-frequency giant magnetoimpedance in glass-coated Co83.2B3.3Si15.9Mn7.6 microwires

The thermal treatment effect on high-frequency giant magnetoimpedance (GMI) in a glass-coated Co 83.2 B 3.3 Si 5.9 Mn 7.6 microwire was investigated. The high-frequency GMI measurement in the form of LC resonators for the microwires annealed at 100 °C, 150 °C, 200 °C, and 250 °C for 1 h in a vacuum was performed on samples with 3 mm in width of electrodes and 9 mm in gap length between electrodes in the frequency range 100 MHz-1 GHz.

GMI measurements in ribbons of Co 70.4Fe 4.6Si 15B 10 in a wide range of frequencies

This paper reports room-temperature giant magnetoimpedance (GMI) data, measured in samples made of pieces of ribbons of amorphous Co 70.4Fe 4.6Si 15B 10 in the microwave frequency range ( f= 2–12 GHz) , for magnetic fields H up to 0.2 T. The measurements were carried out using the samples as one of the conductors of a microstrip line terminated by a short and connected to a network analyzer. We found that values of H, corresponding to the position of the peaks in the impedance spectrum and the peak widths, increase with f. Both the f- and H-dependence of the GMI are in good agreement with a theoretical model based on the dynamics of the domain walls at low f and on the ferromagnetic resonance at high f.

Domains and giant magneto-impedance in amorphous ribbons by magneto-optical Kerr effect

By longitudinal magneto-optical Kerr effect microscopy we analyzed the development of the domain structure in Co70.4Fe4.6Si15B10 amorphous ribbons for which giant magneto-impedance had been observed. Using transverse and longitudinal magneto-optical Kerr effect magnetometry we measured the magnetization direction around the hysteresis loop. These results allowed us to model the domain structure of the sample during the giant magneto-impedance measurements.