Ludek Kraus

Influence of Magnetostriction of NiFe Electroplated Film on the Noise of Fluxgate

One of the sources of fluxgate noise is the noise originating in the magnetic core. Several parameters, such as anisotropy and shape, are traditionally studied and optimized to reduce the noise. In this paper, we show how the magnetostriction influences the noise of the fluxgate. We electroplated NiFe thin-film over a copper layer using different current density. By energy-dispersive X-ray spectroscopy we show that the mutual percentage of Fe and Ni in the film depends on the current density. As a result the magnetostriction is changed from negative to positive values, with minimum magnetostriction found at around Fe19Ni81 composition. When we used these ring cores as base for a fluxgate we observed that the noise rapidly rises as the absolute value of magnetostriction increases, while the minimum noise is achieved at lowest value of magnetostriction. An increase of one order of magnitude of the magnetostriction causes and increment of almost two orders of magnitude in the noise. Therefore, we conclude that it is vital to employ alloys with the lowest possible magnetostriction as core for fluxgates.

Saturation magnetostriction determination of annealed Fe73.5Nb3Cu1Si13.5B9 alloy using two methods

Abstract The saturation magnetostriction ( λ s ) of Fe 73.5 Nb 3 Cu 1 Si 13.5 B 9 alloy samples was determined using the capacitance method and the transverse susceptibility method. The values of λ s decreased from (24.3–26.3) × 10 −6 to (0.5–2.5) × 10 −6 and then increased to 5.6 × 10 −6 for the sample annealed at 650°C. The differences of the two methods were from 0 to 20% in the amorphous state and somewhat higher for the nanocrystalline state.

Piezomagnetic properties of a stress-annealed amorphous Fe-Nb-Si-B alloy

Creep induced magnetic anisotropy in correlation with /spl Delta/E effect was investigated in the ribbons of the Fe/sub 74.7/Nb/sub 3/Si/sub 12.8/B/sub 9.5/ amorphous alloy upon annealing treatment. It was found that hard-ribbon-axis anisotropy developed in the alloy due to the annealing under tensile stress of 207 MPa in the temperature interval 543 K-633 K. The maximum values of the magnetoelastic coupling coefficient 0.35 and 0.29 were obtained at the annealing temperatures 543 K and 633 K, repetitively. Model calculations suggested that the domain structure and piezomagnetic properties are strongly influenced by the internal stress level. >

Nonlinear Magnetoimpedance in Field- and Stress-Annealed Amorphous Ribbons

The diagonal and off-diagonal nonlinear magnetoimpedance in amorphous ribbons with well defined magnetic anisotropy is investigated. Field- and stress-annealing are used to produce easy-ribbon-axis and hard-ribbon-axis anisotropy, respectively. The dependence of the first three harmonic components of impedance on applied magnetic field and dc bias current is discussed. The symmetry properties of diagonal and off-diagonal impedance components as functions of applied magnetic field and dc bias current are substantially different. While all the components of diagonal impedance are mainly symmetric functions of field, the off-diagonal impedance is mainly antisymmetric. It is found that the odd harmonics of diagonal impedance can be related to the second harmonic of the off-diagonal impedance and vice versa. A theoretical explanation of the observed behavior, based on the symmetry properties of nonlinear Landau-Lifshitz-Gilbert equation, is proposed.

Effect of Stress-Induced Anisotropy on the Noise of Ring-Core Fluxgate

In this paper, we show how an anisotropy perpendicular to the direction of excitation is favorable to obtain low noise in fluxgate sensors. We produced ring cores by electroplating NiFe thin film (6 μm) over a bended copper substrate. After the electroplating process, the sample was released and back-stress-induced anisotropy in the magnetic film due to magnetostriction of the material. As a result, we obtained rings with regions showing hard behavior and orthogonal regions showing soft behavior. Then, we used such rings as core for fluxgate and measured both noise and sensitivity for different orientation of the ring in the pick-up coil. When the pick-up coil covers the area with anisotropy perpendicular to the direction of excitation, the sensitivity is lower due to lower permeability, but finally, the noise is lower. On the other hand, rotating the core by 90°, the anisotropy becomes parallel to the direction of excitation and thus the sensitivity increases. Nevertheless, the noise also increases, showing that such anisotropy parallel to direction of excitation is unfavorable even if it brings higher sensitivity.

Bolometric Detection of Ferromagnetic Resonance in Amorphous Microwires

Ferromagnetic resonance (FMR) of glass-coated amorphous Fe₇₈Ni₈P₁₀B₁₀ and Co₆₇Fe₄Cr₇Si₈B₁₄ microwires was investigated at frequency 9.54 GHz by the standard FMR and the electrically detected FMR techniques. Static magnetic field was applied parallel to the wire axis and the resonance was driven by the microwave current passing through the wire. The field dependences of electrical resistance measured at different levels of microwave power were compared with the resonance curves obtained by the standard FMR method. It was found that the change of resistance observed at the resonance is caused by the increase of temperature due to the energy dissipation by the Gilbert damping of spin-precession motion.

Stress-Induced Anisotropy in Electroplated FeNi Racetrack Fluxgate Cores

Magnetic anisotropy plays an important role in the behavior of a fluxgate core. It is desired to have an anisotropy orthogonal to the direction of the excitation field to move from one saturated state to the opposite saturated state without abrupt change of magnetization. In this paper, we present a method for electroplating a Permalloy film in the shape of a racetrack core with built-in anisotropy. The core is electroplated under bending and later released so that the resulting stress on the film generates the anisotropy. We show how the BH loop of the film changes as we increase the bending radius. Moreover, we demonstrate that electroplating under bending extends the frequency range before the inductance drops. Therefore, the fluxgates based on such cores can be used for higher frequencies (at the expense of lower sensitivity given by lower permeability).

Study of magnetization in compacted amorphous and nanocrystalline alloy Fe/sub 73.5/Cu/sub 1/Nb/sub 3/Si/sub 13.5/B/sub 9/

The bulk amorphous FINEMET was prepared from ribbon and powder by shock wave compaction. The magnetization of compacts was measured up to 550/spl deg/C. The Curie and crystallization temperature was 350/spl deg/C and 495/spl deg/C respectively. From the magnetization curves of as-compacted and annealed samples measured at room temperature in the external magnetic field up to 1.8 T, the effective anisotropy and the law of approach to saturation was derived The deviation from saturation was ascribed to the stress related structural defects in ribbon compacts with prevailing effect of the point-like defects. In the powder compacts the deviation was caused by the intrinsic magnetostatic fluctuations due to spatial inhomogeneity. >

Inverse Wiedemann Effect Sensor with Pulse Excitation

Recently, a new type of inverse Wiedemann effect (IWE) field sensor with combination of sinewave and DC current excitation was presented. In this paper, the investigation on the sensor driven by unipolar pulses is reported. The sensor was excited by pulses of 40 to 60 mAp with duration from 1 to 5 mus. The best sensitivity and linearity of the sensor were obtained for the pulses with the highest DC component. Additional improvement of the sensitivity was reached by tuning the pick-up coil by parallel capacitor. It has been also proven that controlled integration is more favorable detection technique for the pulse excitation than synchronous demodulation.

Bimagnetic Microwires, Magnetic Properties, and High-Frequency Behavior

Bimagnetic microwires are cylindrically multilayered systems consisting of two magnetic metallic microlayers, a cylindrical nucleus, and an external shell, separated by an insulating layer. Such microwires are synthesized by combined quenching and drawing, sputtering, and electrodeposition, and the magnetic configuration of each phase can be suitably tailored to result in soft/soft, soft/hard, or hard/soft biphase microwires. Several families of alloy composition for each phase are considered in this overview: magnetostrictive Fe-based and non-magnetostrictive CoFe-based amorphous alloys for the nucleus and soft FeNi and harder CoNi alloys with polycrystalline character for the shell. The phenomenology of the magnetic behavior of the different microwires under low-frequency applied field is firstly described. Particularly the influence of the thickness of layers and that of thermal annealing are presented. A specific study is performed as a function of the measuring temperature in the range of below (15–300 K) and above (300–1000 K) room temperature. Magnetic and structural phase transitions are determined.