FeCuNbSiB thin films have been deposited using RF sputtering. Characterizations have shown that oxygen contamination and residual stress are mainly responsible for magnetic hardening. The sputtering and annealing conditions have been optimized and films with coercive field as low as 10 A m-1 (0.125 Oe) have been achieved. In addition, the influence of film thickness on the magnetic properties has been studied. Thus, magnetic field microsensors based on the magneto-impedance effect have been fabricated by stacking up Finemet/copper/Finemet films. The highest sensitivity (4000 V/T/A) is reached for 750 nm thick films. It is in the same range as cm-sized macroscopic devices realized using 20 µm thick ribbons.
The resistive and reactive components of magneto-impedance (MI) for Finemet/Copper/Finemet sandwiched structures based on stress-annealed nanocrystalline Fe 75 Si 15 B 6 Cu 1 Nb 3 ribbons as functions of different fields (longitudinal and perpendicular) and frequencies have been measured and analyzed. Maximum magnetoresistance and magneto-inductance ratios of 700% and 450% have been obtained in 30-600 kHz frequency range respectively. These large magneto-resistance and magneto-inductive ratios are a direct consequence of the large effective relative permeability due to the closed magnetic flux path in the trilayer structure. The influence of perpendicular bias fields (H per ) in the Longitudinal Magneto-impedance (LMI) configuration greatly improves the MI ratios and sensitivities. The maximum MI ratio for the resistive part increases to as large as 2500% for H per ~ 1 Oe. The sensitivity of the magneto-resistance increases from 48%/Oe to 288%/Oe at 600 kHz frequency with the application of H per ~ 30 Oe. Such high increase in MI ratios and sensitivities with perpendicular bias fields are due to the formation the favourable (transverse) domain structures.
This work presents the fabrication of magnetic field microsensors based on the magneto-impedance phenomenon and dedicated to NDC applications. The multilayer structure, ferromagnetic/conductive/ferromagnetic, is composed of a copper layer sandwiched with two Finemet Ò alloy films. The later, initially an amorphous material, is nanocrystallized by heat treatment. The fabrication process has been optimized in order to minimize coercivity and induce transversal anisotropy. The technological defects induced by the lift-off and sputtering processes change the magneto-impedance properties of the sensors. Eliminating these defects permits the sensor to reach to a sensitivity of 1,200 V/T/A at 30 MHz with a bias field larger than the anisotropy field and without hysteresis. The angular dependence of the sensitivity shows that the sensor is only sensitive to the axial component of the magnetic field.
Magnetoelastic effects in ultra soft nanocrystalline alloys are investigated theoretically and experimentally. From H c measurements, extraction of magnetoelastic contribution is carried out using a formalism obtained revisiting random anisotropy model (RAM) in the light of domain walls (DW) displacements, our approach based on theoretical investigations on the way of a reversal of a correlated volume (CV) located in the vicinity of a DW. Modelling of magnetoelastic effects shows that even in perfectly relaxed samples, a magnetoelastic contribution exists due to elastic frustration experienced by a CV during its magnetization reversal. Magnitude of this energy is large enough to drive coercivity of samples featuring grain diameter D around 10 nm, which are of major interest for applications. r
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