Shape Anisotropy in Magneto-Impedance NiFe-Based Microsensors

Abstract

The magneto-impedance response of narrow multilayered NiFe/Cu/NiFe structures realized by micromolding has been characterized as a function of both the excitation frequency and the magnetic field. It has been found that intrinsic magnetic properties do not depend on the ferromagnetic geometry, but properties linked to the applied magnetic field, i.e. anisotropy field and magneto-impedance value do. The results are consistent with numerical simulations and show that the anisotropy field varies with the ferromagnetic film thickness. The highest magneto-impedance variation and anisotropy field, 170% and 4200 A/m were found for the narrowest and thickest structure. Magneto-impedance effect is based on the impedance variations of a ferromagnetic material due to changes of the skin depth while supplying the material with an AC current and applying an external magnetic field. The skin depth itself varies with the transverse (in case of a film) or circular (in case of a wire) magnetic permeability, thus to the external applied magnetic field:where f is the excitation current frequency going through the material, σ the electrical conductivity and μ t or μ c the magnetic permeability. The impedance depends on the skin effect and the highest variations are obtained for frequencies where the skin depth approximately matches the wire radius or the film half thickness. Values of several hundreds of % have been reported with microwires of amorphous Fe-based alloys, 1,2 thanks to the particular configuration of magnetic domains in rapidly quenched wires. Indeed, the rapid cooling of the material leads to a longitudinal magnetization in the core and a circular magnetization in the shell. The permeability is thus particularly sensitive with the external applied field. Such high magneto-impedance variations together with the use of soft materials lead to high sensitivities to the external magnetic field. Thus, that presents potential applications to small magnetic field measurement or magnetic nanoparticles detection. 3Thin film technology has also been investigated in order to benefit from the ability of collective fabrication process. However, the use of flat configurations and the contact with the substrate can create pinning sites for magnetic domains displacement or can induce stress which hardens the material and lowers the magneto-impedance properties. However large variations have been obtained by using a multilayered structure.4 This structure involves a conductive track between two ferromagnetic layers. Fe-based amorphous or nanocrystalline ferromagnetic materials, 5-10 particularly NiFe, 4,11-14 have been studied leading to magneto-impedance up to 400% at excitation frequencies in a range of several tens to several hundreds of MHz. However, these high frequencies, linked to under-μm thicknesses, make difficult the integration of an associated electronics. In order to decrease the excitation frequency, thick film technology has been investigated. In this way, electrodeposition of NiFe and Cu thin films has been used for ...

Current controlled switching of impedance in magnetic conductor with tilted anisotropy easy axis and its applications

“…Nevertheless we believe that this concept can be extended to other materials and geometries exhibiting a high MI effect and possessing the required magnetic anisotropy. Thus, for example, a high MI effect was demonstrated in thin films and ribbons2829. Moreover, the hysteresis behavior was shown30.…”

Magnetoimpedance effect in nanocrystalline Fe73.5Cu1Nb3Si13.5B9 single-layer and bilayer ribbons

“…This is due to the fact that domain wall displacements are strongly damped owing to eddy currents and only rotations of magnetic moments takes place. Hence the transverse magnetic permeability diminishes, which causes an observed decrease of the MI ratio [34,35]. Therefore, following the above arguments the maximum in (∆Z/Z)% is achieved when the thickness of the ribbon is comparable to the skin depth (t ≈ δ m ).…”

Giant magneto-impedance effect adjusted by electrolytic polishing and thinning of Co-based amorphous ribbons

“…Besides, as observed from the extracted results shown in figure 3(b), the initial susceptibility (χ i ) gradually increases from 7.8 emu/(cm 3 Oe) to 18.8 emu/(cm 3 Oe) with the increasing of t poli . The increasing in χ i here is mainly related to the improvement of dynamic magnetization conditions caused by demagnetization effect [32,44]. For the electrochemical polishing and thinning samples, longer t poli means a thinner of the thickness, this implies a larger demagnetization effect along the out of ribbon plane, which finally forces the in-plane magnetization thus leading to the improvement of susceptibility.…”

“…(H c ). As t poli increases from 0 to 5 min, the saturation field (H s ) slightly decreases from 200 to ∼70 Oe, which is mainly related to the changing of demagnetization effect caused by the decrease of thickness [44]. The derived H c shown in figure 3(b) keeps almost unchangeable and far below 1 Oe.…”