Magnetoimpedance effect of soft magnetic amorphous wires : doctoral disertation

[J. M. Orelj]

VI, 99 listova

U okviru disertacije su istraživani fenomeni koji pripadaju naučnoj oblasti razvoja savremenih senzora na bazi magnetoimpedansnih (MI) elemenata načinjenih od amorfnih metalnih legura na bazi kobalta i legura na bazi gvožđa (žice sastava Co68.15Fe4.35Si12.5B15, Co72.5Si12.5B15 i Fe73.5Nb3Cu1Si13.5B9). Sve legure pripadaju materijalima sa magnetno mekim svojstvima, što je prvi preduslov za izraženi MI-efekat (promena impedanse pod uticajem spoljašnjeg jednosmernog magnetnog polja Hex pri određenim frekvencijama f naizmenične struje koja protiče kroz uzorak). Dakle, centralna istraživanja su sprovedena u domenu funkcionalne zavisnosti impedanse Z (Hex, f). Za potpunu analizu MI-efekta pored modula impedanse, detaljno su analizirane i zavisnosti realnog MR (Hex, f) i imaginarnog dela MX (Hex, f) impedanse. Time je omogućena analiza promene dubine magnetnog prodiranja δm(Hex, f)korišćenjem eksperimentalnih podataka za magnetootpornost MR (Hex, f), dok je za analizu magnetne permeabilnost μ (Hex, f) neophodno i poznavanje zavisnosti magnetoreaktanse MX (Hex, f). Uporedna istraživanja zavisnosti magnetne dubine prodiranja od frekvencije i magnetnog polja δm (Hex, f) kod amorfnih žica sastava Co68.15Fe4.35Si12.5B15i Fe73.5Nb3Cu1Si13.5B9 su pokazala da se kod legure na bazi kobalta dostižu za red veličine niže vrednosti dubine prodiranja i za red veličine više vrednosti MI-odnosa (nekoliko stotina procenata kod legure na bazi kobalta naspram nekoliko desetina procenata kod legure na bazi gvožđa). Optimalne radne frekvencije MI-elemenata su definisane potrebom izbegavanja dominantnog uticaja longitudinalne anizotropije, tj. dinamike magnećenja domena u unutrašnjosti amorfnih žica, koja značajno smanjuje osetljivost mikromagnetnog senzora. Potrebno je ostvariti dominantan uticaj cirkularne anizotropije, tj. mehanizam rotacije vektora magnetizacije u domenima raspoređenim po obodu žice. Najveće vrednosti osetljivosti MIelementa su evidentirane kod amorfnih žica legure Co68.15Fe4.35Si12.5B15 pri frekvencijama 4-5 MHz, a dostignute vrednosti su oko 0,4 %/A/m.

The development of advanced sensors based on magnetoimpedance (MI) elements prepared on cobalt-based (Co68.15Fe4.35Si12.5B15 and Co72.5Si12.5B15) and iron-based (Fe73.5Nb3Cu1Si13.5B9) amorphous alloys was investigated. All investigated alloys are soft magnetic materials, which is the first prerequisite for a pronounced MI effect (impedance change under the influence of an external DC magnetic field Hex at certain frequencies f of alternating current flowing through the sample). Thus, the central research in the dissertation is the behavior of the impedance on magnetic field and frequency Z (Hex, f). In addition to the investigation of the MI-effect by MI-modulus Z (Hex, f), a complete analysis of two crucial MI-parameters: penetration depth δm (Hex, f), and relative magnetic permeability of wire μw (Hex, f) were performed by exploiting all three electrical MIelement components MR (Hex, f), MX (Hex, f), and Z (Hex, f). A theoretical impedance model for a cylinder, which includes both magnetoresistance MR and magnetoreactance MX, has been successfully used to estimate the magnetic permeability values μ (Hex, f). Comparative studies of the dependence of the skin depth on the magnetic field and frequency δm (Hex, f) in amorphous wires of Co68.15Fe4.35Si12.5B15 and Fe73.5Nb3Cu1Si13.5B9 alloys have shown that the cobalt-based alloy attains an order of magnitude lower values of penetration depth and for an order of magnitude higher MI-ratio (several hundred percent in cobalt-based alloy vs. several tens of percent in iron-based alloy). The optimal frequencies of MI-elements are selected to avoid the dominant influence of longitudinal anisotropy, i.e. dynamics of magnetization of domains in the core of wires, which significantly reduces the sensitivity of the micromagnetic sensor. Therefore, it is necessary to realize the dominant influence of the skin effect on circular anisotropy, i.e. mechanism of magnetization vector rotation in domains distributed on the shell of the cylinder. In these conditions of strong skin effect, the values of the skin depth are about one-tenth of the wire radius, and the influence of the magnetic field on the increase of the penetration depth (i.e. on the decrease of the impedance) is pronounced. The highest sensitivity of the MI-elements was recorded with amorphous wires of Co68.15Fe4.35Si12.5B15 alloy at frequencies of 4-5 MHz, and the attained sensitivity is about 0.4 % /A/m.

CoFeSiB, CoSiB i FeNbCuSiB amorfne žice, magnetoimpedansni (MI) efekat, MI-senzor, magnetna dubina prodiranja, magnetna permeabilnost, osetljivost

CoFeSiB, CoSiB, and FeNbCuSiB amorphous wires, magnetoimpedance (MI) effect, MI-element, skin depth, magnetic permeability, sensitivity.

537.622(043.3)

Fizika

Tekst