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eské vysoké u ení technické v Praze Fakulta elektrotechnická Katedra m ení Školní rok 2005/2006 ZADÁNÍ DIPLOMOVÉ PRÁCE Student Obor Miroslav Dvo ák M ení a p ístrojová technika Název tématu: Elektronika

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eské vysoké u ení technické v Praze Fakulta elektrotechnická Katedra m ení Školní rok 2005/2006 ZADÁNÍ DIPLOMOVÉ PRÁCE Student Obor Miroslav Dvo ák M ení a p ístrojová technika Název tématu: Elektronika pro magnetoimpedan ní senzory Zásady pro vypracování: Navrhn te a realizujte budící a detek ní obvody pro ízení magnetoimpedan ních senzor magnetického pole, založených na principu ob í magnetoimpedance (GMI) a inversního Wiedemannova jevu (IWE). Zm te základní parametry zhotovené elektroniky. Vytvo enou elektroniku použijte pro porovnání parametr p edložených magnetoimpedan ních senzor magnetického pole. Annotation Anotace: Tato diplomová práce se zabývá magnetoimpedan ními jevy, p esn ji ob í magnetoimpedancí (GMI) a inversním Wiedemannovým jevem (IWE), v magneticky m kkých materiálech a jejich využitím pro m ení slabých magnetických polí. Zmín né magnetoimpedan ní jevy byly zkoumány na páscích z magneticky m kkých, amorfních materiál, poskytnutých Fyzikálním ústavem AV R. V první fázi, po seznámení se s principem innosti t chto senzor, byly navrženy elektronické obvody, umož ující buzení a detekci signálu vytvo ených senzor magnetického pole, založených na inversním Wiedemannov jevu. V této ásti práce byly navrženy a zhotoveny dv budící a jedna detek ní jednotka. V druhé fázi byly zkoumány vlastnosti p edložených magnetoimpedan ních senzor ; jejich citlivost, teplotní stabilita a šumové vlastnosti. Annotation: This thesis deals with magnetoimpedance effects, rather the Giant magnetoimpedance (GMI) and inverse Wiedemann effect (IWE) in magnetically soft materials and its applications in measurements of weak magnetic fields. The mentioned effects were investigated in amorphous ribbons, kindly supplied by the Institute of Physics of Czech Academy of Science. At first the principle of magnetoimpedance sensors was studied. After that the electronic circuits for excitation and signal detection were designed. In this part two excitation units and one detection unit were developed. In the second part of the thesis the properties of two types of IWE sensor were investigated; sensitivity, temperature stability and noise parameters of the developed magnetometer. - I - Prohlašuji, že jsem tuto diplomovou práci vypracoval samostatn, a použil jsem pouze podklad, uvedených v p iloženém seznamu. I declare that I have elaborated this thesis individually and I have used only the literature given in the list. V Praze dne 13.ledna 2007 Prague, January 13 th, 2007 Miroslav Dvo ák - II - Acknowledgments Pod kování: V této ásti bych cht l pod kovat lidem, bez jejichž pomoci by tato práce nemohla vzniknout. V první ad bych rád pod koval svým rodi m za jejich trp livost a podporu b hem celé doby mého studia. Další pod kování pat í mému vedoucímu diplomové práce Ing. Michalu Malátkovi, bez jehož pomoci by tato práce jen t žko vznikala. S jeho pomocí byla provád na a koordinována v tšina m ení a jeho cenné p ipomínky a rady pomáhaly autorovi utvá et alespo ur itý náhled do problematiky principu magnetoimpedan ních senzor. Nemén významné pod kování pat í panu Ing. Lu ku Krausovi, CSc., z Fyzikálního Institutu Akademie V d R, za poskytnutí amorfních materiál. Sou asn bych rád pod koval všem len m magnetické laborato e, za jejich ob tavou a ochotnou pomoc. Acknowledgments: In this part I would like to give thanks to all the people, without their help this thesis could not be arisen At first I would like to express the thanks to my parents, for their patience, gratefulness and encouragement during my study. Special thank belongs to the supervisor of the thesis, Ing. Michal Malátek. With his help all the measurements were executed and coordinated. His valuable advices helped to the author to create a certain preview into the theory of magnetoimpedance sensors. Another thanks belong to Ing.Lud k Kraus, CSc., from the Institute of Physics of the Czech Academy of Science for the supply of magnetoimpedance samples. Thanks also belong to all members of the magnetic laboratory, for their devoted and goodwill help. - III - Table of contents Table of contents: 1 Introduction Motivation and objectives Organisation of the thesis Magnetoimpedance phenomena GMI effect Very low frequencies Low and medium frequencies High frequency regime Asymmetric GMI Asymmetry due to DC bias Asymmetry due to AC bias field Asymmetry due to exchange bias Matteucci and inverse Wiedemann effect Wiedemann effect Inverse Wiedemann effect, Matteucci effect Studied materials Amorphous ribbons Magnetic field sensor prototypes, based on magnetoimpedance effects Configuration with two MI elements Asymmetric GMI effect Inverse Wiedemann effect Configuration with one MI element Excitation unit Sinewave signal generator Wien bridge oscillator Sine-wave generator using MAX The DC voltage component (DC bias) Voltage-current transducer Parameters of the excitation electronic Detection unit IV - Table of contents 6.1 Signal conditioning Synchronous detection block Current feedback loop Value of the measured magnetic field Performance tests of the sensor prototypes Sensitivity of the sensor Testing measurements Sensor IWE_ Variation of the DC bias current I dc = 15 ma I dc = 17.5 ma I dc = 20 ma I dc = 25 ma Variation of amplitude of the AC component Sensor IWE_ Variation of DC bias component I dc = 2.5 ma I dc = 5 ma I dc = 7.5 ma I dc = 10 ma Variation of amplitude of the AC component Sensitivity tempco of the sensor Temperature offset drift of the sensor Noise characteristics of the magnetometer AGMI sensor Conclusion References Appendices...I 10.1 The mutual characteristics of tested sensors...i 10.2 Scheme of the magnetometer, printed circuit boards...ii 10.3 Photos of the designed electronics...vii - V - Introduction 1 Introduction 1.1 Motivation and objectives Magnetic field sensors are part of our daily live, even though we are not aware of it. We can meet them practically in all branches of industry, for measuring either the magnetic field itself or a physical quantity, transformed to magnetic response. The most precise magnetic field sensors are SQUIDs. These sensors are able to measure the magnetic field variations only and in order to obtain the high resolution, these sensors operate in cryogenics temperatures (approx. 10K). The so called Low-temperature SQUIDs are able to measure field variations in ft range. The most precise magnetic field sensors, at room temperature are fluxgate sensors, which can measure DC or low-frequency AC fields in range of to 10-4 T approx. [1]. In the early 1990 s the effect of Giant Magnetoimpedance (GMI) was reappeared. This phenomenon is characterized by huge change of both, real and imaginary parts of the impedance, when exposed to the static magnetic field. Lately, other phenomena, similar to GMI, were observed. The inverse Wiedemann effect (IWE) with asymmetrical response has become very interesting for the sensory purpose. The first results obtained on these sensors indicate that the IWEs could be able to compete to the fluxgate sensors [2]. During its relatively short history, the GMI and IWE effect were studied in many various types of ferromagnetic materials. The very prosperous results were obtained for magnetically soft, amorphous and nanocrystalline materials [2]. This thesis deals with magnetic field sensors, based on the asymmetric GMI (AGMI) and asymmetric inverse Wiedemann effect, in amorphous ribbons. The aim was to develop the electronic circuits that can be used for operation of such sensors. The further goal was to investigate the properties of the prepared prototypes of IWE and AGMI sensors Introduction 1.2 Organisation of the thesis In this chapter the motivation and objectives for this thesis were introduced. The following chapter deals with the fundamental of the magnetoimpedance effects. The each phenomenon is described by basic formulas. In chapter 3, the detail information about the used materials is discussed. In chapter 4 the magnetic field sensor prototypes, based on magnetoimpedance effects are introduced. Chapter 5 and 6 deal with the development of the electronic circuits those are necessary to control the sensor. In chapter 7 the field responses, temperature parameters and noise characteristics of two IWE sensors are presented. In chapter 8 the conclusions of the thesis are written Magnetoimpedance phenomena 2 Magnetoimpedance phenomena 2.1 GMI effect The true observation of the phenomenon, which is now called Giant Magnetoimpedance (GMI) is ascribing to Heisenberg and Landau, who described it in their basic works on ferromagnetism. Although the right interpretation of this phenomenon was introduced in 1935, this topic was almost forgotten till the beginning of 1990 s. In 1994 two scientific teams (Mohri, Pannina and Beach, Berkovitz) published their works, dealing with the GMI [3], [4]. The both groups proved the earlier physics of this phenomenon. Even though some people had been dealing with this topic earlier than two mentioned teams (Makhotkin et al., Machado, Mandal and Ghatak, [2]), the year 1994 can be considered as the start of GMI golden age [2]. The giant magnetoimpedance is a phenomenon, which appears in conductive, ferromagnetic materials, which are driven by an AC current i and are exposed to external DC magnetic field H 0. Under these conditions, the permeability of the ferromagnetic conductor changes and therefore the complex longitudinal impedance Z of the sample changes as well. This impedance Z is defined as the ratio of AC voltage u z, measured between the ends of the ferromagnetic sample (longitudinal voltage) and AC driving current i z,(1). The index z means that the quantity is measured in the z-axis (axis in parallel with longitudinal axis of the ferromagnetic conductor). Z ( ) = u z /i z = R + jx (1) The GMI effect is quantitatively described by the GMI ratio, according to Eq. (2). The Z 2 = R 2 + X 2 is the impedance modulus and the H 0max is the maximum measured field, at which the sample is considered to be magnetically saturated. The GMI ratio can reach values up to 800% [2]. Z Z Z H 0 Z H 0 max Z H 0 max (2) However this ratio is widely used for quantifying the GMI effect, there are some drawbacks in the evaluation that should be pointed out: - 3 - Magnetoimpedance phenomena Information about the phase shift is lost The ratio strongly depends on the H 0max value, which differs much among the GMI experiments. Moreover, even though the sample is magnetically saturated at field H 0max, it does not mean that the GMI is also saturated With regard to these facts the ratio Z/R DC, where R DC is the DC resistance of the sample, seems to be more adequate. The phenomenon responsible for the GMI at medium and high frequencies (MHz-GHz ranges) is the skin effect. When the high frequency current i z flows through the conductive ferromagnetic material, the current is not uniformly distributed in the volume of the conductor. It is concentrated in the outer shell of the conductor. This fact is described by the skin depth. This parameter determines the distance from the conductor surface towards its interior, in which the magnitude of the current density vector j falls to the 1/e of the magnitude at the surface. The skin depth is expressed by (3), 1 (3) f 0 where is the conductivity of the material, f is the frequency of the driving current and μ 0 is the material permeability. To observe the GMI effect in the sample, it is necessary to expose it to an external DC magnetic field. The DC field causes a change of the permeability, consequently of the skin depth and so the longitudinal impedance of the sample. The permeability, which is studied in GMI, is the transverse permeability μ t (in case of wires the circumferential permeability μ ). This permeability must be large enough and must be significantly modified by the static DC field [2]. According to the frequency f of the driving AC current, the GMI effect can be divided in three regimes; very low frequencies (1 10 khz), low and medium frequencies (10 khz several hundreds of MHz) and high frequencies (GHz). These three frequency regimes are in detail described in [2], so here only the main ideas and differences among them are mentioned Magnetoimpedance phenomena Very low frequencies Each conductor has its self-inductance, which depends on the distribution of the circumferential or (transverse) permeability. When an AC current i z passes through the sample, an AC voltage u z appears between the ends of the sample. This voltage consists of resistive part (according to Ohm s law) and inductive part (Faraday s law of induction), (4), u z = u R +iu L = R dc i ac + i L i i ac (4) where R dc is the DC resistance of the sample, and L i is the internal inductance of the sample. When the sample is exposed to the DC magnetic field, the circular components of magnetisation and the permeability change and so the inductive component of the voltage u z changes as well. This correlation is due to the direct proportion of the inductive component to the circumferential (transverse) permeability. Therefore the low frequency GMI is also called magnetoinductive effect. At these frequencies the skin depth is always larger than half thickness of the sample, considered as weak skin effect [2]. If we accept the physics of the GMI the skin effect, phenomenon that is observed at these frequencies should not be considered as true GMI Low and medium frequencies In order to understand the GMI response of a particular sample, it is necessary to find the dependences of its permeability on applied DC magnetic field H 0 and the frequency f. In case of classical (nonmagnetic) skin effect, the permeability is considered to be a scalar. In ferromagnetics the situation is more complex. The relation between vector of magnetic induction B and vector of magnetic field H is described by permeability tensor. This tensor is generally complex and its values depend on the following factors (among others); frequency of driving current, magnitude of the DC magnetic field and magnetic properties of the sample (shape, magnetic anisotropy etc.). The transverse permeability consists of two components according to Eq. (5), where μ t rot is contribution of the magnetisation vector rotation and μ tdw is contribution of the domain wall movement Magnetoimpedance phenomena μ t = μ t rot + μ tdw (5) At lower frequencies both components contribute to the total transverse permeability. At medium frequencies (around 1 MHz), the domain wall movement is damped by eddy currents and the rotation of magnetisation vector becomes the dominant contributor to the total permeability High frequency regime At very high frequencies of orders of GHz, the changes of impedance are caused by transition of the sample between the state of ferromagnetic resonance (FMR) and ferromagnetic antiresonance (FMAR). It is worth to mention that the distribution, according to the frequency range of the AC driving current is not decisive and some research groups use another criteria [2]. The other one, which is possible, is the ratio of skin depth to transverse dimension of the sample, /a. The strong skin effect is indicated by the ratio /a» 1 whereas the /a «1 indicates weak skin effect. Typical GMI response, the Z(H) curves are shown in Fig. 1 and Fig. 2. The characteristics could be either double-peak or single peak shape. The shape of the response depends on the induced anisotropy in the ferromagnetics. When magnetic anisotropy axis is in parallel with the longitudinal axis of the sample, the GMI curve has the single-peak. When a transverse anisotropy field is induced, the double-peak behaviour is observed. lzl [Ohm] Response of 8116/5743 (Co67Fe4Cr7Si8B14) material 12,5 11,5 lzl [Ohm] Response of 8116/5743 (Co67Fe4Cr7Si8B14) material ,5 12 9,5 10 8, H [A/m] H [A/m] Fig. 1. Double peak GMI response. Fig. 2. Single peak GMI response Magnetoimpedance phenomena The magnitude of the GMI effect does not depend only on the driving current i, frequency f and material properties μ,. Another important parameter is a thickness (diameter) of the studied sample. If two samples of the same material and different thickness are tested, larger GMI is observed in case of the thicker sample. On the contrary, the minimal thickness is limited by the skin depth. When the skin depth is higher than the half thickness of the material, the GMI effect disappears. [2]. 2.2 Asymmetric GMI In the previous text, the GMI response was assumed to be symmetric according to the applied field. The asymmetric GMI response can be obtained by applying an appropriate mechanism. There are three different ways how to obtain the asymmetric GMI response, [2] Asymmetry due to DC bias By applying of a DC bias current I DC through the sample, the symmetrical double-peak GMI response changes. Depending on the direction of the DC current, one peak is enhanced, whereas the second one diminishes. When the current is reversed, the asymmetry also reverses. Asymmetrical behaviour can be observed also for materials with single-peak GMI curve. This occurs thanks to the combination of the circumferential DC field H and a helical magnetic anisotropy of the material. The DC circumferential field is generated by the DC bias current. In case the sample does not have the helical anisotropy, this one must be induced by an appropriate mechanism, described in chapter 4. Theoretical analyses of this asymmetry, based on the electromagnetic models, could be solved by approximate methods Asymmetry due to AC bias field In this case the asymmetry of the GMI response is caused by combination of a helical anisotropy and an axial AC field. The AC axial field h z is generated by a coil, wound around the ferromagnetic core and driven by an AC current i. The coil is connected in - 7 - Magnetoimpedance phenomena parallel with the core. Both core and the coil are excited by a combination of AC and DC current, generating AC and DC circumferential magnetic field h, H, respectively. The voltage measured along the wire is given by Eq. (6). u ac iac Ziac L zz z hz l (6) where i ac is the AC component of excitation current, l is the length of the sample, h z is axial magnetic field, zz is diagonal component of the surface impedance tensor and z is it s off-diagonal component Asymmetry due to exchange bias This asymmetry was firstly observed in CoFeNiSiB ribbon, which was field annealed in the air atmosphere. The same sample annealed in protective atmosphere does not show the asymmetry. So the phenomenon was ascribed to oxidation, which helps the surface crystallisation. When the oxidation process starts close below to the crystallisation temperature of the amorphous alloy, chemical element B and Si diffuse towards the surface, where form oxides. Thanks to this diffusion a thin layer underneath the surface is depleted of B and Si. The crystallisation temperature of this underlayer decreases and the surface crystallisation starts. Such near-surface crystalline layer has much higher coercivity than the bulk, which remains nearly amorphous Thus the surface remains magnetically ordered, according to applied magnetic field during the annealing process. The exchange interactions between the bulk amorphous material and the crystalline surface produce an effective unidirectional surface anisotropy. This anisotropy is believed to be responsible for AGMI response in this case. 2.3 Matteucci and inverse Wiedemann effect Although, this chapter is devoted to Matteucci and inverse Wiedemann effects, it would be advisable to mention the Wiedemann effect at first Magnetoimpedance phenomena Wiedemann effect When a current i pass through a conductive magnetic material in a direction of a magnetisation vector M, this magnetisation twists around the current axis. If th
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