Samir EldemrdashA thesis submitted to University of science and technology
in partial fulfillment of the degree of
MASTER OF PHILOSOPHY
School of physics
Amorphous polycrystalline magnetic materials have attracted more attention since last few decades due to its great magnetic properties which is crucial to implement for sensing or medical applications.
Recently, scientists gave more attention to these microwires, at the point when transform into improvement of attractive microwires got from a notable materials and composites. Amorphous magnetic microwires are glass-covered ferromagnetic microwires which is used for sensing application have a very individual magnetic and anisotropy properties. Magnetic anisotropies of different microwires as consequence of the inverse magnetostriction effect (positive or negative) at the interface between the glass surface and the metal core, in the presence of the internal stress come during the Taylor-Ulitovski preparation method. Therefore, the glass cover is not only for isolation and protection, but also is considered as one of the most parameters which influence on the physical and mechanical properties of microwires??? ?????? ?? 2 ??????
TOC o “1-3” h z u Chapter I A review on glass-coated ferromagnetic microwires PAGEREF _Toc512924285 h 31.1.Introduction PAGEREF _Toc512924286 h 31.2.History of microwire PAGEREF _Toc512924287 h 31.3 Fabrication Techniques for amorphous microwires PAGEREF _Toc512924288 h 51.3.1 Taylor-Ulitovsky method PAGEREF _Toc512924289 h 51.3.2 Melt-Extracted Microwires PAGEREF _Toc512924290 h 71.3.3 In-Rotating Water method PAGEREF _Toc512924291 h 81.3.2 changes in the chem-physi-and mechanical properties of microwire during preparing PAGEREF _Toc512924292 h 81.3.3 reference material used for preparing of microwire PAGEREF _Toc512924293 h 91.4Magnetic characteristics of glass-coated microwires PAGEREF _Toc512924294 h 101.4.1chemical structures of glass coated microwires PAGEREF _Toc512924295 h 101.4.2Internal and residual stresses of glass-coated microwires PAGEREF _Toc512924296 h 101.4.3Magneto-Impedance effect in amorphous wires PAGEREF _Toc512924297 h 111.4.4 Giant Magneto-Impedance (GMI) PAGEREF _Toc512924298 h 111.4.5 Magnetostriction in microwires PAGEREF _Toc512924299 h 131.4.6 Structural relaxation and induced magnetic anisotropies. PAGEREF _Toc512924300 h 141.4.7 Domain Wall structure PAGEREF _Toc512924301 h 161.4.8 Coefficient of thermal expansion of microwire PAGEREF _Toc512924302 h 171.4.9 Crystallization process in glass-coated microwires PAGEREF _Toc512924303 h 171.4.10The origin of magnetic softness PAGEREF _Toc512924304 h 181.5 Classification of microwires PAGEREF _Toc512924305 h 181.5.1 According to hysteresis loop PAGEREF _Toc512924306 h 18
HYPERLINK l “_Toc482564688” Chapter I A review on glass-coated ferromagnetic microwires
1.2.History of microwireIn the course of recent decades, the field of attraction has changed our seeing astoundingly. Beforehand, we used to describe the soft magnetic materials to change the viability of their uncommon magnetic properties and the cost as well. Be that as it may, the current innovation and logical approach of present day society of delicate magnetic materials have taken us towards the progression in the field of connected physical science, material sciences and substantially more 10. It helped us to establish numerous other comparative things in life which helped us to ad lib our work, however it was not exactly that simple.
The huge change in present day magnetic innovation (sensors and devices) has seen an incredible advance in the realm of research due to their profoundly enhanced magnetic properties, small dimensional and little size of magnetic materials. Physics Researchers have as of late accomplished an incredible advance in the creation of novel smaller scale and nanomagnetic materials i.e. microwires, thin films, and so forth. Despite the fact that the modern parts require shabby materials with little in estimate, yet at the same time have the high attractive properties and to accomplish this quality, a very refined innovation must be utilized 16. But unfortunately, , on numerous events, the magnetic properties of these materials are poor than the properties of bulk magnetic materials, for example, amorphous wires, ribbons and so on.
in 1924 Taylor is the first scientist who made The first thin metallic filaments (from liquid phase of the metal) using glass cover for protect and stabilization 1.3. The technique for getting microwires was enhanced by Makovsky in 1940. The standard of this strategy lays in the melting the alloy inside a glass tube with a gas burner till the softening of the tube and the liquefying of the metal took after by quick extending of the tube. But unfortunately this technique did not get wide acknowledgment, as it created microwires of restricted lengths and their parameters were wild. Another strategy for preparing the microwires was created by Ulitovski in 1948 1.4. The embodiment of this strategy is that a persistent melted metal filling the capillary is drawn out from a vertically situated glass tube. A metal sample is suspended in a high- frequency electromagnetic field, which warms it by the induced current and conceivably allows getting microwires of long sections. The following significant advance was the foundation of the Research Institute of Electro-Instrumentation and the manufacturing plant “Microwire” in 1958-59 in Chisinau, Moldova. This made the conditions for completing complex examinations of the casting procedure of microwire, prompting mechanical innovation for creation and an extensive investigation of the properties and the advancement of techniques and hardware for research and control. The size of these works was related with a steady increment in the volume of mechanical creation of parts and gadgets made of microwires. In the mid 1960’s some exploration work in the field of microwires likewise showed up in Britain, Germany, France, USA and Japan. A Japanese organization, UNITIKA Ltd, mass produces amorphous microwires however utilizing distinctive innovation and without a glass shell.
1.3 Fabrication Techniques for amorphous microwiresSince 10th years, there are many techniques for microwires casting and each technique has some advantages, morphology which affects the magnetic properties of microwires. Even though, the common feature of all technique is same: all of them involve melting of the ingot of desirable chemical materials and accordingly fast quenching from the melt.
Upcoming pages we have briefly explained the most known fabrication methods of theses microwires.
1.3.1 Taylor-Ulitovsky methodTaylor in 1924 invented the best way to fabricate the microwire as follows: obtaining thin metal wire is by pulling of molten metals in a viscous glass shell 1.31. the melting point of which was 150-200 °C below the softening temperature of the glass used 1.61.
In 1948 Ulitovski et al produced a new method of glass-coated cast microwire production 1.41. The nature of formation of glass-coated microwire in his method (see Fig. 1.1) can be presented in brief as follows. A few grams of the metals with a few grams (see Fig. 1.1(a)) are placed in a tube made from glass which is turning to introduced into an inductor powered by a high-frequency generator (more than few kiloHertz). Eddy currents remotely induced in the metal sample by the inductor heats it to the melting point and also soften adjoining walls of the glass tube. The metal quantity in the micro-bath, can be continuously restored by a metal rod fed into it, which is depending on the properties of the metal used and the dimension of the obtained microwire, as shown in Fig. 1.1. The processes for obtaining microwire are classified as either “drop” or “continuous.”, as a result of the mentioned methods of metal filling.
Figure SEQ Figure * ARABIC 1 Obtaining microwire in glass shell from melted metals by the Ulitovski method.
The form shape and the homogeneity of the generated amorphous alloy microwires can be amended by adjusting the ejection angle, superheat, and the drum rotation speed. The best performance continuous wires produced over 100-micro long having diameter ranging from 60um to 320um 162.
The unusual properties for example magnetic bistability, electrical properties and fast domain wall propagation were being studied with huge interest and later the discovery of GMI effect attracted a good attention with the end of 80s. Mainly, the drum?s rotation speed (50cm in diameter) is ranging from 150to400 rev/min and the amount of alloy which melted in one run was about 1 gram. The cooling rate is often around 105 K/s. Obtaining microwires with alloy compositions can be considered as The main advantage of using the rotating method which is difficult to produce by other manufacturing methods.
For mass production and research work as well, this fabrication method has been preferred due to 232:
The microwire properties has the same repeatability and reproducibility for mass production
Has the ability to control the shape and structure parameters during the production process
Can be used to produce a continuous microwire up to 10,000 m.
Figure 2 Casting machine which is employed in the Taylor-Ulitovski method.
It should mentioned that another fabrication method has been invented in 1975, by initially Maringer and Mobley 172 to manufacture the microwires by melt-extracted technique typically 30-60micro, which was later amended by Rudkowski 182 in 1991. He again modified the technique in 1995 to improve the fabrication method of wires.
1.3.2 Melt-Extracted Microwires
In last previous century, initially Maringer and Mobley 172 created a method to produce microwires by melt-extracted technique typically 30-60um, which was later improved by Rudkowski 18 2in 1991. He again modified the technique in 1995 to improve the fabrication of wires.
In particular, this technique takes the material into a rod shape a few millimeters in diameter melting the tip by a clean heat source such as RF induction o CW infrared. The extraction of wires is done by mean of rapid rotating sharpened wheel which is made from a refractory metal like molybdenum. It moves at tangential speed between 10 and 50 m/s 17-18. However, some more modifications have been reported in 2003 by Zhukova 19. The technique can produce wires up to 10m in length.
1.3.3 In-Rotating Water methodIn early 80s, the production of microwires was first reported by rapid quenching, and the first study was concentrated on the mechanical properties of wires 152. This method consists of preparation of alloy which is loaded into a quartz spout and following induction melting. Molten metal was ejected by a jet under a specific pressure into the rotating water layer through the orifice of the quart nozzle. The prepared amorphous microwires with different diameters depending on the slot size of the quartz nozzle which has a range 0.06 to 0.34mm. The structure and the homogeneity size of the produced amorphous alloy microwires can be improved by adjusting the ejection angle, superheat, and the drum rotation speed. The good continuous wires made over 100-m long having diameter ranging from 60um to 320um 162.
1.3.2 changes in the chem-physi-and mechanical properties of microwire during preparingFirstly let us define what glass-coated microwires mean are. On the whole, microwires are composite materials made of a metallic elements (amorphous metal) covered by a glass-coating shell (see Fig. 3).
The change in the chemical substance used for the microwires production and variety of dimensions are easily available by the modified Taylor-Ulitovksy fabrication technique as discussed in the previous section 3.1. mechanical properties are determined by the high rate of crystallization and the more smaller of microwire size.
Towards the fabrication methodology type, a complex radial distribution of internal stresses with circular, radial, and axial substance are made inside the metallic core parts
Because of the different quenching rates between the outer shell of surface and the core of the microwire. Besides, the difference in thermal expansion coefficients of the glass shell and the metallic core always plays an additional role in the induced stresses of as-prepared microwires 34, 353.
Figure SEQ Figure * ARABIC 3 SEM image of glass covered ferromagnetic amorphous Co70.5Fe4.5Si15B10 microwire of a diameter17.8 micro
The change in chemical and physical properties can be explained as, glass-coated microwires have an amorphous structure however the row material used has a crystalline structure due to their low magnetic anisotropy and therefore their magnetic properties distinguish by magnetoelastic and anisotropy. Also, amorphous microwires shows great properties like magnetoelastic anisotropy, large shape anisotropy and the most important have soft magnetic properties such as small magnetic field anisotropy (Hs), considerable values of saturation magnetization (Ms) and a high permeability 322.
1.3.3 reference material used for preparing of microwireThe presence of glass which has the required viscosity in the working range of the fabrication process, plays an important role in the microwire casting from a particular components. On the other hand dependent on the melting point of the parent metal. Currently the obtainable glasses row materials only allow the use of metals and alloys with a melting point less than 1500 °C for fabrication of microwire. The casting technology itself does not put limitations on the melting point, and the development of materials with higher melting temperature is dependent on the improvement of the proper glass.
Nowadays, alloys with major transition metals, usually used to produce the core of microwire however Pyrex glass, usually used to manufacture the glass shell.
On the whole, difference in the expansion temperature coefficients of the metal and glass should be decreased, mainly, when fabricating of microwire with a core diameter above 25 microns. For the creation of magnetic anisotropy, this difference in expansion coefficient becomes important.
Materials used for producing microwires should have a relatively small value of surface tension, large adhesion value and coefficient of expansion. The stronger the forces of the chemical bond at the glass-metal border, the better the glass tube will be filled with metal. In this case, a smooth formation of the intermediate phase, and surface tension between the metal and glass phases in a certain temperature range, become important.
The choice of glass for microwire fabrication is depend on many parameters: the difference between the Coefficients of Thermal Expansion (CTE) between metal and glass, viscosity in the working temperature range of the fabrication process, interfacial tension, crystallisation ability, quality of tubes, etc. Glass viscosity must also be taken as one of the factors that affect on the output parameters of microwire, in particular, the thickness of the outer shell. Increase in the viscosity increases the thickness of the insulation.
1.4Magnetic characteristics of glass-coated microwires1.4.1chemical structures of glass coated microwiresMagnetic materials can be classified in many ways; for example it can be categorized based on crystallinity of samples:
i- Crystalline magnetic materials or
ii- Amorphous materials which are show excellent magnetic properties.
The magnetic amorphous microwires have enormous applications ranging from low to high frequencies because of their abnormal properties. In contrast, nanocrystalline magnetic materials offer much more notable soft magnetic properties than their amorphous materials. on the whole, such materials show a biphasic structure with soft magnetic nanocrystalline grains embedded in a magnetic/non-magnetic phase 25-262.
It is should be mentioned that nanocrystalline alloys of microwires can be produces from amorphous alloys precursors by different techniques like electro-deposition, heat annealing, melt quenching, vapour quenching 272.
i- Nanocrystalline magnetic microwires
Nanocrystalline magnetic microwires are alloys based on transition metals consist of tiny grains of ferromagnetic materials established in an amorphous matrix. Such nanocrystalline materials show excellent soft magnetic properties as follows, very small coercivity values, low energy spoil, high permeability. These type of microwires can be produced by doing crystallizing the amorphous precursor using various techniques (like rapid solidification, vapour deposition, plasma processing) from liquid state which can be synthesized for such alloys 282. Moreover, the composition of nanocrystalline alloys is constrained to Co-based or Co-Fe-based alloys for high temperature applications due to high Curie temperature of these metals. Magnetization and magneto-crystalline anisotropy can be modified by change the Co/Fe percentage.
Ii – Amorphous magnetic alloys
Amorphous magnetic microwires are alloys based on transition metals like Iron (Fe), Nickel (Ni) and Cobalt (Co) used that have been melted and cooled suddenly, because of this a random arrangement of atoms with no crystalline grains directions. Magnetism can be determined in amorphous magnetic materials by exchanging the interactions and randomly oriented magnetic anisotropies 122.
1.4.2Internal and residual stresses of glass-coated microwiresTo determine the internal stress occurs in the magnetic microwires 33, 36-393, one should follow two mechanism happened in the same time of the fabrication process. Firstly, the glass transition of the metal that is assumed to happened simultaneously with the hardening of the glass at the glass temperature Tg 363.
The internal stresses for that case are occurred because of the solidification of metal as the solidification front proceeds radial inward to the centre of the wire. Such method creates the radial and circular stresses. Finally, is that the cooling of the metal-glass from (Tg) to room temperature. throughout the cooling method, axial, circumferential, and radial internal stresses are introduced because of the contraction of each substance (glass and metal) characterized by totally different thermal expansion coefficients obeying the subsequent equation:
? (T) = Em (?g – ?m) ?T (2.1)
Where Em is the Young moduli of the metallic nucleus; ?g and ?m are the thermal expansion coefficient of glass-coating and metallic nucleus, respectively, and ?T the difference between molten alloy temperature and as-prepared microwire temperature.
1.4.3Magneto-Impedance effect in amorphous wires
The strong effect of dc magnetic field on an AC voltage in soft magnetic amorphous microwires has generated a considerable interest. From practical point of view this phenomenon is analogous to Giant Magnetoimpedance (GMI). It is reported that the field effect on an AC voltage in terms of a field dependence of impedance Z=Z? + jZ” due to the skin effect. Thus, this phenomenon is being called as a Magneto-Impedance (MI) 462.
The impedance of a magnetic wire with a domain structure can be obtained by considering the ac current distribution inside the wire due to both the outer shape and domain wall displacements. The problem can be considered in an effective medium approximation, in which the microscopic eddy currents, imi, generated by moving domain walls are averaged on the domain wall scale. The problem then reduces to a homogeneous one with an effective magnetic permeability ? which incorporates imi. However, the impedance of a magnetic wire with a domain structure can be obtained by considering AC current distribution inside the wire which is due to both outer shape and domain wall displacement 472.
1.4.4 Giant Magneto-Impedance (GMI)
In magnetic sensing applications, one of the most attractive properties is the so-called giant magneto-impedance (GMI) effect. Since 1994, the GMI effect became one of the promising topic on intensive research and the GMI theory is extensively described in few reviews and original papers 482. The most attention gaining feature of GMI effect is their extraordinary high magnetic field sensitivity. It can change few hundred percent of impedance at very low applied magnetic field, that is quite interesting for magnetic sensors and magnetometers. Moreover, presently Japan is utilizing the GMI effect of thin amorphous glass-coated wires integrating in acceleration sensors and magnetic compass for cell phones 472.
When any soft ferromagnetic material is subjected to a small alternating current (AC), a large change achieved in the AC complex impedance of the material when applying external magnetic field. That change in impedance is known as Giant Magneto-Impedance (GMI) effect. The relative change in impedance (Z) with respect to the applied magnetic field (H), which is defined as GMI effect, is expressed by the following expression:
?Z/Z%=100% XZHmaxZ(Hmax) Where, Hmax is expressed as the external magnetic field to saturate the impedance. The Magneto-Impedance (MI) effect is usually understood as a significant change of impedance Z=Z’+iZ” where Zthe real part (resistance) is and Z”is imaginary part (reactance) of a magnetically soft material under the influence of an external magnetic field HE 492.
The GMI materials (like ribbons, wires or films) are usually enormously soft magnetic materials and this magnetic softness is directly related to the GMI effect 482. The magnetic field dependence of GMI spectra is mainly determined by the type of magnetic anisotropy. Thus, the circumferential anisotropy leads to the observation of the maximum of real component of wire impedance as a function of wire impedance 512. However, in the case of axial anisotropy, the maximum value of GMI ratio corresponds to zero magnetic fields. It is worth mentioning that the GMI effect origin is being explained on the basis of classical electrodynamics theory. The skin effect in microwires, which is responsible for GMI effect at medium and/or high frequencies, is well described in this theory many years ago 522.
As mentioned above the fundamental features of GMI, the circumferential magnetic anisotropy is desirable for GMI as it will provide a large permeability change even for low magnetic fields. In ferromagnetic materials having high circumferential anisotropy the magnetic permeability always possesses a tensor nature and the classical form of impedance which is not authenticated. Obtaining reasonable GMI effect lies in the dependence of ?? on an axial magnetic field which results in a change of ?m 53.2
However, it is necessary to reduce skin depth effect by choosing magnetic materials having large µo (or µT) and small ?m and Rdc, to obtain large GMI values. A large permeability reduces the skin depth that is later increased by the applied field as shown in fig. 7. In fact, the real and imaginary components of Z change with the applied dc field, Hdc. In a first order approximation, the in-plane component or resistance, R, can be expressed as 542:
This means that such changes in ?m caused by Hdc via µo (or µT) will help to modify R and hence Z. Therefore, the skin depth can be assessed as a function of Hdc through the measurement of R 542.
1.4.5 Magnetostriction in microwires
Amorphous glass-coated microwires are characterized by low anisotropy due to their amorphous structure. Therefore, their magnetic properties are mainly given by anisotropy shape and magnetoelastic. During the production of microwires by drawing quenching, magnetoelastic anisotropy arises from the interaction of magnetic moments with applied stress which is also due to different thermal expansion coefficient of glass coating and metallic core 44, 452. This distribution of mechanical stress is crucial for magnetic properties of wires. Whereas, the distribution of stress dependence of magnetostriction either be applied or internally induced could be relevant in low magnetostrictive compositions 452. The stress dependence of magnetostriction is expressed as:
?s (?) = ?s (0) – B? (1)
Where ?s (?) is the magnetostriction constant under stress; ?s (0) is the magnetostriction constant under zero-stress; B is a positive coefficient of order 10?10 MPa, and ? is stresses. The observed change of the magnetostriction can be related with both either internal ?internal, or/and, ?applied, applied stresses (? total = ? applied + ? internal). For low-magnetostrictive compositions (with ?s (0) ?10-7) and induced internal stresses of the order of 1000 MPa, the second term of Eq. 1 is almost of the same order as the first term. Likewise, to tune up the magnetic properties in microwires, the magnetostriction is one of the main and important key factor as well as it can bring out unexpected drastic changes in magnetic responses for the same alloy compositions with different dimensional parameters 452. Glass-coated microwires are mainly divided into three main groups depending on the sign of magnetostriction, which are positive magnetostrictive, negative magnetostrictive, and low magnetostrictive.
i-Magnetic microwires with negative magnetostriction:
CoSiB composition based amorphous glass-coated microwires are mainly characterized by relatively high and negative magnetostriction. The sign of magnetostriction can be determined because of stress distribution and the easy axis in these microwires will be circular 462. Such composition microwires have unhysteretic loop and the magnetization is proportional to the applied field and moves in axial direction through reversible rotation of magnetic moments inside domains. This sort of microwires are best for the construction of miniature sensors, transformer etc.
ii-Magnetic microwires with low magnetostriction
Microwires with low or zero magnetostriction are usually CoFeSiB (3-5 at.% of Fe) are characterized by very low but negative magnetostriction and results in small circular magnetoelastic anisotropy. Such microwires are characterized by circular domain structure below the surface of the metallic core and axial domain structure in the center of the wire as shown in figure 472
The hysteresis loop of such microwires with low magnetostriction has very low coercivity and high initial permeability. To study the domain wall propagation of these wires external parameters (like external stress, temperature, magnetic field, etc) can be employed to construct very sensitive sensors. Especially, microwires with low and negative magnetostriction are one of the most promising applications that are being used widely in sensors of magnetic field based on GMI effect.
iii-Microwires with positive magnetostriction
Amorphous magnetic microwires having large and positive magnetostriction displays a bistable behavior and this had been intensively studied by various groups in last few years. Main reason is due to their magnetoelastic interaction between the magnetic moments and the stress distribution which is introduced whiles the production of microwires. The domain structure consists of a large single domain in the center 15 of the metallic core which is covered by a radial domain structure as shown in fig. 5 48.2 This means that the magnetization jumps from negative remanent to positive remanent state when the wire is subjected to a positive applied field which is larger than or equal to a certain value called switching field, and usually denoted as H. This switching field is potentially dependent on external parameters (like external tensile stress, temperature, external magnetic field, etc).
Moreover, such microwires have perfect rectangular hysteresis loop as shown in figure below and the magnetization may have only two values ± Ms (where Ms is the saturation magnetization). It is easy to detect, therefore these magnetic microwires can be easily employed for miniaturized sensors
????? ???????? ???? ??? 2
1.4.6 Structural relaxation and induced magnetic anisotropies.
. Amorphous nature of glass-coated microwires gives them huge sensitive to external parameters. Doubtless, the stress induced throughout the fabrication method, as mentioned before, defines well a crucial supply of anisotropies. These anisotropies may be relaxed to an oversized extent by annealing. The compelling consequences of annealing processes need understanding of the origins of induced anisotropies. This eventually, results in partly relax the internal stresses induced by the fabrication technique, and offers rise to further sources of induced anisotropy. Dominant the anisotropy is extraordinarily vital for any technological application.
As a result of the rapid quenching, glass-coated microwires are metastable not solely with relevance crystallization however additionally with relevance to structural relaxation within an amorphous phase. The metallic atomic redistributions inside the glass cover tend step by step to reach the perfect amorphous Glass-coated microwires65, 663 Although the rate of change is commonly negligible at room temperature it becomes faster at higher temperatures below the crystallization temperature.
The dependence of the measured physical property on annealing time, tann, and temperature, Tann, is a complex function of thermal history of the sample. The change of any magnitude with either tann, at a given, Tann, based on a number of factors. The relaxation phenomena can be classified into at least two type 673.
i-The first type how is irreversible and monotonic relaxation behavior except very close to and above the glass transition temperature Tg. This section of relaxation phenomena comprises of changes in the volume, and diffusivity or viscosity of a metallic glass.
ii-The second type includes relaxations in the an-elasticity (creep-anisotropy), Curie temperature, field induced magnetic anisotropy, and thermal resistivity. This sction of relaxation phenomena is characterized by a saturation of the change after prolonged annealing, indicating the attainment of a pseudo-equilibrium state 683.
The saturated state is called the pseudo-equilibrium state, since it is only metastable against crystallization. Usually the pseudo-equilibrium state is dependent upon the annealing temperature, so that when the annealing temperature, Tann, is changed the system can move reversibly from one equilibrium state to another. Since this reversibility poses a striking difference compared to the first group, these relaxation phenomena are also known as reversible relaxation phenomena. Annealing can be performed in lots of different ways: conventional annealing, stress annealing, field annealing, or annealing by electrical current (also known as Joule-heating).
However, in case of glass-coated microwires, any single process of annealing can be considered as a twofold process (x-annealing + stress annealing). In other words, the presence of glass layer reinforces strong additional stresses, as a consequence, even only conventional annealing must be considered as conventional + stress annealing too 68, 693. There are several proposed mechanisms for this induced anisotropy 70, 71 3which include: atomic pair ordering resulting in directional order in the sample; induced texturing which line up easy axes; structural relaxation and rearrangement of free volume in amorphous materials; influence over the shape anisotropy of crystallites due to mechanical alignment. Thermal annealing at modest temperature and time affect the magnetoelastic anisotropy: after annealing the magnetoelastic anisotropy drastically 24 Fundamentals: Chapter 2 decreases. Hence, annealing at elevated temperature but below Curie temperature induces a macroscopic magnetic anisotropy with a preferential axis determined by the direction of magnetization during the annealing process 71, 723. Field induced anisotropy found to be increased as high as the annealing temperature. This behavior has been experimentally observed in metallic glasses with different composition 71-743. The microscopic origin of this field induced anisotropy has been successfully explained considering the directional ordering of atomic pairs mechanism developed by Néel et al. 753. This model predicts a dependence of the field-induced anisotropy with the annealing temperature as:
Ku (T) = k (Ms)n (T) (2.9)
being n constant, the value of which can be assumed to be equal 2 if the microscopic origin is the directional ordering of atomic pairs. Theoretical predicted value of the index n was experimentally found in Fe-Ni based metallic glasses 753. Nevertheless, deviations of such theoretical value have been obtained in Co-Fe based metallic glasses 76, 773. In this case, an additional contribution coming from the single-ion (initially n = 3) is considered. Moreover depending of the annealing temperature each contribution could be different according to the content of magnetic elements. Therefore macroscopically isotropic amorphous alloys can exhibit macroscopic magnetic anisotropy in the case if they are subjected to suitable annealing treatments at the presence of either a magnetic field (field annealing) or a mechanical stress (stress annealing).
1.4.7 Domain Wall structure
In amorphous magnetic microwires, the domain wall propagation has also got a considerable attention, which has a good future for micro and nanotechnology related applications used for sensing and storage devices, and logic operation. But to develop this type of devices and make it usable for different application, we should have ability to control the domain wall (DW) propagation34-352
It is necessary to understand the origin of fast DW propagation in amorphous microwires and the way to improve the Domain Wall velocity in other materials 352. Many research paper mentioned that the cylindrical microwires show DW propagation more than 10 km/s and has spontaneous magnetic bistability 36.2 It is should be mentioned that the difference of thermal expansion co-efficient in glass and the metal generated a considerable internal stress which cannot be neglected. This is due to while production the microwires (Taylor-Ulitovsky Method) involves simultaneous rapid quenching the metallic core inside the glass coating 38-392.
The soft magnetic properties of amorphous material are affected by both the produced internal stress and by the external applied stress 402. As a consequence, magnetoelastic anisotropy is the main factor to determine the DW propagation as well as the magnetic properties of amorphous microwires when there is no magneto-crystalline anisotropy.
Nowadays, it has been shown that the remagnetization process of magnetically bistable microwires adequately affected by microwires? inhomogeneities.
Recently, there has been reported that the DWs in microwires can be trapped and slowdown under the effect of an additional anti-parallel local magnetic field 412 and on collision driven by an external applied field 432.
1.4.8 Coefficient of thermal expansion of microwire
Heating of microwire is accompanied with the change of its length.
Heating has a great effect on microwire as a results change of microwires’s length occurs That type of dependence of the omicrowire length on its temperature is normally taken to be linear relation and is written as:
L2=L1( 1+??t) 1
Where ?t=T2- T1 , T1 andT2 are the initial and final temperatures respectively, L1andL2 are the initial and final lengths of the microwire respectively, and ? is the coefficient of linear thermal expansion of the microwire in a given temperature range . I should be noted that in Eq. (1), L2, is the system displacement for the whole two-component composite structure.
1.4.9 Crystallization process in glass-coated microwires
An important property of soft magnetic materials is that consisting of nanocrystallites randomly embedded in a soft amorphous matrix. This type of soft two-phase materials can be
occurred by crystallization of conventional Fe-Si-B amorphous alloys with small addition of Cu
and Nb as a result of the great work done by Yoshizawa et al. 793. So far, the crystallization of amorphous metals was rather known to significantly deteriorate the soft magnetic properties and to yield a relatively coarse microstructure with grain sizes of about 0.1– 1 microns. Therefore, Yoshizawa ultimately mentioned that such unusual combination of this chemical design is a key for particular ultrafine grain structure and as a result superior soft magnetic properties. The created microstructure possessed of small (around 10 nm grain size) nanocrystalites embedded in a residual amorphous phase after annealing the amorphous precursor between 500-600°C for 1 hour. Using that method, the devitrification process of these amorphous alloys came up with the basis features of excellent soft magnetic properties indicated by the high value of permeability about (1 x 105) and alternatively, low coercivity.
Nanocrystalline Fe-Si-B-Cu-Nb alloys have been patented under the trademark entitled Finemet,
this name derives from the combination of “fine” and “metal” which indicates the material’s
features of being formed with fine crystal grains and having excellent magnetic properties. The
next section will likely destined out the origin of this excellent magnetic softness as a unique
advantage of nanocrystalline materials.
1.4.10The origin of magnetic softnessThe main concept of highest magnetic softness property is thought to be originated because the magnetocrystalline anisotropy vanishes and very small magnetostriction value obtained when the grain size approaches 10 nm as theoretically estimated by Herzer 80, 813.
Herzer has successfully applied Alben model 483 to understand the origin of magnetic softness seen in the nanocrystalline phase. According to this model, low coercivity in the nanocrystalline phase is ascribed to tiny effective magnetic anisotropy (Keff 10J/m3). This property is a combination the structural correlation length which is much smaller than the ferromagnetic correlation length. Somehow like in amorphous metals, the magnetocrystalline anisotropy is randomly averaged out by exchange interaction.
While at the same time the averaging effect of exchange interaction in the small grain size system allows to combine the individual properties of different structural phases which expand the variability of property tailoring over that of alloying single phases. Furthermore the suppressed magnetocrystalline anisotropy, low magnetostriction values give the basis for the superior soft magnetic properties seen in particular compositions. Low values of the saturation magnetostriction are important to avoid magnetoelastic anisotropies produced from internal or external mechanical stresses. These findings make the essential difference to large grained materials where the magnetization follows the randomly oriented easy axes of individual grains and, accordingly, the magnetization process is adjusted by the full magneto-crystalline anisotropy of the grains.
1.5 Classification of microwiresAs it was noted in previous sections, ferromagnetic microwires can be categorized into different aspects. Maybe classified according to chemical composition, GMI behavior, magnetic phase, and hysteresis loop
1.5.1 According to hysteresis loopWhen a ferromagnetic material exist into external magnetic field, their magnetization changes in a complex behavior 43, 453, which is described by a magnetization curve as depicted in Fig. 4 Starting from a demagnetized state (M = H = 0), the magnetization increases with increasing the field along the doted curve and finally reaches the saturation magnetization, which is normally presented by Ms. The B-H curve is the curve characteristic of the magnetic properties of a material or element or alloy. It tells you how the material responds to an external magnetic field, and is a critical piece of information when designing magnetic circuits. In the plots below, for a vacuum an H of 800 At/m creates a B of 1 mT. With a sheet steel core, an H of 800 At/m creates a B of 1.2 T. A huge increase in B for the same H! The hysteresis comes into play when the material has been magnetized. The B within the material does not go back to what it was before, but is dependent on the history of its magnetization .For non-magnetic materials that do not saturate, the curve has a fixed slope approximately equal to µ0
i. Diamagnetic materials have a slightly smaller slope
ii. Paramagnetic materials have a slightly greater slope
Figure SEQ Figure * ARABIC 4Typical hysteresis loop of soft magnetic material
The residual magnetism, Br, or remanence (or retentivity), is the flux density that is left within the material after it has been magnetized. ii. A material with a high Br is desired for permanent magnets.
iii. The coercivity, Hc, is the magnetic field intensity that is required to demagnetize the material after it has been magnetized. iv. A material with a high Hc is desired for permanent magnets to prevent them from being easily demagnetized. v. Rare earth magnets have a much higher Hc than Alnico magnets. vi. The saturation effect of the material occurs when all of the magnetic domains within the material have become aligned with the external magnetic field that surrounds it.
e. Characteristics of soft magnetic material(M1)
i. A material with a very low Br and Hcii. It does not retain a strong magnetic field (does not make a good permanent magnet), and is easy to demagnetizeiii. The area enclosed by the B-H curve is small, so it has low hysteresis losses or core lossesiv. This material is desired for use in transformers, motors and electromagnets where the magnetic field is always changing.v. Electrical steels, which contain about 1-2% Si, is a soft magnetic material.
e. Characteristics of hard magnetic material (M2)
i. A material with a very high Br and Hcii. It retains a strong magnetic field (makes a good permanent magnet), and is difficult to demagnetizeiii. The area enclosed by the B-H curve is large, so it has high hysteresis losses or core lossesiv. This material is desired for use in permanent magnets.v. Alloys such as AlNiCo and NdFeB are hard magnetic materials. ???? ????? ????
Figure 5 hysteresis loop for feroomagnetic material
1.5.2 According to GMI aspect
Early advances and research growth
In the last century, the giant magneto-impedance (GMI) phenomena was first observed in
Ni-Fe alloy in 1935 by Harrison et al. 953, yet at that time, it did not attract a great
attention till 1994 as Panina and Mohri et al rediscover it once more with clearly. 963 as well as by Beach and
Berkowitz et al. 973. GMI was analyzed that time more clearly in soft magnetic Cobalt-based amorphous wires of around 120 microns for the diameter. The term “giant” was employed following the well-known giant magneto-resistance (GMR), which is a large variation of the material’s resistance with applied of an external magnetic field. While GMI determine the changes of the material complex impedance as a function of an external applied magnetic field. Since the first insights are very similar either GMI or GMR, for example, in both cases one can investigate a considerable change of the voltage drop across the samples with applied of an external magnetic field, but, the physical origin of both phenomena is completely different.
From the very basic point of view, in our case of GMI, the overall effect of the applied magnetic field is to induce strong modifications in the effective magnetic permeability, the main purpose that is related to define the field and current distribution within the sample. When a small external magnetic field is applied to a sample of soft magnetic material, a strong changing in the internal fields and electrical current density, and as a result, on the sample’s impedance as was reported in these ref. 96, 97,3 because of the magnetic permeability can change orders of magnitude when a rather small field is applied.
The effect, therefore, is strongly dependent on the frequency of the applied current and the magnetic anisotropies present in the material, which leads to discover a great number of interesting new magnetic phenomena.
Nowadays, many ongoing scientific reviews have dealt with different aspects of GMI either
as a research tool to observe some intrinsic and extrinsic magnetic properties of novel
industrial grown soft magnetic materials with different structure and shape 98, 993, or as a leading theory to assess deeper understanding of the mechanism behind GMI as well as to find out some expected behaviors under particular assumptions 100-1033. Each of these assumptions make a vital contribution properties, being interesting for several practical applications. It is should be noted that many of these applications are already proposed and tested in laboratory prototypes (i.e. portable digital display of the terrestrial magnetic field, brain tumor sensor, sensor for the induction motor control, car passing measurement and recording disk,finger-tip blood vessel pulsation, etc.) 98, 993. In the next few paragraphs we discussed the major principles of GMI that will help fully understande the GMI phenomena of glass-coated microwires.
As it was known, if a soft ferromagnetic material is presented to a small alternating current AC, a
huge change in the AC complex impedance of the material can be happened under an applied external magnetic field. The relative variation of the impedance, Z, with applied field, H, denotes the GMI effect (also known as GMI relation), and is expressed by:
?ZZ%= ZH – ZH MAXZH MAX*100where Hmax is usually the external magnetic field sufficient to saturate the sample magnetization.
The complex impedance of a linear electronic element, Z = R + j?L ( where R and L are the
resistance and inductance, respectively) is given by the ratio of the voltage amplitude Vac, measured on the element, to the current amplitude Iac of a sinusoidal current Iac sin ?t, passing through it.
Based on the values of frequency f of the AC current Iac passed through the sample,
Many types of GMI regimes can be observed:
Low frequency range (1–10 KHz): The skin depth is usually larger than the radius or the
sample (weak skin effect or negligible). The impedance changes, consequently, at these frequencies are of an inductive character because of a circular magnetization process exclusively and might not be considered properly as GMI effect as it has been mostly called magneto inductive effect.
moderate frequency from 10KHz to 10 MHz: The GMI obtained basically from variations of the magnetic penetration depth because of extreme changes of the effective magnetic permeability resulted from a DC magnetic field
For higher frequencies range from 10–1000 MHz: The GMI effect is also originated
by the skin effect of the soft magnetic conductor, i.e. must be attributed to the GMI. At these
frequencies the domain walls are strongly inhibited. As a result the magnetization rotation must be considered as responsible for the magnetic permeability change induced by an external magnetic field 96-983.
huge values of frequencies GHz band frequencies: gyromagnetic effect extremely effect on the magnetization rotation at these ranges 103,3 where strong changes of the sample’s impedance
have been attributed to the ferromagnetic resonance FMR 103, 1043. Upon increasing the
frequency, the GMI peaks are shifted to higher magnetic fields values for being the sample
In meantime the investigation of glass-coated microwires for high-frequency GMI effect, quite
striking and prodigious results have been measured either in as-prepared case or after
presenting the microwres to an external variety of annealing or stress, the latter are performed in order to induce specific magnetic anisotropies and inducing magnetic permeability, in turn, modifying GMI response. Around 600% GMI ratio has been measured by optimizing the magnetic anisotropy of nearly zero magnetostrictive Co-based amorphous microwires 108, 1093. The value of the magnetic field at which maximum GMI ratio is occurred increases as the diameter of the magnetic nucleus decreases with comparison to the diameter of glass shell. This is attributed to the different anisotropies induced by the stress from the coating.???? ???? ?????? ???? ???????
As already discussed benefits of using microwires and extensive progress in this research proves in future it will be grapping more attention due to its great properties. The most important advantages of microwire-based sensors are based on their ultra-small size and weightless, multiple utilization possibilities microwires sensors and may be used in potential applications for instance, in embedded systems, technical applications, and biomedical sensors.
As it is known that the nature of glass-coated microwires , having circular symmetry, superior magnetic properties, and can be produced commercially easily and cheap method, all these awesome properties make the microwires attractive for many prospective application in many field medical ,flight, and sensors .
Among very widely application trends in the field of magnetic materials: those cognate to sensing operations are proximately utilized in all engineering and industrial sectors. The rudimentary concept of many magnetic sensors are tied up with the well-known phenomena giant magneto-impedance (GMI).
Furthermore, also some of these applications are likely rooted to a fast and controllable domainwall (DW) motion or propagation throughout magnetic bistability. Yet, glass-coated microwires offer such existence of these two functionalities either GMI or magnetic bistability in a broad variety of compositions of each of two main families: Co-based or Fe-based alloys, respectively.
Obviously, the use of any magnetic material (including glass-coated microwires) in a certain type of application needs first to cross many barriers and specific criteria. Hither, we discuss some basic features of these two abovementioned phenomena GMI and magnetic bistability of glass-coated microwires since apparently they are the two main application trends of this class of soft magnetic materials.
1.5.1 Applications of microwires in flight industry
Magnetic microwires-based sensors or devices are utilized in a range of applications as tensile stress sensor, magnetic field sensors, and temperature sensors or in aviation, medical purpose. the actual fact that the magnetic microwires are used as sensors since it can detect the rising magnetic field disturbing such as from power plants, electrical sources or electric cables based boards whereas other sensors can be sensitive to disturbing the magnetic fields. We have a tendency to say that there are many advantages of using these
Glass-coated microwires are suitable to be used as a sensor for any environment difficultly. Magnetic microwire-based sensors can be used in a wide range of aviation applications as magnetic field sensors, tensile stress sensors or temperature sensors etc. They are very suitable to utilize on small unmanned aerial vehicles (UAVs) 602.
They are very suitable to utilize on small unmanned aerial vehicles (UAVs). An example of the placement of selected types of microwire-based sensors is shown in fig. 8. An example of the placement of selected types of microwire-based sensors is shown in fig. 8.
Figure 6: Magnetic Microwire-based sensor utilization in UAVs 602
Microwires-based sensors used in UAV as shown above having three sensors; magnetic microwires based sensors magnetic field sensor, magnetometer and tensile stress sensor.
1.5.2 Tensile stress sensors
Because the high sensitivity of microwire for external stress, microwire based tensile stress sensors is widely used to detect the operational load.
Hence, when any stress is applied on the sensor that can be calculated in terms of sensitivity. They can be used in structural health monitoring or aircraft construction monitoring as well because of their promising stress dependence affects.
In addition, because of its ultra-small size they can easily be embedded directly into materials. These sensors can also prove their worth in crack detection, in which we can easily calculate the possibilities of crack using these stress sensors. They can be easily fabricated for many different types of structure depending on their applications without influencing on the material properties. There is only one condition for using these microwires in that application, as in addition to a lot of advantage of it should be used only inside of nonmagnetic materials. 612.
1.5.3 Magnetic field sensors
Magnetic field sensors are often employed in form of many applications like for navigating purpose. They’re additionally being replaced by existing compasses. Another example of microwire-based inertial measurement units along side together with accelerometer and gyroscopes. Microwire-based sensors are often accustomed monitor the disturbance in any sensor which may be sensitive to external magnetic field arising like from any electrical supply, electrical circuits placed on board. These microwire-based sensors will facilitate to compensate the mentioned disturbing magnetic field which may be affecting the sensors sensitivity 622.Another possible way of using this microwire can be to determine the electric power consumption by flowing current though the wires. it’d facilitate to calculate the characteristics of a power plant?s performance.
The refinement of high-performance magnetic sensors has been trended based on GMI effect either diagonal or off-diagonal in thin wires. A number of magnetic sensors based on GMI effect and stress-impedance (SI) , have been reported by Aichi Steel Co. in Japan by Honkura and Morhi et al. 110, 1113, and Cobeño et al. 1123 which were in this case relevant for the detection of even low magnetic fields. in brief, a magnetic compass for cell phones and other portative devices utilizing off diagonal GMI effect with linear response to external magnetic field has been developed and prepared for a mass production 1133. The key factor is the ultra high sensitivity of GMI effect and the linear response to external magnetic field. Further development of GMI wire-base magnetic sensor by Uchiyama et al. 1143 has resulted in achieving a sensitivity.
above all, Zhukova et al has studied 1153 a number of sensors based on gathering magnetoelastic properties and SI effect of microwires, such as GMI-magnetoelastic sensor, and temperature sensor based on GMI effect in microwires with small Curie temperature, Tc. Within the utilization of thin glass-coated microwires with different structures and compositions.Biosensors 116-1183, however, primarily based on GMI effect in microwires had been developed and used to stumble on one-of-a-kind kinds of biomolecules binding on magnetic particles. New type of unfastened-area microwave sensing approach primarily based on embedded brief or lengthy ferromagnetic glass-coated microwire inclusion into the composites additionally took part of those developments 1193. while the dialogue inside the previous paragraph is but no longer complete, we will back once more via the give up of this chapter to close some boundaries of these abovementioned findings. Now, we pass directly to easily stretch the ones applications of glass-lined microwires but at the bases of some other phenomena, magnetization switching (bistability) and DW propagation.
1.5.4 Biomechanical applications
One of the most important advantages of Microwire is having ultra-small size which is make it more unique to be used as embedded sensors installed in biomechanical applications. Stress-sensitive microwires can be embedded as a tuneable micro-antenna to control the remote stress 652. It should be noted that this procedure can help to diagnose the instant adhesion quality with the wire and the composite?s matrix. For example, stress-sensitive microwires can be use in prosthetic devices to analyze it at micro-level to give the necessary diagnosis at interfacial conditions 662 as shown in figure below.
Figure 7A proposed stereotype prosthetic device with microwires embedded into implants 652. Courtesy of Dr. Makhnovskiy and Dr. Larissa Panina of Plymouth University
1.5.5 Other applications
many applications of the ferromagnetic microwires is that it can be embedded in different matrices. for example, these microwires are embedded right into a cellulose matrix and using a detector to comfy special documents, may be detected the papers without microwires as proven in determine 10 (a) (sure / no response technique of authentication) 672.
Figure 8(a) System for detecting the microwires from special securised papers and (b) Composite textile with inserted microwires for electromagnetic shielding 67.
ferromagnetic glass-coated microwires may be inserted in a fabric matrix or embedded in a polymeric matrix (constituted from epoxy resins or siliconic). the brand new composite materials, with ferromagnetic microwires as fillers, prove which helps to shield houses in opposition to electromagnetic radiations generated via exceptional sources defined in fig 10 (b).
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