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Influence of annealing on structural transformations and magnetic properties of the Fe72Cu1V4Si15B8 alloy
No full textAPSTRAKT:
U ovoj disertaciji su prikazani rezultati ispitivanja legure Fe72Cu1V4Si15B8, koja
pripada familiji FINEMET-tipa legura. Legura je izrađena ultrabrzim hlađenjem
rastopa legure na rotirajućem disku. Dobijena legura ima već formirane nanokristalne
faze (α-Fe(Si) i Fe23B6 faze), koje se nalaze u amorfnoj matrici, što je ustanovljeno
XRD analizom i Mesbauerovom spektroskopijom. Pored toga, Mesbauerovom
spektroskopijom je ustanovljeno postojanje znatno više kristalnih faza koje zbog male
zastupljenosti nisu detektovane XRD analizom. Legura je termički tretirana, što je
indukovalo strukturne promene koje su doprinele promeni magnetnih svojstava. DTA
analizom je legura zagrevana do 1073 K, različitim brzinama (β= 5K/min, 10K/min i 20
K/min), kojom su ustanovljena dva kristalizaciona pika: (i) od 750 K do 780 K i (ii) od
875 K do 900 K (za β= 5 K/min). Oba pika su nesimetrična, što ukazuje na složenost
procesa kristalizacije. Ovi pikovi odgovaraju jednom termomagnetnom piku u
intervalu od 745 K do 875 K (za H=7960 A/m i β= 4 K/min). Termomagnetnim merenjima
je ustanovljena Kirijeva temperatura amorfne faze legure koja iznosi oko 615 K i
Kirijeva temperatura iskristalisane legure koja iznosi oko 875 K.
Nakon toga su uzorci oblika trake odgrevani različitim temperaturama (Tan= 573 K –
973 K). Ustanovljeno je da je pri Tan= 773 K došlo do najvećeg povećanja magnetizacije
zasićenja, Ms uz minimalno povećanje koercitivnog polja, Hc. Povećanje Ms se objašnjava
uvećanjem zastupljenosti α-Fe(Si) faze, sa porastom Tan, usled transformacije amorfne
u kristalnu fazu. Prosečna veličina novoformiranih kristalita α-Fe(Si) faze ista je
kao i veličini kristalita u neodgrevanoj leguri do Tan=773 K. Pri Tan>773 K dolazi do
naglog povećavanja prosečne veličine kristalita ove faze, što uzrokuje slabiju
pokretljivost magnetnih domena, zbog čega se smanjuje Ms. Isti trend promena Ms uočen
je i za longitudinalnu i normalnu (perpendikularnu) orijentaciju uzorka u magnetnom
polju u odnosu na temperaturu odgrevanja.
Konstantan porast Hc sa porastom Tan, povezano je sa boridnim fazama gvožđa, čiji se
udeo sa odgrevanjem takođe povećava. Naime, odgrevanjem se metastabilna faza Fe23B6
transformiše u stabilnu Fe2B fazu, a proces se završava na Tan= 723 K. Nemagnetni
atomi bora (B) koji okružuju α-Fe smanjuju izmensku interakciju atoma Fe, što je razlog
degradacije magnetno mekih svojstva.
Odgrevane trake su analizirane i Mesbauerovom spektroskopijom, gde su rezultati
pokazali da ni jedan od podspektara u potpunosti ne odgovara identifikovanim fazama
Fe23B6 i Fe2B. Ovo se objašnjava rastvorljivošću elemenata Si i V u kristalnoj rešetki
Fe, kao i bliskog atomskog faktora rasejanja atoma Fe i V, pa se pretpostavlja da u
kristalnim rešetkama ovih faza ima u manjoj meri ugrađenih atoma V i Si. Pored toga
na osnovu podspektara koji korespondiraju Fe(Si) fazi, pretpostavlja se da u pojedinim
delovima ove faze ima različitog rasporeda prvih suseda atoma Fe, kao i ugrađenog V,
što ukazuje na postojanje i drugih oblika kristalnih rešetki ove faze. Ove
pretpostavke su potvrđene skenirajućim elektronskim mikroskopom gde su EDS
analizom detektovana određena zrna bogata vanadijumom. Takođe, ovom metodom
pronađena su i mesta bogata bakrom, što sugeriše na postojanje klastera atoma bakra
koji se prvi izdvajaju prilikom kristalizacije FINEMET sistema i isti pospešuju
nukleaciju α-Fe(Si) faze...ABSTRACT:
This dissertation presents the results of examination of the alloy Fe72Cu1V4Si15B8, which
belongs to the FINEMET-type family of alloys. The alloy was prepared with a rapid quenching
of the melt on a rotating disc. The as-prepared alloy already contains nanocrystals of α-Fe(Si)
and Fe23B6 phases, in an amorphous matrix, that is observed by XRD analysis and by
Mössbauer spectroscopy. In addition, Mössbauer spectroscopy revealed the existence of
significantly more crystal phases that, due to their low presence, were not detected by XRD
analysis. The alloy was thermally treated, which induced structural changes that contributed to
the changes in magnetic properties. First, the alloy was heated to 1073 K, at different rates (β =
5K/min, 10K/min and 20 K/min), and two crystallization peaks were found by DTA analysis:
(i) from 750 K to 780 K, and (ii) from 875 K to 900 K (for β = 5 K/min). Both peaks are
asymmetric, which indicates the complexity of the crystallization process. These peaks
correspond to a single thermomagnetic peak in the range from 745 K to 875 K
(for H = 7960 A/ m and β = 4 K/ min). Thermomagnetic measurements revealed the Curie
temperature of the amorphous phase of the alloy at 615 K, and the Curie temperature of the
crystallized alloy at 875 K.
After that, the ribbon-shaped samples were annealed at different temperatures (Tan = 573 K -
973 K). It was found that at Tan = 773 K there was the huge increase in saturation magnetization
(Ms) with a minimal increase in the coercive field (Hc). The increase in Ms is explained by the
increase of the abundance of α-Fe (Si) phase, with the increase of Tan, due to the transformation
of amorphous into crystalline phase. The average size of the evolved crystallites of α-Fe (Si)
phase is the same as the size of crystallites in the as-prepared alloy up to Tan = 773 K. At Tan >
773 K there is a sharp increase in the average size of crystallites of this phase, which causes
slow mobility of magnetic domains due to reduced width of the domain walls, that is why the
Ms decreases. The same trend of changes in Ms was observed for both longitudinal and
perpendicular orientation of the sample in the magnetic field with increase of the annealing
temperature.
The constant increase of Hc with the increase of Tan is connected with the boride phases of iron,
whose weight fraction also increases with the annealing. By annealing, the metastable phase
Fe23B6 is transformed into a stable Fe2B phase, and the process ends at Tan = 723 K. Nonmagnetic boron atoms (B) surrounding α-Fe reduce the exchange interaction of Fe atoms, and
consequently, the degradation of magnetically soft properties occurs.
The annealed ribbons were also analysed by Mössbauer spectroscopy, where the results showed
that none of the sub-spectra fully corresponded to the identified Fe23B6 and Fe2B phases. This
is explained by the solubility of the elements Si and V in the crystal lattice of Fe, as well as by
the similar atomic scattering factor of Fe and V atoms. It is assumed that some of V and Si
atoms are embedded in the crystalline lattices of these phases. In addition, based on the subspectra corresponding to the Fe (Si) phase, it is assumed that in some parts of this phase there
is a different arrangement of the first neighbors of the Fe atom, as well as a built-in V, which
indicates the existence of other crystalline forms of this phase. These assumptions were
confirmed by scanning electron microscopy where certain vanadium-rich grains were detected
by EDS analysis. Also, with this method, copper-rich sites were found, which suggests the
existence of clusters of copper atoms that are formed at the onset of crystallization of the
FINEMET system and they accelerate the nucleation of the α-Fe (Si) phase..
Influence of annealing on structural transformations and magnetic properties of the Fe72Cu1V4Si15B8 alloy
Get PDFAPSTRAKT:
U ovoj disertaciji su prikazani rezultati ispitivanja legure Fe72Cu1V4Si15B8, koja
pripada familiji FINEMET-tipa legura. Legura je izrađena ultrabrzim hlađenjem
rastopa legure na rotirajućem disku. Dobijena legura ima već formirane nanokristalne
faze (α-Fe(Si) i Fe23B6 faze), koje se nalaze u amorfnoj matrici, što je ustanovljeno
XRD analizom i Mesbauerovom spektroskopijom. Pored toga, Mesbauerovom
spektroskopijom je ustanovljeno postojanje znatno više kristalnih faza koje zbog male
zastupljenosti nisu detektovane XRD analizom. Legura je termički tretirana, što je
indukovalo strukturne promene koje su doprinele promeni magnetnih svojstava. DTA
analizom je legura zagrevana do 1073 K, različitim brzinama (β= 5K/min, 10K/min i 20
K/min), kojom su ustanovljena dva kristalizaciona pika: (i) od 750 K do 780 K i (ii) od
875 K do 900 K (za β= 5 K/min). Oba pika su nesimetrična, što ukazuje na složenost
procesa kristalizacije. Ovi pikovi odgovaraju jednom termomagnetnom piku u
intervalu od 745 K do 875 K (za H=7960 A/m i β= 4 K/min). Termomagnetnim merenjima
je ustanovljena Kirijeva temperatura amorfne faze legure koja iznosi oko 615 K i
Kirijeva temperatura iskristalisane legure koja iznosi oko 875 K.
Nakon toga su uzorci oblika trake odgrevani različitim temperaturama (Tan= 573 K –
973 K). Ustanovljeno je da je pri Tan= 773 K došlo do najvećeg povećanja magnetizacije
zasićenja, Ms uz minimalno povećanje koercitivnog polja, Hc. Povećanje Ms se objašnjava
uvećanjem zastupljenosti α-Fe(Si) faze, sa porastom Tan, usled transformacije amorfne
u kristalnu fazu. Prosečna veličina novoformiranih kristalita α-Fe(Si) faze ista je
kao i veličini kristalita u neodgrevanoj leguri do Tan=773 K. Pri Tan>773 K dolazi do
naglog povećavanja prosečne veličine kristalita ove faze, što uzrokuje slabiju
pokretljivost magnetnih domena, zbog čega se smanjuje Ms. Isti trend promena Ms uočen
je i za longitudinalnu i normalnu (perpendikularnu) orijentaciju uzorka u magnetnom
polju u odnosu na temperaturu odgrevanja.
Konstantan porast Hc sa porastom Tan, povezano je sa boridnim fazama gvožđa, čiji se
udeo sa odgrevanjem takođe povećava. Naime, odgrevanjem se metastabilna faza Fe23B6
transformiše u stabilnu Fe2B fazu, a proces se završava na Tan= 723 K. Nemagnetni
atomi bora (B) koji okružuju α-Fe smanjuju izmensku interakciju atoma Fe, što je razlog
degradacije magnetno mekih svojstva.
Odgrevane trake su analizirane i Mesbauerovom spektroskopijom, gde su rezultati
pokazali da ni jedan od podspektara u potpunosti ne odgovara identifikovanim fazama
Fe23B6 i Fe2B. Ovo se objašnjava rastvorljivošću elemenata Si i V u kristalnoj rešetki
Fe, kao i bliskog atomskog faktora rasejanja atoma Fe i V, pa se pretpostavlja da u
kristalnim rešetkama ovih faza ima u manjoj meri ugrađenih atoma V i Si. Pored toga
na osnovu podspektara koji korespondiraju Fe(Si) fazi, pretpostavlja se da u pojedinim
delovima ove faze ima različitog rasporeda prvih suseda atoma Fe, kao i ugrađenog V,
što ukazuje na postojanje i drugih oblika kristalnih rešetki ove faze. Ove
pretpostavke su potvrđene skenirajućim elektronskim mikroskopom gde su EDS
analizom detektovana određena zrna bogata vanadijumom. Takođe, ovom metodom
pronađena su i mesta bogata bakrom, što sugeriše na postojanje klastera atoma bakra
koji se prvi izdvajaju prilikom kristalizacije FINEMET sistema i isti pospešuju
nukleaciju α-Fe(Si) faze...ABSTRACT:
This dissertation presents the results of examination of the alloy Fe72Cu1V4Si15B8, which
belongs to the FINEMET-type family of alloys. The alloy was prepared with a rapid quenching
of the melt on a rotating disc. The as-prepared alloy already contains nanocrystals of α-Fe(Si)
and Fe23B6 phases, in an amorphous matrix, that is observed by XRD analysis and by
Mössbauer spectroscopy. In addition, Mössbauer spectroscopy revealed the existence of
significantly more crystal phases that, due to their low presence, were not detected by XRD
analysis. The alloy was thermally treated, which induced structural changes that contributed to
the changes in magnetic properties. First, the alloy was heated to 1073 K, at different rates (β =
5K/min, 10K/min and 20 K/min), and two crystallization peaks were found by DTA analysis:
(i) from 750 K to 780 K, and (ii) from 875 K to 900 K (for β = 5 K/min). Both peaks are
asymmetric, which indicates the complexity of the crystallization process. These peaks
correspond to a single thermomagnetic peak in the range from 745 K to 875 K
(for H = 7960 A/ m and β = 4 K/ min). Thermomagnetic measurements revealed the Curie
temperature of the amorphous phase of the alloy at 615 K, and the Curie temperature of the
crystallized alloy at 875 K.
After that, the ribbon-shaped samples were annealed at different temperatures (Tan = 573 K -
973 K). It was found that at Tan = 773 K there was the huge increase in saturation magnetization
(Ms) with a minimal increase in the coercive field (Hc). The increase in Ms is explained by the
increase of the abundance of α-Fe (Si) phase, with the increase of Tan, due to the transformation
of amorphous into crystalline phase. The average size of the evolved crystallites of α-Fe (Si)
phase is the same as the size of crystallites in the as-prepared alloy up to Tan = 773 K. At Tan >
773 K there is a sharp increase in the average size of crystallites of this phase, which causes
slow mobility of magnetic domains due to reduced width of the domain walls, that is why the
Ms decreases. The same trend of changes in Ms was observed for both longitudinal and
perpendicular orientation of the sample in the magnetic field with increase of the annealing
temperature.
The constant increase of Hc with the increase of Tan is connected with the boride phases of iron,
whose weight fraction also increases with the annealing. By annealing, the metastable phase
Fe23B6 is transformed into a stable Fe2B phase, and the process ends at Tan = 723 K. Nonmagnetic boron atoms (B) surrounding α-Fe reduce the exchange interaction of Fe atoms, and
consequently, the degradation of magnetically soft properties occurs.
The annealed ribbons were also analysed by Mössbauer spectroscopy, where the results showed
that none of the sub-spectra fully corresponded to the identified Fe23B6 and Fe2B phases. This
is explained by the solubility of the elements Si and V in the crystal lattice of Fe, as well as by
the similar atomic scattering factor of Fe and V atoms. It is assumed that some of V and Si
atoms are embedded in the crystalline lattices of these phases. In addition, based on the subspectra corresponding to the Fe (Si) phase, it is assumed that in some parts of this phase there
is a different arrangement of the first neighbors of the Fe atom, as well as a built-in V, which
indicates the existence of other crystalline forms of this phase. These assumptions were
confirmed by scanning electron microscopy where certain vanadium-rich grains were detected
by EDS analysis. Also, with this method, copper-rich sites were found, which suggests the
existence of clusters of copper atoms that are formed at the onset of crystallization of the
FINEMET system and they accelerate the nucleation of the α-Fe (Si) phase..
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Magnetoimpedance effect of soft magnetic amorphous wires : doctoral disertation
No full textU 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Magnetoimpedance effect of soft magnetic amorphous wires : doctoral disertation
No full textU 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
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