magnetic materials.doc

8/14/2019 Magnetic Materials.doc

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MAGNETIC PROPERTIES

Lecture 1:

Learning objective(Aim: To un!er"tan! #$ati" meant b% magnetic materia&" an! t$eir

Origin o' ermanent magnetic moment" in "o&i!":

1. orbital magnetic moment of electrons

2. spin magnetic moment of electrons

3. spin magnetic moment of nucleus

We will consider only spin magnetic moment of electrons

)igure *1 Origin o' magnetic !io&e": (a T$e "in o' t$e e&ectron

ro!uce" a magnetic 'ie&! #it$ a !irection !een!ent on t$e

+uantum number m"* (b E&ectron" E&ectron" orbiting aroun! t$e

nuc&eu" create a magnetic 'ie&! aroun! t$e atom*

Magnetic '&u, !en"it% (-:is defined as the number of magnetic field lines

passing unit area of a surface surrounding the source of magnetic field.

Magnetic lines of force are expressed in units of Weber and flux density has

units of Weber/meter2or Tesla.

Magnetic 'ie&! "trengt$ (.:When a medium is exposed to a magnetic

field of intensity H it causes an induction ! in the medium. "i.e.# BH


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or HB = . where $% is the permeability of the medium. &f the surrounding

medium is 'acuum or air HB o= where $o%is the permeability of 'acuum.

$H% is expressed in units of (/m "(mpere per meter#.

Magneti/ation (M) When a material is placed in a magnetic field a net

magnetic moment is created due to all magnetic dipoles. Magneti*ation is

defined as the magnetic moment per unit 'olume. &t has the same unit as

$H%.

Magnetic ermeabi&it% (r an! "u"cetibi&it% (m)These quantities denote the ease with which a material allows magnetic lines of force to

pass through it.HB = for a magnetic material and HB o= for air or 'acuum.

+ne defines a ratio

r

o

=

as the relati'e permeability of the medium with respect to that

of 'acuum. Magneti*ation of a material is proportional to the magnetic

field intensity applied. "i.e.# HM or HM m= where mis defined as

the susceptibility. &t is a dimensionless ,uantity.

RELATIONS.IP -ET0EEN - . M ran! :

)MH(HHB oor +===

-sing HM m= we get )(HH moor += 1

gi'ing us mr += 1


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)igure 12*3 A current a""ing t$roug$ a coi& "et" u a magnetic

'ie&! H#it$ a '&u, !en"it% B* T$e '&u, !en"it% i" $ig$er #$en a

magnetic core i" &ace! #it$in t$e coi&*

-o$r magneton -

The magnetic moment due to spin of a single electron is called the !ohr

magneton !

! .203 x 12 ( m2

4et moment of two electrons of opposite spins

Lecture 3:

T%e o' Magnetic Materia&":

5oft Magnetic materials

Hard magnet

C&a""i'ication o' Magnetic Materia&"


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6erromagnetism

7ara Magnetism

8iamagnetism

(nti ferromagnetism

6erri magnetism

)ERRO MAGNETISM:

&t is a special case of 7aramagnetism.

There is a special form of interaction called exchange coupling between ad9acent

dipoles coupling their magnetic moments together in rigid parallelism. 8omain

structure is characteristic of 6erromagnetism. ( specimen may ha'e different

domains and in each domain all the dipoles are in one direction. (lignment of themagnetic moments of atoms in the same direction so that a net

magneti*ation remains after the magnetic field is remo'ed.

1. 5pontaneous magneti*ation is characteristic of 6erromagnetism.2. Hysteresis is exhibited.3. &t is a 'ery strong effect: a 6erromagnetic material is strongly attracted to

a con'entional magnet.. 5usceptibility is 'ery large and depends on temperature.;. (bo'e a certain temperature $Tc% called the e 6erromagnetism anti6erromagnetism is associated with domain

structure and exchange coupling between ad9acent spins but the alignment


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1. 6errimagnetic materials possess net magnetic moment.2. (bo'e e 6erromagnetic materials and ha'e

hysteresis domain structure etc. but being oxide compounds they ha'elarge resisti'ities.A. They are called 6errites.

0. (bo'e the


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Type of

MagnetismSusceptibilityAtomic / Magnetic Behavior

Example /

Susceptibility

Diamagnetism

Small &

negative.

Atoms have

no magneticmoment

Au

Cu

-2.74x10-6

-0.77x10-6

ParamagnetismSmall &

ositive.

Atoms have

ran!oml"

oriente!

magnetic

moments

#-Sn

Pt

$n

0.1%x10-6

21.04x10-

6

66.10x10-

6

erromagnetism

'arge &

ositive(

)unction o)

alie! )iel!(

microstructure

!een!ent.

Atoms have

arallel

aligne!

magnetic

moments

e *100(000

Anti)erromagnetismSmall &

ositive.

Atoms have

mixe!

arallel an!

anti-arallel

aligne!magnetic

moments

Cr +.6x10-6
http://www.aacg.bham.ac.uk/magnetic_materials/type.htm#Diamagnetismhttp://www.aacg.bham.ac.uk/magnetic_materials/type.htm#Paramagnetismhttp://www.aacg.bham.ac.uk/magnetic_materials/type.htm#Ferromagnetismhttp://www.aacg.bham.ac.uk/magnetic_materials/type.htm#Antiferromagnetismhttp://www.aacg.bham.ac.uk/magnetic_materials/type.htm#Diamagnetismhttp://www.aacg.bham.ac.uk/magnetic_materials/type.htm#Paramagnetismhttp://www.aacg.bham.ac.uk/magnetic_materials/type.htm#Ferromagnetismhttp://www.aacg.bham.ac.uk/magnetic_materials/type.htm#Antiferromagnetism


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Lecture 6:


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6igure 1.; "a# ( ,ualitati'e s>etch of magnetic domains in a

polycrystalline material. The dashed lines show demarcation

between different magnetic domains: the dar> cur'es show thegrain boundaries. "b# The magnetic moments in ad9oining atoms

change direction continuously across the boundary between

domains.


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Lecture 7:

SO)T 8 .AR4 MAGNETIC MATERIALS:

.ar! magnet" So't magnet"

1. Ha'e large hysteresis loss. 1. Ha'e low hysteresis

loss.

2. 8omain wall moment is difficult 2. 8omain wall moment is

relati'ely

easier.

3. elaluminum alloys D. ?xamples) &ron silicon

alloys

copper nic>el iron alloys. ferrous nic>el alloys.


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copper @ nic>el @ cobalt alloys. ferrites garnets.


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Lecture 9:


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)ERRITES: (magnetic roertie" 8 a&ication"

6errites are ferrimagnetic materials which ha'e the chemical formula M2E6e3E+2

where M is a di'alent element li>e 6e


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where Tcis the


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&ron crystalli*es in bcc crystal structure. (ll the dipoles in iron are aligned

parallel to I1J direction. Hence when a magnetic field is applied along

I1J direction of iron it gets easily magneti*ed. &t reaches saturation

magneti*ation e'en for a small 'alue of magnetic field. +n the other hand

when iron is magneti*ed by applying the field along the I111J direction it

re,uires somewhat a larger 'alue of magnetic field to magneti*e it.

Generally it needs magnetic field to magneti*e along the I111J direction

nearly four times than that of the field re,uired for I1J direction while the

I11J direction of iron re,uires a medium 'alue of magnetic field to

magneti*e. This dependence of magnetic beha'ior on crystallographic

directions is called Kmagnetic anisotropyL. The magneti*ation of iron and

nic>el along different directions are shown in below figures.

The direction I1J is the easiest direction of magneti*ation for iron. The

direction I111J is the hard direction. The direction I11J is the medium

direction.

6or nic>el the easy direction is I111J the hard direction is I1J. The

excess energy re,uired per unit 'olume of a substance to magneti*e it along

a particular direction with respect to an easy direction is >nown as

anisotropy energy.

Magneto"triction E''ect:

The change in the dimension of a ferromagnetic material when it is

magneti*ed is >nown as magnetostriction. The deformation is different along

different crystal directions but it is independent of the direction of the

applied field. 8epending on the nature of the material the dimension mayeither increase or decrease. 6or a nic>el rod the length decreases while for a

permalloy the length increases in the presence of magnetic field.

When placed inside alternating field the rod 'ibrates with a fre,uency

twice that of the fre,uency of alternating field. &f the fre,uency of the

alternating field coincides with the natural fre,uency of 'ibration of the rod

due to resonance the amplitude of 'ibration increases. This magnetostriction


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effect is used in generation of ultrasonic wa'es. Thus the magnetostriction

energy is the energy due to the mechanical stresses generated by domain

rotation. The magnetic permeability is related to magnetostriction. 6or high

permeability materials application the magnetostriction effect must be

small.

Energ% ro!uct o' magnetic materia&:

Hard magnetic materials possess 'ery large coerci'ity and permanent

magnetic fields. The hard magnetic materials are used for processing the

permanent magnets and hence the energy stored in the magnetic materials

is 'ery large. The energy stored in the hard magnetic material is used to do

wor>. The energy stored in a magnetic material depends on the maximum

area of the rectangle that fits the !H cur'e in second ,uadrant as shown in

fig.

The energy stored in the magnetic material is approximately gi'en by?max !xHxN

Where !H and N are the flux density field and 'olume of the magnetic

material. The magnetic energy density is proportional to the product of ! and

H of the magnetic material and "!H#maxrepresents the maximum energy of

the magnetic material. Therefore "!H#max is the important parameter for

comparing the hard magnetic materials.

So't magnetic materia&":

!oth ferromagnetic and ferrimagnetic materials are classified as either

soft or hard on the basis of their hysteresis characteristics.

5oft magnetic materials are used in applications re,uiring fre,uent

re'ersals of the directions of magneti*ation such as cores of transformers

motors inductors and generators. &n soft magnetic materials the hysteresis

losses must be small. More o'er the soft magnetic materials must ha'e a

high initial permeability and a low coerci'ity. ( material possessing these

properties can reach its saturation magneti*ation with low applied field.


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to restrict the motion of domain walls and thus increase the coerci'ity.


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Colling is the techni,ue by which sheet transformer cores are fabricated.

( flat sheet that has been rolled is called rolling texture or rolling sheet i.e.

all grains orient in the direction of rolling. 6or this type of texture during

rolling operation for most of the grains in the sheet a specific

crystallographic plane "h > l# becomes aligned parallel or nearly parallel to

the surface of sheet. &n addition a direction Iu ' wJ in that plane lies parallel

or nearly parallel to the rolling direction.

Thus a rolling texture is indicated by the planedirection combination "h >

l#Iu ' wJ. 6or body centered cubic alloys including 6e5i alloy the rolling

texture is "1 1 #I 1J which is represented in below fig. Thus transformer

cores of this ironsilicon alloy are fabricated such the direction in which the

sheet was rolled is aligned parallel to the direction of applied magnetic field.

.ar! Magnetic Materia&": Hard magnetic materials are used in permanent magnets which must

ha'e a high resistance to demagneti*ation. (lso a hard magnetic material

has a high remanance high coerci'ity high saturation flux density as well as

a low initial permeability and high hysteresis energy losses. The hysteresis

cur'e for hard magnetic material is as shown in fig.

The two most important characteristics related to applications for these

materials are the coerci'ity and energy product "!H#max. The 'alue of the

energy product represents the amout of energy re,uired to demagneti*e a

permanent magnet. &f "!H#maxis large that material will be hard in terms of

its magnetic characteristics.

Hysteresis beha'ior depends upon the mo'ement of domain walls. The

mo'ement of domain walls depends on the final microstructure i.e. the si*e

shape and orientation of crystal domains and impurities. +f course

microstructure will depend upon how the material is processed. The hard

magnetic materials are prepared by heating the magnetic materials to the

re,uired temperature and then suddenly cooling them by dipping in a cold

li,uid. &n a hard magnetic material impurities are purposely introduced to

ma>e it hard. 8ue to these impurities domain walls can not mo'e easily. &n

this way we can increase the coerci'ity and decrease the susceptibility'alues by obstructing domain wall motion. 5o large external field is re,uired

for demagneti*ation.

There are two types of hard magnetic materials. Those are

1.


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e

the Gadolinium doped 5m2


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magnets are far superior to electromagnets in that their magnetic fields are

continuously maintained with out the necessity of expending electrical

power. 6urther more no heat is generated during operation. Motors using

permanent magnets are much smaller than their electromagnets motors.

6amiliar motor applications) &n cordless drills and screw dri'ers in

automobiles li>e fan motors washer wiper window winder in audio and

'ideo recorders and in cloc>s spea>ers in audio systems light weight

earphones and computer peripherals.

Ne# magnetic materia&" 'or re&a% !e"igne":

Modern relays use permanent magnets. These magnets must maintain

their strength under all temperatures.

5amariumcobalt magnets ha'e demonstrated stable field strength into

the range of 3;


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Most relay applications demand stable performance in the 2< range.

s.

Recor!ing $ea! materia&":

( recording head material should be made up of a soft magnetic materialha'ing low coerci'ity and high saturation magneti*ation. !ecause its

magneti*ation has to follow the input signal Icurrent or magnetic field

strengthJ.

?x) 7ermalloy"4i6e alloys# 5endust"6e(l5i alloys#

5oft ferrites) MnFn and 4iFn ferrites.

Recor!ing roce"":

Magnetic recording of a signal on a tape or dis> is shown in fig. The tape

is a polymer pac>ing tape that has a coating of magnetic material on it. The

audio signal or 'ideo signal to be recorded is con'erted into current signal

and is passed through a toroid type electromagnet with a small air gap. Thisair gapped core electromagnet is called recording head. Whene'er the

current signal is passed through a coil which is wound around the

electromagnet core it produces a magnetic field in that material. This

current signal also produces a magnetic field in the air gap.


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recording head or writing head magneti*es the magnetic material present in

the tape. The magneti*ation produced on the tape is proportional to the

current signal. This magneti*ation is retained in the material when field is

remo'ed or current signal is off. Thus the signal gets stored in the tape.

(ctually the data to be stored or recorded is in the form of time se,uence

of binary digits "one and *ero# or bits. These bits are con'erted into an

electric current wa'e form that passes through a coil of writing head. ( $one%

bit corresponds to a change in current polarity while a *ero bit corresponds

to no change in polarity of the writing current signal. Thus a mo'ing dis> or

tape is magneti*ed in the positi'e direction for positi'e current and in the

negati'e direction for negati'e direction for negati'e current flow.

Rea!ing roce"":

The recording head used for recording on tape is also used for reading "or

playing audio cassette# the tape. The reading process in a tape is based on

the principle of 6araday%s law of induction. ( portion of the magnetic field

present in the tape penetrates through the recording head when the tape is

in touch with the head. (s the tape is mo'ing with a constant 'elocity

'oltage is induced due to change in magnetic field. This can be amplified andthen con'erted bac> into its original form.


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Storage o' Magnetic 4ata: "Tapes 6loppy and Magnetic 8isc 8ri'es#

Magnetic data can be stored in de'ices li>e tapes floppy and hard dis>.

The magnetic materials used for storage should retain the information

recorded in them when magnetic field is remo'ed. This re,uires high

remanent magneti*ation. Therfore the storage media should be made up of a

material ha'ing high remanent magneti*ation and optimum 'alue of

coerci'ity. The coerci'ity determines the stability of recording. The coerci'ity

cannot be too high since it can stop the reading. Cegarding magnetic

properties the hysteresis loops for these magnetic storage media should be

relati'ely large and s,uare.

There are two types of magnetic storage media. Those are particulate

and thin film. 7articulate media consist of 'ery small needle li>e or acicular

particles.

?x) R6e2+3ferrite ing efficiency of thin film domains is greater than acicular particles.

There is 'oid space in between particles. !s'alues for particulate media lie in

between . and .ATesls. 6or thin films !s'alues lie in between .A and

1.2Tesla.

Lecture 251


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