Magnetic materials

04/15/2023 J.Subrahmanyam Confidential 1

Magnetic materials

1) Magnetic Induction or Magnetic Flux density (B): The magnetic

induction or magnetic flux density is the number of lines of magnetic force

passing through unit area perpendicularly. Where Φ is the magnetic flux

and A is the area of cross section. Units: Weber/m2 or Tesla.

2) Magnetic Field Intensity or Intensity of Magnetic Field (H):

Magnetic Field Intensity at any point in the magnetic field is the force

experienced by an unit north pole placed at that point. Units: A/m.


DELL

IMPORTANT APPLICATION

3) Magnetic Permeability (µ): It describes the nature of the material i.e.

it is a material property. It is the ease with which the material allows

magnetic lines of force to pass through it or the degree to which magnetic

field can penetrate a given medium. Mathematically it is equal to the ratio

of magnetic induction B inside a material to the applied magnetic field

intensity H. Units: H/m.


Defnition

4) Magnetization: Process of converting a non magnetic material into

magnetic sample.

5) Intensity of Magnetization (M): It is a material property. It is

defined as magnetic moment per unit volume in a material. Units: A/m.


04/15/2023 M V V K Srinivas Prasad Confidential 5



• Created by current through a coil:

• Relation for the applied magnetic field, H:

L

INH

applied magnetic fieldunits = (ampere-turns/m)

current

Magnetic Properties

magnetic field H

current I

N = total number of turnsL = length of the coil


• Magnetic induction results in the material

Response to a Magnetic Field

current I

B = Magnetic Induction (tesla) inside the material




Origin of magnetic dipoles

The spin of the electron produces a magnetic field with a

direction dependent on the quantum number ml.

The spin of the electron produces a magnetic field

with a direction dependent on the quantum number

ms.

Origin of magnetic dipoles



Electrons orbiting around the nucleus create a magnetic

field around the atom.

04/15/2023

M V V K Srinivas Prasad

Confidential 14

ORIGIN OF MAGNETISM IN MATERIALS

Nuclear spin

Orbital motion of electrons

Origin of Magnetism Spin of electrons

A moving electric charge, macroscopically or “microscopically” is responsible for Magnetism

Weak effect

Unpaired electrons required for net Magnetic Moment

Magnetic Moment resultant from the spin of a single unpaired electron→ Bohr Magneton = 9.273 x 1024 A/m2

This effect is Strong.

Permanent Dipoles

Alignment of dipoles

Direction of dipoles

Magnitudes of dipoles

Dia magnetic materials

Para, Ferro, Anti ferro,Ferri magnetic materials

YesNo

Para

Random Uniform

Ferro, Anti ferro, Ferri

Same

Ferro Anti ferro, Ferri

Sam

eAnti ferro

Ferri

Opposite

Different

Classification of magnetic Materials

04/15/2023 Confidential 15J.Subrahmanyam


Diamagnetic Materials


Properties • No permanent dipoles are present so net magnetic moment is

zero.

• The number of orientations of electronic orbits is such that the

vector sum of the magnetic moments is zero.

• External field will cause a rotation action on the individual

electronic orbits.

• Dipoles are induced in the material in presence of external

magnetic field.

04/15/2023 Confidential 18

No Applied

Magnetic Field (H = 0)Applied

Magnetic Field (H)

none

oppo

sing



• The external magnetic field produces induced magnetic

moment which is due to orbital magnetic moment..

• Induced magnetic moment is always in opposite direction of

the applied magnetic field.

• So magnetic induction in the specimen decreases.

• Magnetic susceptibility is small and negative.

• Repels magnetic lines of force.

• Diamagnetic susceptibility is independent of temperature and

applied magnetic field strength.


• Susceptibility is of the order of -10-5.

• Relative permeability is less than one.

• It is present in all materials, but since it is so weak it can be

observed only when other types of magnetism are totally

absent.

• Examples: Bi, Zn, gold, H2O, alkali earth elements (Be, Mg,

Ca, Sr), superconducting elements in superconducting

state.


paramagnetic Materials


Properties • If the orbital's are not completely filled or spins not balanced,

an overall small magnetic moment may exist.

• i.e. paramagnetism is because of orbital and spin magnetic

moments of the electron.

• Possess permanent dipoles.

• In the absence of external magnetic field all dipoles are

randomly oriented so net magnetic moment is zero.

• Spin alignment is random.

• The magnetic dipoles do not interact


No Applied

Magnetic Field (H = 0)

Applied

Magnetic Field (H)

rand

om

alig

ned


• In presence of magnetic field the material gets feebly

magnetized.

• i.e. the material allows few magnetic lines of force to pass

through it.

• Relative permeability µr >1

• The orientation of magnetic dipoles depends on temperature

and applied field.

• Susceptibility is independent of applied mag. field & depends

on temperature

• C is Curie constant



• With increase in temperature susceptibility decreases.

• Susceptibility is small and positive.

• These materials are used in lasers.

• Paramagnetic property of oxygen is used in NMR technique

for medical diagnose.

• The susceptibility range from 10-5 to 10-2.

• Examples: alkali metals (Li, Na, K, Rb), transition metals, Al,

Pt, Mn, Cr etc.




Ferromagnetic Materials


Properties • Origin for magnetism in Ferro mag. Materials are due to Spin

magnetic moment.

• Permanent dipoles are present so possess net magnetic

moment

• Material shows magnetic properties even in the absence of

external magnetic field.

• Possess spontaneous magnetization.

• Spontaneous magnetization is because of interaction between

dipoles called EXCHANGE COUPLING.


alig

ned

alig

ned

No Applied

Magnetic Field (H = 0)Applied

Magnetic Field (H)

• Magnetic susceptibility is as high as 106.

• So H << M. thus B = µoM


Magnetic induction

B (tesla)

Strength of applied magnetic field (H) (ampere-turns/m)

Ferromagnetic


• When placed in external mag. field it strongly attracts magnetic

lines of force.

• All spins are aligned parallel & in same direction.

• Susceptibility is large and positive, it is given by Curie Weiss Law

• C is Curie constant & θ is Curie temperature.

• When temp is greater than curie temp then the material gets

converted in to paramagnetic.

• They possess the property of HYSTERESIS.

• Material gets divided into small regions called domains.

• Examples: Fe, Co, Ni.

M V V K Srinivas Prasad04/15/2023 Confidential 35

Ferro magnetic Materials

Even when H = 0, the dipoles

tend to strongly align over

small patches.

When H is applied, the domains

align to produce a large net

magnetization.

04/15/2023M V V K Srinivas Prasad

Confidential 36

Thermal energy can randomize the spin

Ferromagnetic ParamagneticTcurie

Heat

Tc for different materials: Fe=1043 K, Ni=631 K,

Co=1400 K, Gd= 298 K


Curie Temperature

The temperature above (Tc) which ferromagnetic material become

paramagnetic.

Below the Curie temperature, the ferromagnetic is ordered and

above it, disordered.

The saturation magnetization goes to zero at the Curie

temperature.


Antiferro magnetic Material


Properties • The spin alignment is in antiparallel manner.• So net magnetic moment is zero.• Susceptibility depends on temperature.• Susceptibility is small and positive.• Initially susceptibility increases with increase in

temperature and beyond Neel temperature the susceptibility decreases with temperature.

• At Neel temperature susceptibility is maximum.

• Examples: FeO, MnO, Cr2O3 and salts of transition elements.

Nm TT

C




Ferrimagnetic Materials


Classification of Ferrimagnetic Materials

Ferrimagnetic Materials

Cubic Ferrites

MeFe2O4

Hexagonal Ferrites

AB12O19

Garnets

M3Fe5O12


Properties • Special type of ferro and antiferromagnetic material.• Generally oxides in nature.• Ionic in nature• Ceramic in nature so high resistivity (insulators)• The spin alignment is antiparallel but different

magnitude.• So they possess net magnetic moment.• Also called ferrites.

• General form MFe2O4

• Susceptibility is very large and positive.• Examples: ferrous ferrite, nickle ferrite


Ion

Mn2+ 3d5

E.C Spin Orientation Net Spin S Magnetic Moment

5/2 5µB

Fe2+ 3d6 2 4µB

Co2+ 3d7 3/2 3µB

Ni2+ 3d8 1 2µB

Cu2+ 3d9 1/2 1µB


Unpaired electrons give rise to ferromagnetism in alkali metals

Net magnetic moment

Na 3s1 1 B

Fe 3d64s2 4 B

atom crystal

2.2 B

Co 3d74s2 3 B 1.7 B

Ni 3d84s2 2 B 0.6 B


Ferrimagnetism

• All Fe2+ have a spin magnetic moment.

• Half of Fe3+ have a spin moment in on direction, the other half in the other (decreasing the overall moment to just that contributed by the Fe2+ ions).

Simpler picture showing a net magnetic moment.





Domain Theory of Ferromagnetic Materials



Lots and lots of domains in Ferro- (or Ferri-) Magnets

Domains form for a reason in ferro- and

ferrimagnetic materials. They are not random

structures.

What happens when magnetic field is applied to the ferromagnetic crystal?


Ferromagnetism

• Materials that retain a magnetization in zero field

• Quantum mechanical exchange interactions favour parallel alignment of moments

• Examples: iron, cobalt


• According to Becker, there are two independent processes which take place and lead to magnetization when magnetic field is applied.

1. Domain growth: Volume of domains oriented favourably w. r. t to the field at the expense of less favourably oriented domains.

2. Domain rotation: Rotation of the directions of magnetization towards the direction of the field.


Magnetic domains• Ferromagnetic

materials tend to form magnetic domains

• Each domain is magnetized in a different direction

• Domain structure minimizes energy due to stray fields


Magnetic domains

• Applying a field changes domain structure

• Domains with magnetization in direction of field grow

• Other domains shrink

Domain Structure and the Hysteresis Loop1.Domain growth:

1. Each domain is magnetized in a different direction

2. Applying a field changes domain structure. Domains with magnetization in direction of field grow.

3. Other domains shrink

2.Domain rotation: Finally by applying very strong fields can saturate

magnetization by creating single domain



Bloch walls - The boundaries between magnetic domains.

Domain Structure and the Hysteresis Loop

The entire change in spin direction between domains does not occur in one sudden jump across a single atomic plane rather takes place in a gradual way extending over many atomic planes.

Bloch Wall

The magnetic moments in adjoining atoms change direction continuously across the boundary between domains.


Magnetic domains

• Applying very strong fields can saturate magnetization by creating single domain


Hysteresis Curve

• Means lagging or retarding of an effect behind the cause

of the effect.

• Here effect is B & cause of the effect is H.

• Also called B H curve.

• Hysteresis in magnetic materials means lagging of

magnetic induction (B) or magnetization (M) behind the

magnetizing field (H).



• As the applied field (H) increases...

---the magnetic moment aligns with H.

• “Domains” with aligned magnetic moment grow at expense of poorly aligned ones!

H = 0

Applied Magnetic Field (H)

Mag

netic

in

duct

ion

(B)

0

Bsat

H

H

H

H

H

ferromagnetic or ferrimagnetic material initially unmagnetized


• Notice the permeability values depend upon the magnitude of H.

When a magnetic field is first applied to a magnetic material, magnetization initially increases slowly, then more rapidly as the domains begin to grow.

Later, magnetization slows, as domains must eventually rotate to reach saturation.




Hysteresis loop - The loop traced out by magnetization in a ferromagnetic or ferrimagnetic material as the magnetic field is cycled. OR

Hysteresis Loop

• Removing the field does not necessarily return domain structure to original state. Hence results in magnetic hysteresis.

Applied Magnetic Field (H)

1. initial (unmagnetized state)

B 2. apply H, cause alignment

4

Negative H needed to demagnitize!

. Coercivity, HC

3. remove H, alignment stays! => permanent magnet!

Ferromagnetism: Magnetic hysteresis

Ms – Saturation magnetization

Hc – Coercive force (the field needed to bring the magnetization back to zero)

Mrs – Saturation remanent magnetization

M

H

Mrs

Hc

Ms

04/15/2023 M V V K Srinivas Prasad 66


remanent magnetization = M0

coercivity = Hc


Domain growth reversible boundary displacements.

Domain growth irreversible boundary displacements.

Magnetization by domain rotation

Hysteresis Loop

• Means lagging or retarding of an effect behind the cause of the effect.

• Here effect is B & cause of the effect is H.

• Also called B H curve.• Hysteresis in magnetic

materials means lagging of magnetic induction (B) or magnetization (M) behind the magnetizing field (H).


Hysteresis, Remanence, & Coercivity of Ferromagnetic Materials


“hard” ferromagnetic material has a large M0 and large Hc.

“soft” ferromagnetic material has both a small M0 and Hc.

Hard versus Soft Magnets

High initial permeability.

Low coercivity.

Reaches to saturation magnetization with a

relatively low applied magnetic field.

It can be easily magnetized and demagnetized.

Low Hysteresis loss.

Applications involve, generators, motors, dynamos,

Cores of transformers and switching circuits.

Characteristics of soft magnetic materials:

Soft Magnets:


Importance of Soft Magnetic Materials:

Saturation magnetization can be changed by altering composition of the materials.

Ex:- substitution of Ni2+ in place of Fe2+ changes saturation magnetization of ferrous-Ferrite.

Susceptibility and coercivity which also influence the shape of the Hysteresis curve are sensitive to the structural variables rather than composition.

Low value of coercivity corresponds to the easy movement of domain walls as magnetic field changes magnitude and/ or direction.


Hard versus Soft Magnets

Characteristics of Hard magnetic materials:

Low initial permeability.

High coercivity and High remanence.

High saturation flux density.

Reaches to saturation magnetization with a

high applied magnetic field.

It can not be easily magnetized and

demagnetized.

High Hysteresis loss.

Used as permanent magnets.

Hard Magnets:

Importance of Hard magnetic material: Two important characteristics related to applications of these materials are

(i) Coercivity and (ii) energy product expressed as (BH)max with units in kJ/m3.

This corresponds to the area of largest B-H rectangle that can be constructed within the second quadrant of the Hysteresis curve.

Larger the value of energy product harder is the material in terms of its magnetic characteristics.

Schematic magnetization curve that displays hysteresis. Within the second quadrant are drawn two B–H energy product rectangles; the area of that rectangle labeled (BH)max is the largest possible, which is greater than the area defined by Bd–Hd

Who to get larger area of (BH)max i.e., who to produce Hard magnets?

Energy product represents the amount of energy required to demagnetize a permanent magnet.

Hysteresis behaviour depends upon the movement of domain walls.

The movement of domain walls depends on the final microstructure.Ex: the size, shape and orientation of crystal domains and impurities.

Microstructure will depend upon how the material is processed.

In a hard magnetic material, impurities are purposely introduced, to make it hard. Due to these impurities domain walls cannot move easily.

Finally the coercivity can increase and susceptibility can be decrease.

So large external field is required to demagnetization i.e., difficult to move the domain walls.

Baskar, Naren & G.Srinivas

Hard Magnetic Material Soft Magnetic MaterialHave large hysteresis loss. Have low hysteresis loss.

Domain wall moment is difficult Domain wall moment is relatively easier.

Coercivity & Retentivity are large. Coercivity & Retentivity are small.

Cannot be easily magnetized & demagnetized

Can be easily magnetized & demagnetized.

Magneto static energy is large. Magneto static energy is small.

Have small values of permeability and susceptibility

Have large values of susceptibility and permeability.

Used to make permanent magnets. Used to make electromagnets.

Iron-nickel-aluminum alloys, copper-nickle-iron alloys, copper–nickel– cobalt alloys

Iron- silicon alloys, ferrous- nickel alloys, ferrites, garnets.


Applications of

Magnetic Materials


Simulation of hard drive courtesy Martin Chen.Reprinted with permissionfrom International Business Machines Corporation.

• Head can... --apply magnetic field H & align domains (i.e., magnetize the medium). --detect a change in the magnetization of the medium.• Two media types:

MAGNETIC STORAGE• Information is stored by magnetizing material.


--Particulate: needle-shaped g-Fe2O3. +/- mag. moment along axis. (tape, floppy)


~60nm --Thin film: CoPtCr or CoCrTa alloy. Domains are ~ 10-30nm! (hard drive)



Magnetic hard drives



©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.





RELAYS


• Relays are electromagnetically operated switch.

• A relay is a control device consisting of a small electromagnet which, when energized by a current in its winding, attracts a piece of magnetic material, thus operating a switch in another circuit.


• A relay is a remote controlled switch capable of switching multiple circuits, either individually, simultaneously or in sequence.

• Relays are used where it is necessary to control a circuit by a low power signal or where several circuits are to be controlled by one signal.


Applications• Telecommunication system

• Computer interfaces

• Domestic appliances

• Air conditioning

• Traffic control

• Control of motors

• Business machines

• Electric power control


• Consists of a coil of wire surrounding a soft iron core and a movable iron armature and one or more set of contacts.

• When electric current is passed through the coil, it generates a magnetic field that attracts armature and a contact is made.


• Modern relays to use a permanent magnet for assisting both the energized and the deenergized conditions.

• These magnets must maintain their strength under all temperature and vibration extremes.

• Loss of magnetic field strength could cause the relay to change key operating


• MAGNETIC MATERIALS

• The three primary types of magnetic materials used are;

• A) Ceramic Types

• B) Alnico Types

• C) Rare Earth Types


Ceramic Type

• Ceramic magnets are composed of Strontium or Barium Ferrite.

• Ceramic magnets are hard and brittle and are extensively used in consumer products.

Advantages

1) They are the least expensive magnets.

2) They are very resistant to corrosion.

3) They are stable up to approximately 300°C.


Disadvantages

1) They are difficult to machine.2) They have a low energy product (3MGOe)3) They have a low/moderate coercively

(2KOe).4) magnets is cost is very low5) The low energy product will drive up the

volume of magnet6) magnetic flux can be lost rapidly with the

introduction of small demagnatising forces.


Alnico Type• Alnico magnets are made of alloys of

Aluminum, Nickel and Cobalt.

Advantages • 1) They are relatively inexpensive.• 2) They are stable up to very high

temperatures (550°C).• 3) They are very resistant to corrosion.


Disadvantages • 1) They are very difficult to machine.• 2) They have a low coercively (1KOe).• 3) They have a moderate energy product

(5MGOe).• Alnico does hold its magnetic properties at

very high temperatures• It can lose it’s magnetic strength under

conditions of shock


Rare Earth Type

• Alloys of the Rare Earths are the most advanced commercialized permanent magnet materials.

• These materials represent a significant improvement in permanent magnet properties.

• The two primary materials are the Samarium-Cobalt family and the Neodymium-Iron-Boron family.


Samarium – Cobalt Family• This family of magnets was developed in the

1970’s.• Applications requiring high magnetic energy

with little volume were1) Very high energy product (30MGOe).

2) Very high coercivity (10KOe).

3) Stable at high temperatures (350°C).

4) They are very resistant to corrosion.

5) They are the most expensive.

6) They are difficult to machine


Neodymium – Iron – Boron

• The discovery of Neodymium-Iron-Boron magnets discovered late in 1983 by Sumitono Special Metals and General Motors.

• These magnets are the highest energy permanent magnets.

• Less expensive than SmCo.


Advantages 1)Exceptionally high energy product (40MGOe).

2) Exceptionally high coercivity (15KOe).

3) Relatively easy to machine.

4) They are relatively inexpensive

Disadvantages

1) They do not resist corrosion.

2) They are not stable above 150°C.


SENSORS


SENSORS ?• American National Standards Institute• A device which provides a usable output in response to a specified

measure

• A sensor acquires a physical quantity and converts it into a signal suitable for processing (e.g. optical, electrical, mechanical)

• Nowadays common sensors convert measurement of physical phenomena into an electrical signal

• Active element of a sensor is called a transducer

Sensor

Input Signal Output Signal


Definition of a sensor• Def.

– A sensor is a device that receives a signal or stimulus and response with an electrical signal.

– Sensor is a device that measures a physical quantity and converts it into a signal which can be read by an absorber or by an instrument.


Magnets can be used to sense

– Position– Force– Torque– Speed– Rotation– Acceleration– current and magnetic field

Engineering

Magnetic materials