SmithCh16

8/13/2019 SmithCh16

1/28

CHAPTER

16Magnetic

Properties

16-1

8/13/2019 SmithCh16

2/28

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Magnetic Materials

Very important in electrical engineering

Soft magnetic materials: Materials that can

be easily magnetized and demagnetized.

Applications:Transformer cores, stator

and rotor materials.

Hard magnetic materials: Cannot be easily

demagnetized (permanent magnets).

Applications:Loud speakers, telephone

receivers.

16-2

8/13/2019 SmithCh16

3/28


Magnetic Fields

Ferromagnetic materials: Iron, cobalt and nickel -

provide strong magnetic field when magnetized.

Magnetism is dipolarup to atomic level.

Magnetic fields are also produced by current carrying

conductors.

Magnetic field of a solenoid is

H = 0.4 n i /l A/m

n = number of turnsl = length

i = current

Figure 15.3a16-3

After C.S. Barrett, W. D. Nix, and A. S. Teteman, Principles of Engineering Materials, Prentice-Hall, 1973, p.459.

8/13/2019 SmithCh16

4/28


Magnetic Induction

If demagnetized iron bar is placed inside a solenoid,

the magnetic field outside solenoid increases.

The magnetic field due to the bar adds to that of

solenoid - Magnetic induction (B) .

Intensity of Magnetization(M) : Induced magnetic

moment per unit volume

B = 0H + 0M = 0(H+M)

0 = permeability of free space

= 4 x 10-7 (Tm/A)

In most cases 0 >0 H

Therefore B =~ M

Figure 15.3b

16-4 After C.S. Barrett, W. D. Nix, and A. S. Teteman, Principles of Engineering Materials, Prentice-Hall, 1973, p.459.

8/13/2019 SmithCh16

5/28


Magnetic Permeability

Magnetic permeability= = B/H

Magnetic susceptibility= Xm= M/H

For vacuum = 0= = 4 x 10-7 (Tm/A)

Relative permeability= r= / 0

B = 0r H Relative permeability is

measure of induced magnetic field.

Magnetic materials that

are easily magnetizedhave high magnetic

permeability.

Figure 15.4

16-5

8/13/2019 SmithCh16

6/28


Types of Magnetism

Magnetic fields and forces are due to intrinsic spin of

electrons.

Diamagnetism:External magnetic field unbalances

orbiting electrons causing dipoles that appose applied

field.

very small negative magnetic susceptibility.

Paramagnetism:Materials exhibit small positive

magnetic susceptibility.

Paramagnetic effect disappears when the applied

magnetic field is removed.

Produced by alignment of individual dipole

momentsof atoms or molecules.

16-6

8/13/2019 SmithCh16

7/28


Ferromagnetism

Ferromagnetic elements (Fe, Co, Ni and Gd) produce

large magnetic fields.

It is due to spin of the 3d electrons of adjacent atoms

aligning in parallel directionsin microscopic domains

by spontaneous magnetization.

Random orientation of domains results in no netmagnetization.

The ratio of atomic

spacing to diameter

of 3d orbit must be

1.4 to 2.7.

Figure 15.616-7

8/13/2019 SmithCh16

8/28


Magnetic Moments of a Single Unpaired Electron

Each electron spinning about its own axis has dipole

moment B

B = e h / 4 m

In paired electrons

positive and negative

moments cancel.

Antiferromagnetism:In presence of magnetic field,

magnetic dipoles align in apposite directions .

Examples:-Manganese and Chromium.

Ferrimagnetism:Ions of ceramicshave different

magnitudes of magnetic moments and are aligned in

antiparallel manner creating net magnetic moments.

B= Bohr magneton

e = electron charge

h = planks constant

m = electron mass

16-8

8/13/2019 SmithCh16

9/28


Effect of Temperature on Ferromagnetism

Above 0 K, thermal energy causes magnetic dipoles to

deviatefrom parallel arrangement.

At higher temperature, (curie temperature)

ferromagnetism is completely lost and material

becomes paramagnetic.

On cooling, ferromagnetic

domains reform.

Examples:Fe 7700C

Co 11230C

Ni 3580C

Figure 15.9

16-9

8/13/2019 SmithCh16

10/28


Ferromagnetic Domains

Magnetic dipole moments align themselves in parallel

direction called magnetic domains.

When demagnetized, domains are rearranged in

random order.

When external magnetic

field is applied the domains

that have moments parallel

to applied filed grow.

When domain growthfinishes, domain rotation

occurs.Figure 15.11

16-10 After R.M. Rosw, L. A. Shepard, and J. Wulff, Structure and Properties of Materials, vol. IV: Wiley, 1996, p.193.

8/13/2019 SmithCh16

11/28


Types of Energies that Determine the Structure

Most stable structure is attained when overall potential

energyis minimum.

Potential energy with a domain is minimized when all

atomic dipoles are aligned in single direction.

Magnetostatic energy:Potential energy produced by

its external field.

Formation of multiple

domain reduces

magnetostatic energy.

Figure 15.13

16-11

C i ht Th M G Hill C i I P i i i d f d ti di l

8/13/2019 SmithCh16

12/28


Magnetocrystalline Anisotropy Energy

Magnetization with applied field for a single crystal

varies with crystal orientation. Saturation magnetization occurs most easily for the

direction of BCC iron.

Saturation magnetizationoccurs with highest appliedfield for direction.

Some grains of polycrystalline

materials need some energy

to rotatetheir resultant

moment.

This energy is magnetocrystalline anisotropy energy.

Figure 15.14

16-12

C i ht Th M G Hill C i I P i i i d f d ti di l

8/13/2019 SmithCh16

13/28


Domain Wall Energy

Domain wallis the region through which the

orientation of the magnetic moment changes gradually.

300 atoms wide due to balance between exchange

forceand magnetocrystalline anisotropy.

Equilibrium wall width is width at which sum of two

energies are minimum.

Figure 15.15

16-13 After C.S. Barrett, W. D. Nix, and A. S. Teteman, Principles of Engineering Materials, Prentice-Hall, 1973, p.485.

Cop right The McGra Hill Companies Inc Permission req ired for reprod ction or displa

8/13/2019 SmithCh16

14/28


Magnetostrictive Energy

Magnetostriction:Magnetically induced reversible

elastic strain.

Energy due to mechanical stress created by

magnetostriction is called magnetostriction energy.

It is due to change in bond lengthcaused by rotation of

dipole moments.

Equilibrium domain configuration is reached when

sum of magnetostrictive and domain wall energies are

minimum.

Figure 15.16 Figure 15.1716-14

Copyright The McGraw Hill Companies Inc Permission required for reproduction or display

8/13/2019 SmithCh16

15/28


Magnetization and Demagnetization

Magnetization and demagnetization do not followsame loop.

Once magnetized, remnant induction Brremainseven after demagnetization.

Negative field Hc(coercive

force) must be applied to

completely demagnetize.

Magnetization loop is

called hysteresis loop.

Area inside the loop

is a measure of work done

in magnetizing and

demagnetizing.

Figure 15.18

16-15

Copyright The McGraw Hill Companies Inc Permission required for reproduction or display

8/13/2019 SmithCh16

16/28


Soft Magnetic Materials

Easily magnetized and demagnetized.

Low coercive forceand high saturation induction are

desirable properties.

Hysteresis energy losses:Due to dissipated energy

required to push the domain back and forth.

Imperfections increases hysteresis.

Eddy current energy losses: Induced electric current

causes some stray electric currents resulting from

transient voltage.

Source of energy loss by electrical resistance

healing.

16-16

Copyright The McGraw-Hill Companies Inc Permission required for reproduction or display

8/13/2019 SmithCh16

17/28


Iron Silicon Alloys

Iron3 to 4% Sialloys are commonly used soft

magnetic materials.

Silicon reduces electrical resistivity, decreases

hysteresis, decreases magnetostriction.

Silicon decreases saturation induction and curie

temperature.

Laminated corefurther reduces eddy current

losses.

Decrease in energy loss is also achieved by using

grain oriented silicon sheet.

16-17


8/13/2019 SmithCh16

18/28


Metallic Glasses

Noncrystalline domains.

Soft magnetic properties, have combination of

ferromagnetic materials with metalloidsB and Si,

Used in low energy core-loss transformers, magnetic

sensors and recording heads.

Produced by rapid coolingas a thin film on a rotating

copper surface mold.

Strong, hard, flexible and corrosion resistant.

Easy movement of domain walls due to absence of

grain boundaries.

Low hysteresis loss.

16-18


8/13/2019 SmithCh16

19/28

Copyright The McGraw Hill Companies, Inc. Permission required for reproduction or display

Nickel-Iron Alloys

Higher permeabilityat lower field.

Used in highly sensitive communication.

50% Ni alloy: moderate permeability, high saturation

induction.

79% Ni alloy: High permeability, low saturation

induction.

Low magnetoanisotropy and magnetostrictive energy.

Initial permeability is increased byannealingin

presence of magnetic field.

16-19


8/13/2019 SmithCh16

20/28

Copyright The McGraw Hill Companies, Inc. Permission required for reproduction or display

Hard Magnetic Materials - Properties

High coercive forceHcand Induction Br.

High hysteresis loss and difficult to demagnetize.

Some energy of the field is converted to potential

energy.

Maximum energy product is a measure of magnetic

potential energy = Max (B x H).

Max (B x H) = area of

largest rectanglethat

can be inscribed in the

second quadrant of the

hysteresis loop.

Figure 15.2516-20


8/13/2019 SmithCh16

21/28

Copy g t e cG a Co pa es, c e ss o equ ed o ep oduct o o d sp ay

Alnico Alloys

Alnico : Aluminum + Nickel + Cobalt

High energy product, high remnant induction and

moderate coercivity.

Produced by casting or powder metallurgy.

Structure:Single phase

BCC at 12500C but

decomposes to and

at 750 to 8500C.

is highly magnetic. If heat treated in magnetic

field, becomes elongated

and hence is difficult to rotateHigh coercivity.Figure 15.26

16-21 After B. D. Cullity, Introduction to Magnetic Materials, Addision- Wesley, 1972, p. 566


8/13/2019 SmithCh16

22/28

py g p , q p p y

Rare Earth Alloys

Very high maximum energy product and coercivity

due to unpaired 4f electrons.

SmCo5single phase magnets:Coercivity is based on

nucleation and pinning down of domain walls at

surfaces and grain boundaries.

Powder metallurgy fabrication: Particlespressed in magnetic field and sintered.

Precipitation hardened Sm(Co,Cu)2.5alloy: Part of Co

substituted by Cu.

Precipitate produced at low temperatures anddomain walls are pinned at precipitates.

Used in medical devices.

16-22


8/13/2019 SmithCh16

23/28

py g p q p p y

Neodymium-Iron-Boron Magnetic Alloys

Produced by powder metallurgy and rapid

solidification melt-spun ribbon process.

Highly ferromagnetic Nd2Fe14Bgrains are surrounded

by nonferromagnetic Nd rich intergranular phase.

High coercivity and energy product due to difficulty in

reverse nucleating.

Used in automotive

starting motors.

Figure 15.29

16-23 After J. J. Croat and J. F. Herbst, MRS Bull., June 1988, p.37.


8/13/2019 SmithCh16

24/28

Iron-Chromium-Cobalt Magnetic Alloys

Structure and properties analogues to Alnico.

16% Fe, 28% Cr, 11% Co.

Single phase at high temperature (12000C).

Precipitates of chromium rich 2phase forms below

6500C.

Domain walls gets pinned into precipitate particles.

Particles are elongatedby forming to increase

coercivity.

Can be cold formed.

Used in permanent

magnets of modern

telephones.Figure 15.30

16-24 After S. Jin et al., J. Appl.Phys., 53:4300 (1982).


8/13/2019 SmithCh16

25/28

Ferrites

Magnetic ceramics made by mixing Fe2O3with other

oxides and carbonatesin powder form.

Domain structure and hysteresis loop similar to

ferromagnets but low magnetic saturation.

Soft ferrites: MO Fe2O3 where M is Fe2+, Mn 2+, Ni 2+

or Zn 2+.

Inverse spinelstructure.

Cubic unit cells with

8 subcells.

Only 1/8thof tetrahedral sites

are occupied in normal spinal

structure.

Figure 15.34

16-25


8/13/2019 SmithCh16

26/28

Net Magnetic Moments in Inverse Spinel Ferrites

Fe 2+ions 4 unpaired 3d electrons.

Fe 3+ions 5 unpaired 3d electrons.

Each unpaired 3d electron has one Bohr magneton

16-26


8/13/2019 SmithCh16

27/28

Properties and Applications of Soft Ferrites

Useful magnetic properties, good insulators.

High electrical resistivitylow eddy current losses.

Applications:Low-signal, Memory-core, audiovisual

and recording head applications.

Recording heads are made up of Mn-Zn and Ni-Zn

spinel ferrites.

Magnetic core memoriesare used in some computers.

16-27


8/13/2019 SmithCh16

28/28

Magnetically Hard Ferrites

General formula:MOFe2O3(hexagonal crystal

structure).

Examples:Barium Ferrite (BaO.6Fe2O3) and

Strontium Ferrite (SrO.6Fe2O3).

Low cost, low density, high coercive force.

High magnetocrystalline anisotropy.

Magnetization takes place by domain wall nucleation

and motion.

Applications:Generators, relays, motors, loudspeakers

and door closers.

16 28

Documents

SmithCh16