Contributed by:
1. Classification of Magnetic Materials
2. Magnetic Dipoles and Magnetic Moments
3. Magnetization, Permeability, and the Magnetic Field
4. Diamagnetic, Paramagnetic, Ferromagnetic, Ferrimagnetic, and Superparamagnetic Materials
5. Domain Structure and the Hysteresis Loop
6. The Curie Temperature
7. Applications of Magnetic Materials
8. Metallic and Ceramic Magnetic Materials
1.
The Science and Engineering
of Materials, 4th ed
Donald R. Askeland – Pradeep P. Phulé
Chapter 19 – Magnetic Materials
1
2.
Objectives of Chapter 19
To study the fundamental basis for
responses of certain materials to the
presence of magnetic fields.
To examine the properties and applications
of different types of magnetic materials.
2
3.
Chapter Outline
19.1 Classification of Magnetic Materials
19.2 Magnetic Dipoles and Magnetic Moments
19.3 Magnetization, Permeability, and the
Magnetic Field
19.4 Diamagnetic, Paramagnetic, Ferromagnetic,
Ferrimagnetic, and Superparamagnetic
Materials
19.5 Domain Structure and the Hysteresis Loop
19.6 The Curie Temperature
19.7 Applications of Magnetic Materials
19.8 Metallic and Ceramic Magnetic Materials
3
4.
Section 19.1
Classification of Magnetic Materials
Ferromagnetism
Ferrimagnetism
Diamagnetism
Antiferromagnetism
Hard magnet
4
5.
Section 19.2
Magnetic Dipoles and Magnetic Moments
The magnetic behavior of materials can be traced to the
structure of atoms.
Bohr magneton - The strength of a magnetic moment of
an electron (μB) due to electron spin.
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6.
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Figure 19.1 Origin of magnetic dipoles: (a) The spin of the
electron produces a magnetic field with a direction dependent
on the quantum number ms. (b) Electrons Electrons orbiting
around the nucleus create a magnetic field around the atom.
6
8.
Section 19.3
Magnetization, Permeability, and the
Magnetic Field
Magnetic permeability - The ratio between inductance or
magnetization and magnetic field. It is a measure of the
ease with which magnetic flux lines can ‘‘flow’’ through a
material.
Magnetization - The total magnetic moment per unit
volume.
Magnetic susceptibility - The ratio between
magnetization and the applied field.
8
9.
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Figure 19.2 A current passing through a coil sets up a magnetic
field H with a flux density B. The flux density is higher when a
magnetic core is placed within the coil.
9
11.
Example 19.1
Theoretical and Actual Saturation
Magnetization in Fe
Calculate the maximum, or saturation, magnetization that we
expect in iron. The lattice parameter of BCC iron is 2.866 Å .
Compare this value with 2.1 tesla (a value of saturation flux
density experimentally observed for pure Fe.)
Example 19.1 SOLUTION
Based on the unpaired electronic spins, we expect each iron
atom to have four electrons that act as magnetic dipoles. The
number of atoms per m3 in BCC iron is:
11
12.
Example 19.1 SOLUTION (Continued)
The maximum volume magnetization (Msat) is the total
magnetic moment per unit volume:
To convert the value of saturation magnetization M into
saturation flux density B in tesla, we need the value of μ0M. In
ferromagnetic materials μ0M >> μ0H and therefore, B μ0M.
Saturation induction in tesla = Bsat = μ0Msat.
12
13.
Section 19.4
Diamagnetic, Paramagnetic,
Ferromagnetic, Ferrimagnetic, and
Superparamagnetic Materials
Ferromagnetism - Alignment of the magnetic moments
of atoms in the same direction so that a net
magnetization remains after the magnetic field is
removed.
Ferrimagnetism - Magnetic behavior obtained when ions
in a material have their magnetic moments aligned in an
antiparallel arrangement such that the moments do not
completely cancel out and a net magnetization remains.
Diamagnetism - The effect caused by the magnetic
moment due to the orbiting electrons, which produces a
slight opposition to the imposed magnetic field.
13
14.
Section 19.4 (Continued)
Antiferromagnetism - Arrangement of magnetic
moments such that the magnetic moments of atoms or
ions cancel out causing zero net magnetization.
Hard magnet - Ferromagnetic or ferrimagnetic material
that has a coercivity > 104 A . m-1.
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15.
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Figure 19.3 The
effect of the core
material on the flux
density. The
magnetic moment
opposes the field in
diamagnetic
materials.
Progressively
stronger moments
are present in
paramagnetic,
ferrimagnetic, and
ferromagnetic
materials for the
same applied field.
15
16.
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Figure 19.4 The
crystal structure of
Mn0 consists of
alternating layers of
{111} type planes of
oxygen and
manganese ions. The
magnetic moments of
the manganese ions
in every other (111)
plane are oppositely
aligned.
Consequently, Mn0 is
antiferromagnetic.
16
17.
Example 19.2
Design/Materials Selection for a Solenoid
We want to produce a solenoid coil that produces an
inductance of at least 2000 gauss when a 10-mA current
flows through the conductor. Due to space limitations,
the coil should be composed of 10 turns over a 1 cm
length. Select a core material for the coil.
17
19.
Example 19.2 SOLUTION
The magnetic field H produced by the coil.
The permeability of the core material must be:
The relative permeability of the core material must be at least:
From Table 19-4, we find that 4-79 permalloy has a
maximum relative permeability of 80,000 and might be a
good selection for the core material.
19
20.
Section 19.5
Domain Structure and
the Hysteresis Loop
Domains - Small regions within a single or polycrystalline
material in which all of the magnetization directions are
aligned.
Bloch walls - The boundaries between magnetic domains.
Saturation magnetization - When all of the dipoles have
been aligned by the field, producing the maximum
magnetization.
Remanance - The polarization or magnetization that
remains in a material after it has been removed from the
field.
Hysteresis loop - The loop traced out by magnetization in
a ferromagnetic or ferrimagnetic material as the magnetic
field is cycled.
20
21.
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Figure 19.5 (a) A qualitative sketch of magnetic domains in a
polycrystalline material. The dashed lines show demarcation
between different magnetic domains; the dark curves show the
grain boundaries. (b) The magnetic moments in adjoining
atoms change direction continuously across the boundary
between domains.
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22.
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is a trademark used herein under license.
Figure 19.6 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. Notice the
permeability values depend upon the magnitude of H.
22
23.
Figure 19.7 (a) The ferromagnetic hysteresis M-H loop showing the
effect of the magnetic field on inductance or magnetization. The
dipole alignment leads to saturation magnetization (point 3), a
remanance (point 4), and a coercive field (point 5). (b) The
corresponding B-H loop. Notice the end of the B-H loop, the B value
does not saturate since B = μ0H + μ0M. (Source: Adapted from
Permanent Magnetism, by R. Skomski and J.M.D. Coey, p. 3, Fig. 1-
1. Edited by J.M.D. Coey and D.R. Tilley. Copyright © 1999 Institute
of Physics Publishing. Adapted by permission.)
23
24.
Section 19.6
The Curie Temperature
Curie temperature - The temperature above (Tc) which
ferromagnetic or ferrimagnetic materials become
paramagnetic.
24
25.
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Figure 19.8 The effect of temperature on (a) the hysteresis
loop and (b) the remanance. Ferromagnetic behavior
disappears above the Curie temperature.
25
27.
Example 19.3
Design/Materials Selection
for a High-Temperature Magnet
Select a permanent magnet for an application in an aerospace
vehicle that must re-enter Earth’s atmosphere. During re-entry,
the magnet may be exposed to magnetic fields as high as 600
oersted and may briefly reach temperatures as high as 500oC.
We want the material to have the highest power possible and
to maintain its magnetization after re-entry.
27
29.
Example 19.3 SOLUTION
It is first necessary to select potential materials having
sufficient coercive field Hc and Curie temperature that re-
entry will not demagnetize them.
The Co5Sm has four times the power of the Alnico 5 and,
based on performance, might be our best choice.
29
30.
Section 19.7
Applications of Magnetic Materials
Soft Magnetic Materials - Ferromagnetic materials are
often used to enhance the magnetic flux density (B)
produced when an electric current is passed through the
material. Applications include cores for electromagnets,
electric motors, transformers, generators, and other
electrical equipment.
Data Storage Materials - Magnetic materials are used for
data storage.
Permanent Magnets - Magnetic materials are used to
make strong permanent magnets
Power - The strength of a permanent magnet as
expressed by the maximum product of the inductance
and magnetic field.
30
31.
Figure 19.9 (a) Comparison of the hysteresis
loops for three applications of ferromagnetic
and ferrimagnetic materials.
31
32.
Figure 19.9 (b) Saturation magnetization and coercivity values for
different magnetic materials. (Source: Adapted from ‘‘Magnetic
Materials: An Overview, Basic Concepts, Magnetic Measurements,
Magnetostrictive Materials,’’ by G.Y. Chin et al. In D. Bloor, M.
Flemings, and S. Mahajan (Eds.), Encyclopedia of Advanced
Materials, Vol. 1, 1994, p. 1423, Fig. 1. Copyright © 1994
Pergamon Press. Reprinted by permission of the editor.)
32
36.
Figure 19.10 (a) The largest rectangle drawn in the second or fourth
quadrant of the B-H curve gives the maximum BH product. (BH)max is
related to the power, or energy, required to demagnetize the
permanent magnet. (b) Development of permanent magnet
materials, maximum energy product is shown on the y -axis.
(Source: Adapted from Permanent Magnetism, by R. Skomski and
J.M.D. Coey, p. 23, Table 1-2. Edited by J.M.D. Coey and D.R. Tilley.
Copyright © 1999 Institute of Physics Publishing. Adapted by
36
37.
Example 19.4
Energy Product for Permanent Magnets
Determine the power, or BH product, for the magnetic
material whose properties are shown in Figure 19.11.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used
Figure 19.11 The fourth
herein under license.
quadrant of the B-H curve
for a permanent magnetic
material (for Example 19.4)
37
38.
Example 19.4 SOLUTION
Several rectangles have been drawn in the fourth
quadrant of the B-H curve. The BH product in each is:
Thus, the power is about 4.2 106 gauss . oersted.
38
39.
Example 19.5
Design/Selection of Magnetic Materials
Select an appropriate magnetic material for the following
applications: a high-electrical-efficiency motor, a magnetic
device to keep cupboard doors closed, a magnet used in an
ammeter or voltmeter, and magnetic resonance imaging.
Example 19.5 SOLUTION
High-electrical-efficiency motor: To minimize hysteresis
losses, we might use an oriented silicon iron, taking
advantage of its anisotropic behavior and its small hysteresis
Magnet for cupboard doors: The magnetic latches used
to fasten cupboard doors must be permanent magnets;
however, low cost is a more important design feature than
high power. An inexpensive ferritic steel or a low-cost ferrite
would be recommended.
39
40.
Example 19.5 SOLUTION (Continued)
Magnets for an ammeter or voltmeter: For these
applications, Alnico alloys are particularly effective. We find
that these alloys are among the least sensitive to changes
in temperature, assuring accurate current or voltage
readings over a range of temperatures.
Magnetic resonance imaging: One of the
applications for MRI is in medical diagnostics. In this case,
we want a very powerful magnet. A Nd2Fe12B magnetic
material, which has an exceptionally high BH product,
might be recommended for this application. We can also
make use of very strong electromagnets made using
40
41.
Example 19.6
Lifting Power of a Magnet
Calculate the force in kN for one square meter area of a
permanent magnet whose saturation magnetization is 1.61
Example 19.6 SOLUTION
We have been given the value of μ0M = 1.61 tesla. We can
rewrite the equation that provides the force due to a
permanent magnet as follows.
41
42.
Section 19.8
Metallic and Ceramic Magnetic Materials
Magnetocrystalline anisotropy - In single crystals, the
coercivity depends upon crystallographic direction
creating easy and hard axes of magnetization.
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43.
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Figure 19.12 Information can be stored or retrieved from a
magnetic disk by use of an electromagnetic head. A current in the
head magnetizes domains in the disk during storage; the domains
in the disk induce a current in the head during retrieval.
43
44.
Figure 19.13 The initial magnetization curve for iron is
highly anisotropic; magnetization is easiest when the 100
directions are aligned with the field and hardest
along [111]. (Source: From Principles of Electrical
Engineering Materials and Devices, by S.O. Kasap, p.
623, Fig. 8-24. Copyright © 1997 Irwin. Reprinted by
permission of The McGraw-Hill Companies.)
44
45.
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Figure 19.14 Demagnetizing curves for Co5Sm and Co5Ce,
representing a portion of the hysteresis loop.
45
46.
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Figure 19.15 (a) The structure of magnetite, Fe304. (b) The
subcell of magnetite. The magnetic moments of ions in the
octahedral sites line up with the magnetic field, but the
magnetic moments of ions in tetrahedral sites oppose the
field. A net magnetic moment is produced by this ionic
46
47.
Example 19.7
Magnetization in Magnetite (Fe3O4)
Calculate the total magnetic moment per cubic centimeter in
magnetite. Calculate the value of the saturation flux density
(Bsat) for this material.
Figure 19.15 (b) The
subcell of magnetite.
47
48.
Example 19.7 SOLUTION
In the unit cell overall, there are eight subcells, so the total
magnetic moment is 32 Bohr magnetons per cell. The size
of the unit cell, with a lattice parameter of 8.37 10-8 cm,
The magnetic moment per cubic centimeter is:
This expression represents the magnetization M at
saturation (Msat). The value of Bsat μ0Msat will be =
(4 10-7)(5.1 105) = 0.64 Tesla.
48
50.
Example 19.8
Design/Materials Selection
for a Ceramic Magnet
Design a cubic ferrite magnet that has a total magnetic
moment per cubic meter of 5.5 105 A/m.
50
51.
Example 19.8 SOLUTION
Assuming that the addition of Mn ions does not appreciably
affect the size of the unit cell, we find from Example 19-7
Let x be the fraction of Mn2+ ions that have replaced the
Fe2+ ions, which have now been reduced to 1 - x. Then,
the total magnetic moment is:
Therefore we need to replace 34.6 at% of the Fe2+ ions with
Mn2+ ions to obtain the desired magnetization.
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52.
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Figure 19.16
material (for
Problem 19.19).
Hysteresis curve
for a hard magnetic
53.
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Figure 19.17 Hysteresis curve for a hard magnetic
material (for Problem 19.30).
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54.
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license.
Figure 19.14 (Repeated for Problem 19.36.) Demagnetiz-
ing curves for Co5Sm and Co5Ce, representing a portion of
the hysteresis loop.
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