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Chapter 20: Magnetic Materials
Chapter 20:
Magnetic Materials
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2. © 2011 Cengage Learning Engineering. All Rights Reserved.
Chapter 20: Magnetic Materials
Learning Objectives
Classification of magnetic materials
Magnetic dipoles and magnetic moments
Magnetization, permeability, and the magnetic field
Diamagnetic, paramagnetic, ferromagnetic, ferrimagnetic, and superparamagnetic materials
Domain structure and the hysteresis loop
The Curie temperature
Applications of magnetic materials
Metallic and ceramic magnetic materials
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20 - 2

3. Classification of Magnetic Materials
Chapter 20: Magnetic Materials
Classification of Magnetic 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
Paramagnetism
The net magnetic moment caused by the alignment of the electron spins when a magnetic field is applied
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4. Classification of Magnetic Materials
Chapter 20: Magnetic Materials
Classification of Magnetic Materials
Antiferromagnetism
Arrangement of magnetic moments such that the magnetic moments of atoms or ions cancel out causing zero net magnetization
Superparamagnetism
In the nanoscale regime, materials that are ferromagnetic or ferrimagnetic but behave in a paramagnetic manner (because of their nano-sized grains or particles)
Permanent magnets
A hard magnetic material
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5. © 2011 Cengage Learning Engineering. All Rights Reserved.
Chapter 20: Magnetic Materials
Figure 20.1
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 orbiting around the nucleus create a magnetic field around the atom.
The magnetic moment of an electron due to its spin is known as the Bohr magneton (B)
B = qh/4me = × A.m2
where q is the charge on the electron, h is Planck’s constant, and meis the mass of the electron. This moment is directed along the axis of electron spin.
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Chapter 20: Magnetic Materials
Table 20.1
The electron spins in the 3d energy level in transition metals with arrows indicating the direction of spin.
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Chapter 20: Magnetic Materials
Figure 20.2
When an electric current is passed through the coil, a magnetic field H is produced, with the strength of the field given by:
H = nI/l
where n is the number of turns, l is the length of the coil (m), and I is the current (A). The units of H are therefore ampere turn/m, or simply A/m by 4 × 10-3
The number of lines of flux, called the flux density, or inductance B, is related to the applied field by
B = 0H
where B is the inductance, H is the magnetic field, and 0 is a constant called the magnetic permeability of vacuum - 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.
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Chapter 20: Magnetic Materials
Table 20.2
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9. Magnetization, Permeability,
Chapter 20: Magnetic Materials
Magnetization, Permeability,
and the Magnetic Field
When an electric current is passed through the coil, a magnetic field H is produced, with the strength of the field given by:
where
n number of turns
l length of the coil (m)
I current (A)
The units of H are therefore ampere turn/m, or simply A/m by 4 × 10-3
20 - 9
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10. Magnetization, Permeability,
Chapter 20: Magnetic Materials
Magnetization, Permeability,
and the Magnetic Field
The number of lines of flux, called the flux density, or inductance B, is related to the applied field by
B = 0H
where
B inductance
H magnetic field
0 constant called the magnetic permeability of vacuum - the ratio between inductance or magnetization and magnetic field
When the material is placed within the magnetic field
B = H
where  is the permeability of the material in the field
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11. Magnetization, Permeability,
Chapter 20: Magnetic Materials
Magnetization, Permeability,
and the Magnetic Field
Influence of the magnetic material by the relative permeability r
where
The magnetization M represents the increase in the inductance due to the core material, B = 0H + 0M
Magnetic susceptibility is the ratio between magnetization and the applied field
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Chapter 20: Magnetic Materials
Figure 20.3
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.
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13. Diamagnetic and Paramagnetic Materials
Chapter 20: Magnetic Materials
Diamagnetic and Paramagnetic Materials
Diamagentic behavior
The effect caused by the magnetic moment due to the orbiting electrons, which produces a slight opposition to the imposed magnetic field.
Paramagnetism
The net magnetic moment caused by the alignment of the electron spins when a magnetic field is applied.
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14. Ferromagnetic Materials
Chapter 20: Magnetic Materials
Ferromagnetic 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.
Susceptibility is given by the following equation known as the Curie-Weiss law:
where
C constant that depends upon the material
Tc Curie temperature
T temperature above Tc
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Chapter 20: Magnetic Materials
Figure 20.4
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Chapter 20: Magnetic Materials
Figure 20.5
Domains are regions in the material in which all of the dipoles are aligned in a certain direction.
Boundaries, called Bloch walls, separate the individual magnetic domains. The Bloch walls are narrow zones in which the direction of the magnetic moment gradually and continuously changes from that of one domain to that of the next.
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Chapter 20: Magnetic Materials
Figure 20.6
Saturation magnetization, produced when all of the domains are oriented along with the magnetic field, is the greatest amount of magnetization that the material can obtain.
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Chapter 20: Magnetic Materials
Figure 20.7
Remanance - The polarization or magnetization that remains in a material after it has been removed from the field. The remanance is due to the permanent alignment of the dipoles.
Coercivity - The magnetic field needed to coerce or force the domains in a direction opposite to the magnetization direction. This is a microstructure-sensitive property.
The dependence of coercivity on the shape of a particle or grain is known as magnetic shape anisotropy.
Hysteresis loop - The loop traced out by magnetization in a ferromagnetic or ferrimagnetic material as the magnetic field is cycled.
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Chapter 20: Magnetic Materials
Figure 20.8
Curie temperature - The temperature above (Tc) which ferromagnetic or ferrimagnetic materials become paramagnetic.
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20. Table 20.3 - Curie Temperatures For Selected Materials
Chapter 20: Magnetic Materials
Table Curie Temperatures For Selected Materials
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Chapter 20: Magnetic Materials
Figure 20.9
(a) Comparison of the hysteresis loops for three applications of ferromagnetic and ferrimagnetic materials.
(b) Saturation magnetization and coercivity values for different magnetic materials.
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22. Table 20.4 - Properties of Selected Soft Magnetic Materials
Chapter 20: Magnetic Materials
Table Properties of Selected Soft Magnetic Materials
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Chapter 20: Magnetic Materials
Table Properties of Typical Magnetic Recording Materials in a Powder Form
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Chapter 20: Magnetic Materials
Table Properties of Selected Hard, or Permanent, or Magnetic Materials
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Chapter 20: Magnetic Materials
Figure 20.10
The magnetic force obtainable using a permanent magnet
F = 0M2A/2
A is the cross-sectional area of the magnet, M is the magnetization, and 0 is the magnetic permeability of free space.
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26. Metallic and Ceramic Magnetic Materials
Chapter 20: Magnetic Materials
Metallic and Ceramic Magnetic Materials
Magnetic alloys
Iron-nickel alloys: Such as Permalloy, have high permeabilities, making them useful as soft magnets.
Composite magnets: Are used to reduce eddy current losses.
Data storage materials: Magnetic materials for information storage must have a square loop and a low coercive field, permitting very rapid transmission of information.
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Chapter 20: Magnetic Materials
Figure 20.13
Magnetocrystalline anisotropy - In single crystals, the coercivity depends upon crystallographic direction creating easy and hard axes of magnetization.
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Chapter 20: Magnetic Materials
Figure 20.14
Demagnetizing curves for Co5Sm and Co5Ce, representing a portion of the hysteresis loop.
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Chapter 20: Magnetic Materials
Figure 20.15
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30. Table 20.7 - Magnetic Moments for Ions in the Spinel Structure
Chapter 20: Magnetic Materials
Table Magnetic Moments for Ions in the Spinel Structure
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31. Metallic and Ceramic Magnetic Materials
Chapter 20: Magnetic Materials
Metallic and Ceramic Magnetic Materials
Magnetostriction
Certain materials can develop strain when their magnetic state is changed.
The magnetostrictive effect can be seen either by changing the magnetic field or by changing the temperature.
The magnetostriction phenomenon is analogous to electrostriction.
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Chapter 20: Magnetic Materials
Key Terms
Ferromagnetic
Ferrimagnetic
Diamagnetic
Paramagnetic
Antiferromagnetic
Superparamagnetic
Permanent magnets or hard magnetic materials
Magnetic moment
Bohr magneton
Magnetic permeability of vacuum
Magnetization
Magnetic susceptibility
Diamagnetism
Paramagnetism
Domains
Bloch walls
Saturation magnetization
Remanance
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33. © 2011 Cengage Learning Engineering. All Rights Reserved.
Chapter 20: Magnetic Materials
Key Terms
Magnetic shape anisotropy
Hysteresis loop
Curie temperature
Power
Magnetocrystalline anisotropy
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