Chapter-08

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From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Definition of a Magnetic Dipole Moment
A magnetic moment
Fig 8.1
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Magnetic Dipole Moment
μ m  IAu n
Magnetic moment
Current
Unit vector normal to the
surface
Area circled by current
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Magnetic moment behaves like a magnet
A magnetic dipole moment puts out a magnetic field just like bar magnet. The field
B depends on µm.
Fig 8.3
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Magnetization and surface currents
The field B in the material inside the solenoid is due to the conduction current I through the
Wires and the magnetization current Im on the surface of the magnetized medium, or
B = B0 + 0M
Fig 8.8
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Ampere’s circuital law
Fig 8.9
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Ampere’s Law and the Inductance of a Toroidal Coil
Ampere’s law
H
dl

I
t

c
Magnetic field inside toroidal coil
B  o  r H 
o  r NI

Inductance of toroidal coil
T otalflux threaded N o r N 2 A
L


Current
I

From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Ferromagnetic Materials
In a magnetized region of a ferromagnetic material such as iron all the magnetic moments
are spontaneously aligned in the same direction. There is a strong magnetization vector M
even in the absence of an applied field.
Fig 8.15
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Antiferromagnetic Materials
In this antiferromagnetic BCC crystal (Cr) the magnetic moment of the center atom is
cancelled by the magnetic moments of the corner atoms (an eighth of the corner atom
belongs to the unit cell).
Fig 8.16
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Ferrimagnetic Materials
Illustration of magnetic ordering in a ferrimagnetic crystal. All A-atoms have their spins
Aligned in one direction an all B-atoms have their spins aligned in the opposite direction. As
The magnetic moment of an A-atom is greater than that of a B-atom, there is net magnetization,
M, in the crystal.
Fig 8.17
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
The exchange integral as a function of r/rd, where r is the interatomic distance and rd the radius
of the d-orbit (or the average d-subshell radius. Cr to Ni are transition metals. For Gd, the x-axis
is r/rf where rf is the radius of the f-orbit.
Fig 8.20
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Magnetocrystalline anisotropy in a single iron crystal. M vs. H depends on the crystal on the
crystal direction and is easiest along [100] and hardest along [111]
Fig 8.24
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
In a Bloch wall the neighboring spin magnetic moments rotate gradually and it takes several hundred
atomic spacings to rotate the magnetic moment by 180.
Fig 8.25
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Potential energy of a domain wall depends on the exchange and anisotropy energies
Fig 8.26
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Magnetostriction
Magnetostriction means that the iron crystal in a magnetic field along x, an easy direction,
elongates along x but contracts in the transverse directions.
Fig 8.27
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Schematic illustration of magnetic domains in the grains of an unmagnetized polycrystalline
iron sample. Very small grains have single domains.
Fig 8.30
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
(a) A typical M vs. H hysterisis curve
(b) The corresponding B vs. H hysterisis curve. The shaded area inside the hysterisis loop
Is the energy loss per unit volume per cycle.
Fig 8.32
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
A Single Domain Fine Particle
A single domain fine particle.
Fig 8.40
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
(a) Point P represents the operating point of the magnet and determines the magnetic
field inside and outside the magnet.
(b) Energy density in the gap is proportional to (BH) and for a given geometry and size of
gap this is maximum at a particular magnetic field Bm* or Bg*.
Fig 8.43
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
Giant Magnetoresistance (GMR)
A highly simplified view of the principle of the giant mangetoresistance effect.
(a) The basic trilayer structure.
(b) Antiparallel magnetic layers with high resistance RAP.
(c) An external field aligns layers, parallel alignment has a lower resistance RP.
Fig 8.58
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
The principle of longitudinal magnetic recording on a flexible medium, e.g. magnetic
tape in an audio cassette
Fig 8.61
From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)

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