19'6 Magnetic materials

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19.6 Magnetic materials

• Magnets
• Magnetic recording

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Overview- Magnetic materials owe their properties
to magnetic dipole moments of atoms. Importance
permanent magnets, electromagnets magnetic
recording, magnetic tape, computer drives, credit
cards
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Atomic magnetism
Classical model orbital angular momentum
Charge rotating around the atomic nucleus acts as
a current loop
N
A current loop creates a magnetic dipole
e-
S
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Quantum mechanical model Spin quantum number
Spin is an intrinsic property of matter.
Particles with spin quantum numbers Sgt0
have magnetic properties. protons S1/2
Magnetic -gt NMR spectroscopy electrons S1/2
Magnetic properties.
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Magnetic properties of matter
diamagnetic
?/?o
Carbon 1-2x10-5 Iron alum
salt 166x10-5 Iron metal 1000-3000
all electrons paired S0
slightly less than vaccum
paramagnetic
unpaired electrons
slightly more than vaccum
Sgt0, but atoms not interacting
ferromagnetic
unpaired electrons coupled spins in large domains
much more than vaccum
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Soft magnetic materials e.g. iron Easily
magnetized but doesnt retain magnetization for
long Used as core for electromagnets Hard
magnetic materials e.g. metal alloys Alnico
(Aluminum, Nickel, Cobalt) Hard to magnetize but
retains the magnetization for a long time Used
as permanent magnets.
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Magnetic Domains
Magnetism due to magnetic domains. Each domain
has millions of atoms with magnetic moments
coupled Separated by domain boundaries
Soft magnetic materials-Boundary movement
10-4m
B
domain boundary shifts in B field to
give magnetization along B field direction
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Hard magnetic materials
B
Magnetic dipoles reorient in the domains to give
a net magnetic moment. Harder to do, i.e requires
higher B field. but also harder to reverse.
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Magnetization (polarization of magnetic dipoles)
Permanent magnet
refrigerator door
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Magnetic recording
alternating current
Electromagnet
Iron core- magnetic fields confined in the core
Gap
Fringe B-fields
Magnetic tape-magnetized by fringe fields
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Magnetic tape Information coded in the
orientation of magnetic particles
Magnetization can be read on playback to generate
a voltage signal
Similar recording for computer hard disks, credit
cards.
Information can be erased by magnetic fields.