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Magnetic Properties

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Title: Magnetic Properties


1
Magnetic Properties
  • Subhalakshmi Lamba

2
Scope of the Lecture
  • History of Magnetism electricity magnetism
  • Quantities used to describe magnetic properties
  • Different types of magnetism

3
Scope of the Lecture
  • Classical Theories
  • The need for a quantum theory
  • Quantum Theory of Paramagnetism

4
A story
  • References to a herdsman Magnes by a Greek
    historian Pliny
  • Known in China and Europe -800 BC

5
LODESTONE
Typical current 1,000,000 Amp.
  • Lodestone contains iron oxide mineral
  • magnetite
  • When lightning strikes the earth it could
    create a magnetic field large enough to saturate
    the magnetization of lodestone .

Once in 1 10 million years
6
THE FIRST MAGNETIC DEVICES
Chinese Portugese compasses (16 th century )
floating fish-shaped iron leaf Wu Ching Tsung
Yao ( written in 1040)
7
Magnets in a car
8
In the future
Quantum Computers
9

Electricity Magnetism
  • Electrified Amber attracts small objects
  • Lodestone attracts iron

A Connection ?
Hans Christian Ørsted ( 1777 1851)
10
Oersteds Experiment
  • placed a wire above the compass needle
  • connected both ends across a battery
  • the needle spun until it was at right angles to
    the wire

11
Farradays Ideas
A quantitative relationship between a changing
magnetic field and the electric field created by
the change
Michael Faraday  (1791-1867)
12
Effect of a changing magnetic field
13
Ampere's Law
The magnetic field in space around an electric
current is proportional to the electric current
which serves as its source.
For any closed loop path, the sum of the length
elements times the magnetic field in the
direction of the length element is equal to the
permeability times the electric current enclosed
in the loop .
14
Magnetic field due to a long wire
15
Biot and Savart's Law
magnetic field due to an arbitrary current
distribution
16
Maxwells Equations
17
Maxwells Equations
James Clerk Maxwell (1831 - 1879)
18
Magnetic Field
  • A force field similar to the gravitational and
    electrical field, detected by a probe.
  • Force experienced by the probe due to the field
  • Sources of a magnetic field are magnetic poles
  • Fictitious points near the ends of a magnet

19
Magnetic Field
  • Magnetic fields are produced by electric
    currents
  • can be macroscopic currents in wires, or
    microscopic currents associated with electrons in
    atomic orbits.
  • The magnetic field B is defined in terms of force
    on moving charge in the Lorentz force law.

20
Magnetic Poles
  • the external magnetic field is strongest at the
    poles
  • The two types of magnetic poles cannot exist
    separately always coupled together as a dipole.
  • Isolated magnetic monopoles have not yet been
    detected.

21
Magnetic Poles
  • The attractive or repulsive force between two
    magnets decreased in proportion to the square of
    the distance between them

cgs
MKSA
22
ELECTROMAGNETIC FORCE
  • One of the four fundamental forces
  • Electric Force and the Magnetic Force
  • Lorentz Force

23
Magnetic Field B
  • Defined from the force on a moving charge
  • Unit of magnetic field Newton seconds /(Coulomb
    meter) or Newtons /Ampere meter - Tesla
  • Smaller unit is Gauss
  • 1 Tesla 104 Gauss

24
Magnetic Field Strength H
  • Magnetic fields are generated by currents and
    measured in B (Tesla)
  • magnetic materials themselves contribute
    internal magnetic fields when a magnetic field
    passes through them.
  • To segregate the two contributions
  • H B/?0 - M

Driving magnetic field
Material response
25
Magnetic Field Strength H
  • M Magnetization of the material
  • B ?0 ( H M )
  • B ? H ? ? ?r ?0

Permeability of space
Relative permeability Of the material
26
Magnetic Susceptibility ?
  • B ?0 ( H M )
  • Replace B ? H ? ? H ?0 ( H M )
  • ? ?r ?0 H ?0 ( H M )
  • ? ?0 M ?0 (?r -1) H
  • ? M (?r -1) H

?
Magnetic Susceptibility
27
Unit of Magnetic Field
  • Unit of H Ampere m -1
  • H B / ?
  • Oersted
  • 1 Ampere m -1
  • 0.01257 oersted

Tesla / N Ampere -2 N / Ampere m /N Ampere
-2 Ampere m -1
28
Atomic magnetism / Magnetism in Matter
  • S.J. Brugmans (1778)
  • cobalt attracted
  • bismuth antimony repelled
  • by the single pole of a magnet
  • Farraday (1845-1849)
  • almost all matter has some magnetic property or
    the other (mostly to a very small degree)

? PARAMAGNETIM
? DIAMAGETISM
29
Magnetic Materials
ROOM TEMPERATURE
30
Explaining Magnetism in Matter
  • Ampere
  • existence of small molecular currents
  • Each atom/molecule would behave as a small
    permanent magnet
  • Would align in the presence of a magnetic field

31
Thermal Properties of magnetic materials
  • M is proportional to the applied field H
  • ? Lim H ? 0 M / H
  • ? C / T

CURIES LAW
PIERRE CURIE
32
Thermal Properties of magnetic materials
  • Does not imply that the magnetization can
    increase infinitely as temperature is lowered
    with increased magnetic field.
  • Once all the magnetic dipoles are aligned the
    magnetization cannot increase further increase
  • SATURATION

33
Curies Law
  • Normal paramagnetic substances obey the Curie Law
  • Examples Aluminum, platinum, manganese,
    chromium

?C/T
?
1/?
1/?T/C
T in K
T in K
34
Diamagnetism
  • The susceptibility, ? is negative
  • It does not change much with temperature
  • Examples water, inert gases

35
Ferromagnetism
Curie Temperature TC
  • M decreases rapidly with H
  • Beyond the Curie temperature it behaves like a
    paramagnetic substance
  • Examples iron, cobalt, nickel

Behaves like a paramagnet
M decreases rapidly with H
36
Antiferromagnetism
  • like paramagnets above a critical temperature TN
    called Neél temperature.
  • Below TN ? is small T-dependence is different
    from paramagnets.
  • Example Cobalt

37
Ferrimagnetism
  • Like ferromagnets, but the effect tends to be
    smaller.
  • The 1/? curve is very close to zero below a
    critical temperature, also called Neél
    temperature.
  • Examples magnetite (Fe3O4) and spinel ferrites

38
A COMPARISON
DIAmagnetic
PARAmagnetic
FERROmagnetic
ANTIFERROmagnetic
FERRImagnetic
39
A COMPARISON
DIAmagnetic
PARAmagnetic
40
A COMPARISON
FERROmagnetic
B
ANTIFERROmagnetic
FERRImagnetic
41
A COMPARISON
(?r -1)? ? ?r ?1
PARAmagnetic
DIAmagnetic
  • ? gt 0, ?r gt 1
  • ? lt 0 , ?r lt 1
  • ?r 0
  • (superconductors)

FERROmagnetic
  • ? gt 0, ?r gtgt 1

42
Novel Magnetism
  • SPIN GLASS A random orientation of frozen
    spins.
  • CLUSTER GLASS spins make small clusters with
    magnetic order but no order between clusters
  • METAMAGNET Field induced magnetic transition
    from a low magnetization state to a relatively
    much higher magnetization state

43
Novel Magnetism
  • Superparamagnet when the size of the
  • magnetic particle is very small domains are
    not formed . Each magnetic particle behaves a
    giant paramagnetic ion.

44
A RECAP
  • Origin of magnetism
  • Connection between electricity and magnetism
    electric currents produce magnetic fields
  • Magnetism in matter
  • Temperature dependence of magnetic properties
    CURIES law

45
A RECAP
  • Temperature dependence of magnetic properties
    CURIES law
  • Different types of magnetic properties
  • Classical theories
  • Their limitations
  • QUANTUM THEORIES
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