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Intrinsic
Magnetism
Updated 2009Mar24
Dr. Bill Pezzaglia
Intrinsic Magnetism
Rough Draft
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Ferromagnetic
Diamagnetic
Paramagnetic
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Discovery of Magnets
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900 BC: Attributed to shepherd Magnus,
who found nails of his sandals pulled out
by rocks atop Mount Ida
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(writings of Pliny the elder,
23-79 AD )
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Ore “Magnetite” (Iron Oxide) is a
common in Magnesia, Thessaly (Greece).
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Amperian Currents
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Ampere was the first to propose an
explanation for intrinsic magnetisms
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He thought that they were due to
microscopic currents in the atoms
(atomic currents).
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This was criticized because iron is a
poor conductor. The resistance would
quickly dissipate any currents into
heat.
André-Marie Ampère
(1775 -1836)
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The Bohr Magnetron
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1911 Rutherford proposes that atom has
electrons orbiting nucleus (i.e. a “current loop”)
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1913 Procopiu (1915 Bohr) calculates the
magnetic moment of electron orbiting atom.
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Bohr Magnetron:
B 
eh
 9.27 10  24 Amp  m 2
2 m
But it turns out this is not the primary cause of
magnetism.
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Induced Magnetism
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Pierre Curie studied magnetism for his
doctorial thesis 1890-1895
Magnetization (magnetic moment per
volume) increases with applied field.
Curie’s Law: Magnetism decreases with
temperature
B
M C
T
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Curie Constant “C”
Spin of Electron
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1925 Uhlenbech & Goudsmit propose that
the electron has spin, and this spinning
charge creates a magnetic moment, which
also turns out to be one Bohr Magnetron
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Atoms which have unpaired electrons will
have magnetic moments, which can line up
with an external field.
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Intrinsic Magnetism
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1905 Models of Paramagnetism and
Ferromagnetism
Paul Langevin (1872-1946) is noted
for his work on paramagnetism and
diamagnetism, and devised the
modern interpretation of this
phenomenon in terms of spins of
electrons within atoms. Student of
Pierre Curie
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I think he would be the one that was
able to tell what the currie constant
was in terms of bohr magnetron.
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Curie Constant “C”
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n=valence electron density
kB =Boltzmann’s Constant
B =Bohr Magnetron (electron magnetic moment)
B
M C
T
n B
C
3k B
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Paramagnetism
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Ferromagnetic
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Curie Temperature
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Above “Curie Temperature” Ferromagnetism ceases,
and the material will become paramagnetic.
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Approximately: 768°C for Iron
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Note, the earth’s iron core is well over 5000°C, so you
can’t explain the earth’s magnetic field by
Ferromagnetism!
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Ferromagnetism
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At room temperature only 3 elements exhibit
ferromagnetism in their pure elemental form:
Iron, Nickel and Cobalt
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Until recently best magnets were AlNiCo (Iron with
Aluminum, Nickel & Cobalt)
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New rare-earth magnets (n.b. NIB=neodymium, iron and
boron) are much stronger, but have lower Curie
temperature (300 to 400°C) than AlNiCo.
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Diamagnetic
Diamagnetism
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1778 S. J. Bergman first to observe that bismuth and
antimony were repelled by magnetic fields
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1845 term "diamagnetism" coined by Michael Faraday,
realizes nearly all materials exhibit effect
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Diamagnetic Levitation (first done
years ago, but in 1990s people
started levitating frogs and mice!)
http://www.ru.nl/hfml/research/levitation/diamagnetic/
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References