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Intrinsic Magnetism Dr. Bill Pezzaglia Updated 2009Mar24.
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Transcript of Intrinsic Magnetism Dr. Bill Pezzaglia Updated 2009Mar24.
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IntrinsicIntrinsic
Magnetism Magnetism
IntrinsicIntrinsic
Magnetism Magnetism
Dr. Bill PezzagliaUpdated 2009Mar24
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Intrinsic MagnetismRough Draft
• Ferromagnetic• Diamagnetic• Paramagnetic
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Discovery of Magnets
• 900 BC: Attributed to shepherd Magnus, who found nails of his sandals pulled out by rocks atop Mount Ida
• (writings of Pliny the elder, 23-79 AD )
• Ore “Magnetite” (Iron Oxide) is acommon in Magnesia, Thessaly (Greece).
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Amperian Currents
• Ampere was the first to propose an explanation for intrinsic magnetisms
• He thought that they were due to microscopic currents in the atoms (atomic currents).
• This was criticized because iron is a poor conductor. The resistance would quickly dissipate any currents into heat.
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André-Marie Ampère(1775 -1836)
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The Bohr Magnetron
• 1911 Rutherford proposes that atom has electrons orbiting nucleus (i.e. a “current loop”)
• 1913 Procopiu (1915 Bohr) calculates the magnetic moment of electron orbiting atom.
• Bohr Magnetron:
But it turns out this is not the primary cause of magnetism.
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mAmpm
ehB
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Induced Magnetism6
• 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
• Curie Constant “C”
T
BCM
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Spin of Electron
• 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
• Atoms which have unpaired electrons will have magnetic moments, which can line up with an external field.
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Intrinsic Magnetism• 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
• I think he would be the one that was able to tell what the currie constant was in terms of bohr magnetron.
• Curie Constant “C”• n=valence electron density• kB =Boltzmann’s Constant B =Bohr Magnetron (electron magnetic moment)
T
BCM
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B
B
k
nC
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Paramagnetism9
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Ferromagnetic10
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Curie Temperature11
• Above “Curie Temperature” Ferromagnetism ceases, and the material will become paramagnetic.
• Approximately: 768°C for Iron
• 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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Ferromagnetism12
• At room temperature only 3 elements exhibit ferromagnetism in their pure elemental form:
• Iron, Nickel and Cobalt
• Until recently best magnets were AlNiCo (Iron with Aluminum, Nickel & Cobalt)
• 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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Diamagnetic13
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Diamagnetism
• 1778 S. J. Bergman first to observe that bismuth and antimony were repelled by magnetic fields
• 1845 term "diamagnetism" coined by Michael Faraday, realizes nearly all materials exhibit effect
• Diamagnetic Levitation (first doneyears ago, but in 1990s peoplestarted levitating frogs and mice!)
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http://www.ru.nl/hfml/research/levitation/diamagnetic/
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References15
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