IntrinsicIntrinsic

Magnetism Magnetism

IntrinsicIntrinsic

Magnetism Magnetism

Dr. Bill PezzagliaUpdated 2009Mar24

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)

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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ehB

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

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

3

2

Paramagnetism9

Ferromagnetic10

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!

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.

Diamagnetic13

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/

References15