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What causes certain rock to be naturally magnetized?

  1. May 19, 2004 #1
    I am looking for an explanation on how bar magnets or lodestone works. After searching the internet I found many articles on magnetic fields, electricity causing magnetic fields, electromagnets, etc. But I wasn’t able to find an explanation on why lodestone naturally attracts other metals.

    The science channel had an hour long show about magnetism but only thirty seconds of the show explained how natural magnets work. The show mentioned all electrons have a north and south polls and in most elements the electrons line up in such a way that they cancel out their magnetic properties. But in lodestone the electrons line up with all north poles in one direction and all south poles in another direction which causes a natural magnet.

    I’m hoping some folks here could confirm and expand on this.

    Thanks,
     
  2. jcsd
  3. May 20, 2004 #2

    ZapperZ

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    What you want to look for is a phenomenon called "ferromagnetism".

    http://hyperphysics.phy-astr.gsu.edu/hbase/solids/ferro.html

    All of the ferromagnetic materials tend to have atoms that have an unpaired electron in the outer-most shell (typically from the d-shell). This causes these atoms to have a net magnetic moment, not simply due to the magnetic moment of the electron, but rather due to the orbital angular momentum of that unpaired electron. But this is only the first ingredient in making a ferromagnet.

    The crystal structure of the material is also crucial. When the material is subjected to an external magnetic field (in nature, it is from the earth's magnetic field - so in lodestone, this is a very slow process since the earth's field is very weak), these small magnetic moments of the atom tend to allign themselves with the external field. The nature of the crystal arrangement for these material favors a configuration in which these allignment are "locked" in place even after you remove the external magnetic field. This is in contrast to paramagnetism in which the material also has all these atoms with individual magnetic moments, but they are oriented randomly and not locked in any direction until you apply an external field.

    As you can guess, there are a lot more complex explanation of the physics of magnetism (which is in the field of condensed matter physics).

    Zz.
     
  4. May 20, 2004 #3
    Thank you ZapperZ, excellent explanation!

    So if the outer most shell had an even number of electrons in it, would the electrons cancel each other’s magnetism effects out?

    Do they actually “spin” or is that just the best way to describe them?

    Thanks again,
     
  5. May 20, 2004 #4

    ZapperZ

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    Remember that I earlier said that it doesn't just depend on the unpaired spin. It has more to do with the orbital angular momentum. If you have an unpaired spin, but this is the s-orbital, you do not have ferromagnetism. This is because this orbital is symmetric. You can only induce a dipole moment using very large fields.

    Typically, the d-shell, due to the "lobes" of the orbital, are the ones that can create a large enough magnetic moment. If you look at all those elements that are ferromagnets, you'll find that they are in the "transition" element region of the periodic table, i.e. you're filling the d-orbital.

    "spin" is a very loose term. Just think of it as something that produces a magnetic moment. Don't think of the object as spinning on its axis.

    Zz.
     
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