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B Magnets and demagnetization

  1. Sep 21, 2016 #1


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    I have been asked how many times a magnet can magnetize a magnetic material before itself get demagnitized.
    The example in hand was the question about a strong neodymium magnet that accidently ruined a ferrite magnet.
    The next question was then why this neodymium magnet appears to not lose its magnetism if the neomagnet magnetize the same ferrite material over and over again.
    I don't have a good answer to that question (And I do not know if this question is qualified as B, I or A prefix)
    Do you guys have a good explanation to this?


  2. jcsd
  3. Sep 21, 2016 #2


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    I can only offer the B answer!
    Magnets don't give part of their magnetism to the other object when they magnetise it. So they lose nothing.

    When a piece of unmagnetised material is placed next to a magnet, the flux in the magnet may actually increase. This is the principle of the keeper that we used to use with our old iron magnets: a piece of unmagnetised high permeability iron is placed across the poles of a horseshoe magnet or two pieces across the ends of a pair of bar magnets. This ensured that during storage there was a high flux through the magnet aligned in the direction that keeps the domains correctly aligned, especially near the poles.

    So when an ummagnetised material is placed near a magnet's pole , it does nothing that would demagnetise the magnet. The unmagnetised material gets some flux through it and when the magnet is removed, some of the flux remains, depending on the remnance of the material. The magnet temporarily enjoys a higher flux level and reverts to its original level when the other material is removed. The process can be repeated indefinitely.

    If the magnet is placed near another magnet with like poles together, the flux is reduced. When the other magnet is removed, it may return to its former level, but it may be permanently reduced, depending on the strength of the other magnet and the properties of the material of which it is made (coercivity.)
    Since Nd magnets are pretty strong, it would take another stronger Nd magnet to demagnetise it. As you found, the Nd magnet being much stronger than the ferrite, was able to demagnetise it and even remagnetise it in a different orientation. This is because the Nd is strong enough to produce a reverse field in the ferrite when the magnetic field strengths are added.
  4. Sep 22, 2016 #3


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    It makes sense :-)
    When I got that question it was just as hard to answer as answering the question about a compressed metal spring that is heated up to the melting point - where do the input energy go when tension release under high temperature? (I know the answer to that last question now :-))

    Thanks again Merlin :-)
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