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If you run a magnet a few times over an unmagnetized piece of a magnetic material (such as an iron nail), you can convert it into a magnet as well. Will using a permanent magnet to magnetize something reduce the field strength of the original magnet?

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    $\begingroup$ By "less of a magnet," do you mean that the strength of the field decreases? $\endgroup$
    – Kyle Kanos
    Commented Jul 9, 2015 at 18:37
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    $\begingroup$ The magnet won't change if you do it right. The energy to magnetize the metal comes from whatever causes the motion. $\endgroup$
    – CuriousOne
    Commented Jul 9, 2015 at 18:41
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    $\begingroup$ Not likely. The magnetism in object that can magnetize and demagnetize comes from aligning and randomizing the grain alignment (internal structure larger than molecules but generally still microscopic) $\endgroup$
    – Rick
    Commented Jul 10, 2015 at 20:05
  • $\begingroup$ Note that you can damage the field strength of a permanent magnet by heating above its Curie temperature, or by shocking it (I mean as in stress. Like hitting it against something, not as in passing a current through it.) $\endgroup$
    – CoilKid
    Commented Jul 19, 2015 at 23:16

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The original magnet should remain unchanged.

The atoms in magnet materials each have a magnetic moment, which is a vector quantity with a positive direction defined by how it responds to a magnetic field. For these purposes, you can think of it as an atom-sized magnet.

When you have a region in a material where all the magnetic moments are aligned the same way, that region is then call a magnetic domain. Magnetic materials are made up of many domains, which give them their magnetic properties.

In permanent magnets most, if not all, of the domains face the same way and are fixed in place by the crystalline structure of the material. This differs from temporary magnets, such as soft iron (like in a nail). In iron the domains are facing random directions, which gives it no net magnetism.

You can realign the domains in ferromagnetic materials (like iron) with a permanent magnet, or a magnetic field produced by a current. When the domains are realigned, what was originally just a nail becomes a magnet until demagnetized or heated above its Curie temperature.

You cannot easily realign the domains in a permanent magnet, although it is possible by exposing it to a large enough magnetic field, or by heating above the Curie temperature of the material. In general though, it is extremely unlikely that using a permanent magnet to magnetize something will result in the demagnetization of the permanent magnet.

Note that the model I described applies to a perfect crystal. In reality, a piece of iron is polycrystalline and contains many crystalline "grains". While the domains in the grains will change and orient to a magnetic field, any loose grains will orient themselves as well. For the most part the result will be the same, but is useful to know the grain structure can also determines whether the material will be magnetic overall.

Related or interesting:

More on how permanent magnets work (Georgia State University physics database.)

What is the specific cause of permanent magnetism? (another physics SE question.)

Georgia State University, Ferromagnetism

A good video description of permanent magnetism (YouTube)

Special Relativity and electromagnets (YouTube)

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