Some time ago there was a discussion about de-magnetizing of loudspeaker magnets and why the get de-magnetized and how to.... and where to....etc..

This is what I found out about magnets and why they become weaker:
(this is a very oversimplified text)

When making the magnetic materials:
During formation of the material the atoms align themselves in the same magnetic direction over a small volume called a "domain". Each domain is a sort of a tiny small magnet by itself, then, but its field is canceled out at a distance by that of other domains, so that the net effect at a distance is zero.
In a magnetic material (eg. Alnico), when an exterior magnetic field is applied all the domain tend to ALIGN with it. Some domains align more easily than others and so the resulting magnetic moment depends on how STRONG the applied field is, up until all possible domain are aligned (material is then saturated).

Now, when the exterior magnetic field is removed these domains are held in aligment itselvf, but some of the most weakly domains will collapse back to randomly realign and then weakened the magnetic material in some degree.

This weaknes is caused by high temperatures, by shock or BY TIME.
So, that´s the oversimplified explanation to why magnets get weaker by time. Of course, they can again get re-align when applying a new, strong enough exterior magnetic field.

This information (oversimplified) came from Joseph J Stupak Jr. at the Oersted Tech. Corp. who was the author of the document:
"Methods of Magnetizing Permanent Magnets". EMCW Coil Winding Show
1 october-2 november 2000, Cincinnati, Ohio. (E-mail [email protected])

When speaking to a re-magnetizing company i Stockholm, Sweden, they told me that when re-magnetizing loudspeaker magnets their experience was they become 10-15% stronger. These magnets was from the 50´s and 60´s
mostly.

Place to re-magnetize loudspeaker magnets in Sweden as far as I know is HYAB in Stockholm. Talk to Mr Lars Aakesson on phone number
+46 8 261 080.

Best regards
Flodstroem