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I have a bunch of magnets (one of those game-board thingies) given to me when I was a school-going lad over 20 years ago, and the magnets feel just as strong as it was the day it was given.

As a corollary to this question Do magnets lose their magnetism?, is there a way to determine how long a permanent magnet will remain a magnet?

Addendum: Would two magnets remain a magnet for a shorter duration if they were glued in close proximity with like poles facing each other?

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The accepted answer to the linked question is at best.... enigmatic. But in terms of this particular question, you probably should be asking something more precise, like what is the timescale associated with the decay of the total magnetic moment of the magnet. – BebopButUnsteady Sep 14 '11 at 20:26
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It loses some properties, but at a, for most practical usage cases, negligible rate. I remember the rate being in the order of 1% every decade or so.

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They lose more if you bang them around. – xpda Sep 15 '11 at 1:58
@xpda - Yes, well, you can do a lot of things to them, and that in return will influence their properties. But this is assuming you leave them in peace somewhere. – Rook Sep 15 '11 at 2:49

If a permanent magnet could "decay" at the rate given in Rook's answer above there would be none found in geological strata.

A permanent magnet has a permanent orientation of the magnetic moments in a specific vectorially additive direction depending on small crystal domains. To change, i.e. be demagnetized, the magnetic moments have to be randomized by either an external magnetic field or excess heat/melting or vibrations possibly. If nothing like that happens it should be stable. Little magnets in a box left undisturbed would not change magnetisation unless a random magnetic field was in the area .

Non magnetic iron left undisturbed will acquire a field from the magnetic field of the earth, so some change in the orientation of the field could happen to these little magnets, depending on how they lay with respect to the weak field of the earth.

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Some magnets, e.g. AlNiCo, require a keeper (essentially, a piece of iron placed between the poles) to keep magnetic flux lines concentrated inside the magnet, to keep from spontaneously demagnetizing (the material reaches an unstable point in the intrinsic B-H curve). They can also be demagnetized by mechanical means (e.g., by being dropped). I think this is because the shock provides energy necessary to change domain boundaries.

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Given the answers above if true, the life of a magnet would be equivalent to the sums of the half lives of the underlying material. In other words, material decay over a long period would result in no magnet.

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protected by Qmechanic Nov 4 '13 at 19:49

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