I'm sorry if my idea will be completely stupid, but I don't find it to be a proof of "magnetization on purpose".
Very small pieces of pure iron (and most steels) are magnetic (ferromagnetic) due to their nature. It looses it's magnetic properties when heated, but regains them when cooled - to what extent depends on the way the cooling is performed. If it's rapid the magnetization after the cooling will probably be small, If it's gradual the sample will fully regain the full magnetization (ordering), which will mostly depend on sample's shape and chemical composition.
Now if the piece is not a microscopic one, then the magnetization is due to the fact that when the large piece is cooled it has to be under the influence of magnetic fields (magnetic field of the earth -this is actually how we know that it did decrease during the past 1000 years, anvil, hammers etc.). So if one has a sword that has nonuniform magnetization it can be that this sword for example was wielded from a few different pieces. It can be that the chemical composition varies from side to side (sulfa, chromium I think do kill magnetization quite easily). It can be that while cooled it was lying next to a huge magnetized hammer, so that the external field itself was extremely nonuniform.
Again, the way cooling is performed is very important. Very rapid cooling usually prevents the formation of a magnetized state. So if the blade for example is differentially heat treated it is possible that it simply has different magnetic states present due to this fact.
Such swords rather then becoming demagnetized with time would first actually become uniformly magnetized, and then would assume some magnetization due to the current earth's magnetic field (if it lies in the same place all the time), however it's even a big question if this _ever_ happens (it well may be that the current magnetized state is so efficient that it should take unphysically long time to change it using weak fields and temperatures).
Sincerely yours,
K.Rivkin
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