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Q & A: Liquid magnets?

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Most recent answer: 10/22/2007
Q:
Can magnetic material be stored in a liquid form with out losing it’s magnatism
- Adam (age 28)
Alexandria, Va
A:
No, for a couple of reasons.

Ordinarily, the tendency of the electron spins (little magnets) to line up with each other is more fragile than the tendency of the atoms or molecules to form crystals. So the Curie temperature, at which the magnetism vanishes (the spins start pointing randomly) is less than the temperature at which the crystal melts. When the crystal is cooled down again, the material can't remember which direction was magnetic North and which was South, so the new magnetism has no memory of the direction of the old magnetism.
There's a deep reason why for any magnetic pattern the exact opposite pattern is equally likely to show up. One is just what you get if you look at the other but with time running backwards. In thermal equilibrium, there's on the average no difference between forward and backward in time, so neither pattern can be more likely than the other.

In one case that we know of a liquid metal remains magnetic, in the sense that the spins still line up into magnetic domains. (www.edpsciences.org/articles/ euro/pdf/1998/19/44117.pdf ) However, even then the direction of the magnetism will be quickly forgotten over time in the liquid as the domains randomly rotate. In a solid, the combination of the regular crystal pattern and random local deviations from the regular pattern make the magnetic directions get stuck in special directions. It takes a very long time in a good magnet for the domains to flip around, so that's why we call those 'permanent' magnets.

One case that is particularly interesting is the magnet inside the Earth that makes compasses line up. It's much more complicated than the simple magnetic materials we talked about, because it is not close to being in thermal equilibrium. Also, it's enormous, which tends to make the typical times for it to change long. Nevertheless, the Earth's magnetism too occasionally changes, leaving a record in the longer-lived magnetism of rocks.

Mike and Tom

(published on 10/22/2007)

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