physics question #587



m, a 14 year old female from the Internet asks on January 23, 2002,

Q:

Does water affect a magnet's strength? Do magnets lose their strength in cold temperatures? Are there any exceptions?

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the answer

James Livingston answered on January 24, 2002, A:

Water does not significantly affect a magnet's strength except that small parts of some magnets may have a chemical reaction with water, so that some magnets have to be coated if they will be immersed in water for an extended period of time.

Most metallic magnets (alnicos, rare-earth & neodymium magnets) actually become stronger in cold temperatures. However, the common "ceramic" magnets made of ferrites (mostly iron barium oxide), used widely in applications where low cost is more important than maximum properties, do lose some of their strength in very cold temperatures.

James Livingston answered on January 28, 2004, A:

Reader Lizette, asks on January 22, 2004:
Why are magnets stronger when they are in colder temperatures?

Dr. Livingston answers on January 28, 2004:
Most ferromagnets gradually lose a bit of their magnetism as temperature increases. Ferromagnetic ordering results from the "exchange force" between net spins on neighboring atoms, but increasing temperature leads to an increase in thermal disorder - a classic example of the competing goals of lower energy (via increased spin order) and higher entropy (via increased spin disorder). Entropy eventually wins at a sufficiently high temperature. Low temperature favors order, high temperature favors disorder. Above the Curie temperature, individual iron atoms still retain a magnetization from net spin, but the spins are now randomly directed and iron is paramagnetic.

That might be enough, unless you want to add that the effect of temperature is usually treated theoretically with thermodynamic models and minimization of free energy E-TS, which includes both energy E and entropy S.

Increasing temperature of course also leads to increasing atomic vibrations, and in permanent magnets, this can lead to the thermal unpinning of domain walls and a coercivity (called magnetic "hardness" in Driving Force) that decreases with temperature more rapidly than the basic ferromagnetism itself (and its saturation magnetization). So temperature usually has a greater effect on "hard" or permanent magnets than on "soft" or temporary magnets.

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