physics question #3179

Gabby Stec, a 15 year old female from Harrisburg asks on January 16, 2006,


Why, specifically, do ferrite magnets gain strength in low temperatures?

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the answer

James Livingston answered on January 17, 2006, A:

[Editor's introduction: One can get into a lot of detail but the math is very difficult as it has to do with quantum theory. Instead perhaps the best way to understand is by analogy. Magnetism exists because the spins of individual electrons in a material all line up. Imagine a bunch of spinning tops, with their axes of spin all pointing in exactly the same direction. That's a magnet. In ordinary non-magnetic materials, electron spin points every which way and so magnetism is cancelled out.

If some of the tops in a magnetic are bumped or pushed or made to wobble or point in a slightly different direction, then the magnetism won't be so strong. What is heat? Heat can be thought of as the jostling of atoms, even when they are in a solid like a magnet. So even though they are locked into a lattice of rows and columns--the crystal structure of the magnetic metal--they are still wiggling and jiggling in position. The hotter the material, the more they wiggle and jiggle. All this movement can cause the tops (the electrons with nice lined up spin) to get bumped and knocked off perfect alignment. The more heat, the more they get knocked off. When it's nice a cold, they hardly get knocked at all, so the magnetism is the strongest then.]

Dr. Livingston adds: Ferrite magnets are a bit more complicated because they are ferrimagnets, not ferromagnets. In ferrites, some of the atomic magnets point one way, and some point the opposite way, with one direction dominating and the net magnetization being the difference between the two sets of atomic magnets. As a result, ferrite magnets actually are a bit strange in that some magnetic properties actually decrease with decreasing temperature, since the two sets of atomic magnets respond somewhat differently to temperature changes. But at this level of discussion it's not necessary to bother with that complication.

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