Superconductivity and Magnetism
Most recent answer: 10/22/2007
Q:
Why are superconductors not magnetic?
- Ola
Sweden
- Ola
Sweden
A:
Neat question. We think that the answer is that some superconductors
are magnetic. However, most aren’t. The reason is that
superconductivity arises from the behavior of pairs of electrons, where
electrons of opposite spin (and hence opposite intrinsic magnetism)
pair up. If the state they pair up into has no orbital magnetism (i.e.
no circulating current) then there can’t be any magnetism without
breaking the superconducting pairs and losing superconductivity. (In
Type II superconductors, magnetism can penetrate the superconductor,
but only in little vortices within which the superconductivity is
lost.)
However, it is now strongly suspected that there can be types of electron pairing in which the pairing state does have circulating currents, and is intrinsically magnetic. Something very like that is already known for superfluidity in helium. Superconductivity of that type would require magnetism. In any particular sample, it would just be an accident which way the magnetism pointed.
Mike W.
For the first kind of superconductor Mike mentions, you can make a superconducting material stop superconducting by introducing magnetic atoms in it. Superconductors normally expel magnetic fields from their bulk, called the "Meissner effect". Introducing magnetic impurities to a material which otherwise superconduct makes the effect go away. Here is a cool link from Lawrence Berkeley Labs about the different kinds of superconductors:
Tom
However, it is now strongly suspected that there can be types of electron pairing in which the pairing state does have circulating currents, and is intrinsically magnetic. Something very like that is already known for superfluidity in helium. Superconductivity of that type would require magnetism. In any particular sample, it would just be an accident which way the magnetism pointed.
Mike W.
For the first kind of superconductor Mike mentions, you can make a superconducting material stop superconducting by introducing magnetic atoms in it. Superconductors normally expel magnetic fields from their bulk, called the "Meissner effect". Introducing magnetic impurities to a material which otherwise superconduct makes the effect go away. Here is a cool link from Lawrence Berkeley Labs about the different kinds of superconductors:
Tom
(published on 10/22/2007)