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Paramagnetism
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Paramagnetism

 

Paramagnetism is the tendency of the atomic magnetic dipoles, due to quantum-mechanical spin, in a material that is otherwise non-magnetic to align with an external magnetic field. This alignment of the atomic dipoles with the magnetic field tends to strengthen it, and is described by a relative magnetic permeability greater than unity (or, equivalently, a small positive magnetic susceptibility).

In pure paramagnetism, the field acts on each atomic dipole independently and there are no interactions between individual atomic dipoles. Such paramagnetic behaviour can also be observed in ferromagnetic materials that are above their Curie temperature.

Paramagnetic materials attract and repel like normal magnets when subject to a magnetic field. Under relatively low magnetic field saturation when the majority of the atomic dipoles are not aligned with the field, paramagnetic materials exhibit magnetisation according to Curie's Law:

In the above equation, M is the resulting magnetisation, B is the magnetic flux density of the applied field, T is absolute temperature (Kelvin), and C is a material-specific Curie constant. This law indicates that paramagnetic materials tend to become increasingly magnetic as the applied magnetic field is increased, but less magnetic as temperature is increased. Curie's law is incomplete because it fails to predict what will happen when most of the little magnets are aligned (after everything is aligned, increasing the external field will not increase the total magnetization) so Curie's Constant really should be expressed as a function of how much of the material is already aligned.

Paramagnetic materials in magnetic fields will act like magnets but when the field is removed, thermal motion will quickly disrupt the magnetic alignment. In general paramagnetic effects are small (magnetic susceptibility of the order of 10-3 to 10-5).

Ferromagnetic materials above the Curie temperature become paramagnetic.

See also: diamagnetism, ferromagnetism, Pierre Curie.