Paramagnetism

Paramagnetic substances are weakly magnetised in a field and tend to move from a weak-field region to a strong-field one: a magnet attracts them weakly.

Each atom, ion or molecule carries a permanent magnetic dipole moment of its own, but ceaseless thermal motion keeps them randomly pointed, so there is no net magnetisation without a field.

A strong enough applied field B₀, especially at low temperature, lines the atomic moments up along B₀. The lines crowd slightly into the sample and the field inside rises, usually by about one part in 10⁵.

In a non-uniform field a paramagnetic bar moves from weak field to strong.

Examples: aluminium, sodium, calcium, oxygen (at STP) and copper chloride.

χ and μᵣ of a paramagnet depend on the material and, in a simple way, on its temperature. A stronger field or a lower temperature raises M until it saturates, with every dipole aligned.