Paramagnetism is a type of magnetism that occurs in substances with a positive magnetic susceptibility. It results in these substances being weakly attracted by a strong magnet. Paramagnetism is caused by the presence of at least one unpaired electron orbital (giving rise to an unpaired spin) in the atoms, molecules, or ions of the paramagnetic material, which results in these particles having a dipole moment. An applied magnetic field tends to align these dipoles in such a way that for small field and high temperatures the induced field is proportional to the applied field; the magnetization is in the same direction as the applied field.

 

Paramagnetism is normally stronger than diamagnetism, and the effect varies inversely with temperature. Below the Curie temperature, certain paramagnetic materials exhibit ferromagnetism.

 

Some compounds and most chemical elements are paramagnetic under certain circumstances. However, true paramagnets display magnetic susceptibility according to the Curie or Curie-Weiss laws and exhibit paramagnetism over a wide temperature range. Examples of paramagnets include the coordination complex myoglobin, transition metal complexes, iron oxide (FeO), and rare earth ions. Elements that are paramagnetic at room temperature include aluminum (Al), manganese (Mn), platinum (Pt), and oxygen (O₂, gas and liquid).

 

How paramagnetism works

Paramagnetism results from the presence of least one unpaired electron spin in a material's atoms or molecules. In other words, any material that possesses atoms with incompletely filled atomic orbitals is paramagnetic. The spin of the unpaired electrons gives them a magnetic dipole moment. Basically, each unpaired electron acts as a tiny magnet within the material. When an external magnetic field is applied, the spin of the electrons aligns with the field. Because all the unpaired electrons align the same way, the material is attracted to the field. When the external field is removed, the spins return to their randomized orientations.

The magnetization approximately follows Curie's law, which states that the magnetic susceptibility χ is inversely proportional to temperature:

 

M = χH = CH/T

 

where M is magnetization, χ is magnetic susceptibility, H is the auxiliary magnetic field, T is the absolute (Kelvin) temperature, and C is the material-specific Curie constant.