Materials consisting of paired electrons have zero net magnetic moments. However, when these materials are placed under an external magnetic field, the moments opposite to the field are induced. Such materials are called diamagnets. Diamagnetism is the response of the diamagnets when placed in an external magnetic field.
Diamagnetism was discovered by Anton Brugmans in 1778 when he observed that bismuth gets repelled by magnetic fields, thus theorizing that diamagnets get repelled by magnets. When placed under a non-uniform external magnetic field, diamagnets tend to move from the stronger to the weaker magnetic field region. Since diamagnets repel the magnetic field, their magnetic permeability is less than unity.
When a magnetic field is applied to a material, it gets magnetized. The applied field is generally expressed as magnetic field intensity. The resultant magnetic field is the sum of the applied magnetic field and the field due to the magnetization. The magnetization is very small and antiparallel to the applied magnetic field. The ratio of the magnetization to the magnetic field intensity is known as the susceptibility. The susceptibility for diamagnets is a minimal negative value of 10-5–10-6 Since diamagnetism occurs due to the orbital motion of electrons independent of temperature, the susceptibility value is also temperature independent for diamagnets.
When placed in an external magnetic field, all materials have a diamagnetic contribution. For paramagnetic and ferromagnetic materials, the diamagnetic contribution is very small, and thus can be neglected.