Diamagnétisme

Diamagnetism arises when an external magnetic field induces a Lorentz force on orbiting electrons. Electrons accelerate or decelerate depending on their angular velocity direction relative to the field, creating additional currents that generate induced dipole moments opposite to the applied field. This induced magnetization is the defining characteristic of diamagnetic materials.

Diamagnetic materials repel magnetic fields because the induced magnetic moments are always directed opposite to the external field. This antiparallel orientation causes the material to experience a repulsive force, moving from stronger to weaker field regions. The relative permeability of diamagnets is less than unity, reflecting this repulsive interaction.

Magnetic susceptibility measures the ratio of magnetization to magnetic field intensity. For diamagnetic materials, susceptibility is a small negative value between 10⁻⁵ and 10⁻⁶, indicating weak antiparallel magnetization. This negative susceptibility is temperature independent because diamagnetism depends on orbital electron motion rather than thermal effects.

An atom's net magnetic dipole moment is the vector sum of its orbital and spin moments. In diamagnetic materials, paired electrons have zero net magnetic moments. When an external field is applied, induced moments opposite to the field develop, but the material maintains its overall diamagnetic character with negative susceptibility.

Diamagnetic susceptibility is temperature independent because it arises from the orbital motion of electrons in response to an external magnetic field, not from thermal energy or random atomic alignment. The induced Lorentz forces on electrons depend only on the field strength and electron velocity, making the effect independent of temperature variations.

All materials exhibit diamagnetism when placed in an external magnetic field. However, paramagnetic and ferromagnetic materials have much larger magnetic moments from unpaired electrons or permanent atomic dipoles, making their diamagnetic contribution negligible. Diamagnetic materials lack these unpaired electrons, so diamagnetism is their dominant magnetic response.

When an electron's angular velocity opposes the external field direction, the outward Lorentz force decelerates its motion. This deceleration creates an additional current in the clockwise direction, opposite to the initial current. The resulting induced dipole moment is again directed opposite to the external field, consistent with diamagnetic behavior.