Diamagnetism — Definition

Definition

Imagine an atom as a tiny solar system, with electrons orbiting a nucleus. Each orbiting electron acts like a tiny current loop, creating a very small magnetic field, or what we call an 'orbital magnetic moment'. In most atoms, these individual magnetic moments are randomly oriented or cancel each other out, so the material doesn't have a net magnetic moment on its own. This is the starting point for understanding diamagnetism.

Now, what happens when we bring an external magnetic field near such a material? According to a fundamental principle in physics called Lenz's Law, any change in magnetic flux through a loop (like an electron's orbit) will induce a current (or in this case, a change in orbital motion) that creates its own magnetic field to *oppose* the original change.

So, when an external magnetic field is applied, it slightly alters the speed of the orbiting electrons. Some electrons might speed up, and others might slow down, in such a way that their collective orbital motion generates an *additional*, induced magnetic moment.

Crucially, this induced magnetic moment is always directed *opposite* to the applied external magnetic field.

Because this induced magnetic moment opposes the external field, the material experiences a weak repulsive force from the magnet. This is why diamagnetic materials are said to be 'weakly repelled' by magnets. Think of it like trying to push two north poles together – they resist each other. In the case of diamagnetism, the material itself temporarily becomes a tiny 'opposite pole' when a magnet is near.

This phenomenon is universal; all materials exhibit diamagnetism because all atoms have orbiting electrons. However, in many materials, this weak diamagnetic effect is masked by stronger magnetic properties like paramagnetism or ferromagnetism, which arise from other mechanisms (like unpaired electron spins).

Therefore, we only classify materials as 'diamagnetic' if diamagnetism is their dominant magnetic behavior. Examples include water, copper, bismuth, noble gases, and many organic compounds. An important characteristic is that this induced magnetic moment is temporary and disappears as soon as the external magnetic field is removed.

Also, the strength of diamagnetism is largely independent of temperature.