What happens to the magnetic properties of a material when it reaches its Curie temperature?

When a material reaches its Curie temperature, it undergoes a significant change in its magnetic properties. At this temperature, ferromagnetic materials lose their ferromagnetic properties and transition to a paramagnetic state. This occurs because the thermal energy at the Curie temperature is sufficient to overcome the magnetic ordered state that exists below this temperature. As a result, the alignment of the magnetic domains, which is responsible for ferromagnetism, becomes disrupted.

In a ferromagnetic material, the magnetic domains are aligned in the same direction, contributing to a net magnetization. Once the material is heated to the Curie temperature, the thermal agitation causes these domains to lose their alignment, leading to a significant reduction in the material's ability to exhibit permanent magnetism. Instead, it behaves as a paramagnetic material, meaning it will only exhibit magnetic properties in the presence of an external magnetic field and will not retain any magnetization once this field is removed.

Understanding this transition is crucial in magnetic particle testing, as the presence of a ferromagnetic state is necessary for effectively revealing surface and near-surface defects through magnetic particle inspection.