Effect of Temperature

Ferromagnetism is fundamentally a temperature-dependent phenomenon, because it rests entirely on the exchange coupling between neighbouring atomic moments described in section 11.5.3, and that coupling itself weakens as temperature rises. As a ferromagnetic material is heated, this weakening exchange coupling progressively distorts the domain structure -- domains that were sharply aligned along a single axis become less perfectly ordered -- until, at a material-specific temperature, the domain structure collapses ENTIRELY, and the material's magnetic behaviour turns paramagnetic. This transition temperature is called the Curie temperature, $T_C$, of the material.

Above the Curie temperature ($T>T_C$), the material's susceptibility follows the Curie-Weiss law, $\chi=\dfrac{C}{T-T_C}$, a direct extension of the ordinary paramagnetic Curie's law ($\chi\propto1/T$) with the temperature T replaced by $T-T_C$, so that $\chi$ formally diverges as T approaches $T_C$ from above (Fig. 11.10) and falls off with further heating beyond that, exactly as an ordinary paramagnetic material's susceptibility does with temperature. …