Q.What happens to a ferromagnetic material when its temperature increases above its Curie temperature?

[!TLDR] Above the Curie temperature $T_C$, the domain structure collapses and the material stops being ferromagnetic, behaving instead as an ordinary paramagnetic material, with $\chi=\dfrac{C}{T-T_C}$. [!ANSWER] It becomes paramagnetic (the domain structure is destroyed).

Ferromagnetism depends on a strong exchange coupling that locks the moments of all atoms within a domain into a common direction (section 11.5.3). As temperature rises, this exchange coupling weakens, progressively distorting the domain structure; at the material's Curie temperature $T_C$, the domain structure collapses entirely, and above $T_C$ the material simply behaves like an ordinary paramagnetic substance, with atomic moments once again randomised by thermal agitation in the absence of a field. Its susceptibility above $T_C$ follows the Curie-Weiss law, $\chi=\dfrac{C}{T-T_C}$, the direct generalisation of the ordinary paramagnetic Curie's law. The material's atoms have not lost their individual magnetic moments -- what is lost, permanently above $T_C$, is the domain organisation that made the bulk material strongly, cooperatively magnetic. [!ANSWER] It becomes paramagnetic (the domain structure is destroyed).

Recall the definition of Curie temperature and the domain-collapse mechanism from section 11.5.4.

Confusing the Curie temperature (the ferromagnetic-to-paramagnetic transition) with a temperature at which the material would melt, lose all magnetic moment entirely, or become non-magnetic in some permanent, irreversible sense -- cooling the material back below $T_C$ restores its ferromagnetic behaviour.