Q.At Curie temperature the ferromagnetic substance becomes:

Curie Temperature: When a Magnet "Gives Up"

Imagine a room full of people all facing the same direction — that's a ferromagnet. Now imagine the room gets hotter, people start jostling, and soon everyone is facing a random direction. The temperature at which that ordered "all facing one way" state breaks down is the Curie temperature.

The Intuition

In a ferromagnetic material like iron, cobalt, or nickel, there are tiny regions called domains. Inside each domain, the atomic magnetic moments (think of them as tiny compass needles) are all aligned in the same direction. This alignment is what gives the material its strong, permanent magnetism.

What keeps these moments aligned? A quantum mechanical interaction called exchange coupling — it's like a strong social pressure that makes neighbouring moments want to point the same way. But thermal energy works against this. As temperature rises, atoms vibrate more violently, and those vibrations shake the magnetic moments out of alignment.

At some critical temperature, the thermal jostling becomes strong enough to completely overwhelm the exchange coupling. The domains shatter. The material can no longer hold a net magnetisation on its own.

The Precise Statement

Below $T_C$, the material can retain magnetisation even without an external field. Above $T_C$, it behaves like a paramagnet — it only shows magnetism when an external field is applied, and weakly at that.

What Happens Above $T_C$?

Once past the Curie point, the material follows the Curie-Weiss law for its magnetic susceptibility $\chi$:

Here $C$ is the Curie constant (depends on the material). Notice what this says: as $T$ approaches $T_C$ from above, $\chi$ blows up to infinity. That's the signature of the phase transition — just above $T_C$, the material is almost ready to order, so it responds extremely strongly to any external field. …