Question

Assertion (A) : When a ferromagnetic substance is heated to high temperature it becomes paramagnetic in nature.
Reason (R) : The disappearance of magnetisation of a ferromagnet is abrupt and not gradual.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Both Assertion (A) and Reason (R) are false.

Hint:

Ferromagnetic to paramagnetic transition at Curie temperature is a continuous second-order phase transition. Magnetization decreases smoothly to zero as $T \to T_C$, not abruptly.\

Answer 1

The Correct Option is C

Concept:
• Ferromagnetic materials possess domain structures where atomic magnetic dipole moments are strongly aligned in parallel due to exchange coupling.
• As temperature rises, increased thermal agitation disrupts the spontaneous alignment of atomic dipoles within the domains.
• At temperatures exceeding the Curie temperature ($T > T_C$), the domain structure completely breaks down, and the substance transforms into a paramagnetic state.

Step 1: Analyze the Assertion (A)
When a ferromagnetic material (such as iron, cobalt, or nickel) is heated above its characteristic Curie temperature $T_C$, the thermal agitation overcomes the quantum exchange forces holding the magnetic domains together.
The domain structure is destroyed, and the substance exhibits paramagnetic behavior following the Curie-Weiss law: $\chi = \frac{C}{T - T_C}$ for $T > T_C$.
Therefore, Assertion (A) is true.

Step 2: Analyze the Reason (R)
The loss of spontaneous magnetization in a ferromagnetic material as temperature approaches $T_C$ is a continuous (second-order) phase transition.
The spontaneous magnetization decreases gradually and continuously as temperature increases, reaching zero smoothly at $T = T_C$ according to the power law $M_s(T) \propto (T_C - T)^\beta$, where $\beta$ is a critical exponent.
The disappearance of magnetization is therefore gradual over the temperature range and not an abrupt step discontinuity.
Therefore, Reason (R) is false.

Step 3: Conclusion
Assertion (A) is a true statement, whereas Reason (R) is false. Thus, option (C) is the correct choice.