Ferromagnetism vs Temperature – Curie Point Behaviour (Assertion–Reason) Assertion (A): Ferromagnetic materials exhibit ferromagnetic properties only when the temperature is below the (ferromagnetic) Curie temperature θf. Reason (R): For a ferromagnet at temperatures T > θf, the material behaves paramagnetically (spontaneous order is lost).

Introduction / Context:
Ferromagnets show spontaneous magnetization due to exchange coupling among atomic moments. This long-range order is temperature sensitive. Above a critical temperature (Curie point), thermal agitation disrupts alignment and the material transitions to paramagnetic behavior with no spontaneous magnetization. The Assertion–Reason pair explores this fundamental thermal transition.

Given Data / Assumptions:

• Ferromagnetic material with Curie temperature θf (also written Tc).
• Spontaneous magnetization present only below θf.
• Classical/quantum statistical descriptions (Weiss/Heisenberg models) apply.

Concept / Approach:

Below θf, exchange interactions dominate over thermal disorder, producing spontaneous domain alignment (ferromagnetism). At T ≥ θf, thermal energy is sufficient to overcome cooperative ordering; the susceptibility follows the Curie–Weiss law, χ ∝ 1/(T − θ), and the material behaves paramagnetically without remanence or coercivity in the zero-field limit. Hence, both statements are true and the Reason directly explains the Assertion.

Step-by-Step Solution:

Verification / Alternative check:

Experimentally, magnetization–temperature curves show spontaneous magnetization dropping to zero at θf, with susceptibility changing to Curie–Weiss behavior above θf.

Why Other Options Are Wrong:

• A and R not independent; R explicitly explains A by stating the high-temperature phase.
• Claiming R is false contradicts established magnetic phase transitions.

Common Pitfalls:

Confusing the “paramagnetic Curie temperature” sign conventions; forgetting that antiferromagnets have a Néel temperature, not a Curie point.

Final Answer:

Both A and R are true and R is correct explanation of A