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).
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ABoth A and R are true and R is correct explanation of A
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BBoth A and R are true but R is not correct explanation of A
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CA is true but R is false
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DA is false but R is true
Answer
Correct Answer: Both A and R are true and R is correct explanation of A
Explanation
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:
Identify phase: T < θf → ferromagnetic order present.Increase temperature beyond θf → order vanishes; becomes paramagnetic.Therefore, A is true; R correctly states the mechanism/condition causing A.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