Ferromagnetism — identifying correct property statements Which of the following statements about ferromagnetic materials is correct?

Introduction / Context:
Ferromagnetism is characterized by strong magnetic ordering arising from exchange interactions. Understanding basic ferromagnetic properties helps differentiate ferromagnets from ferrites and paramagnets.

Given Data / Assumptions:

• “Spontaneous magnetisation” refers to nonzero magnetization without an external field.
• Curie temperature is the threshold above which ferromagnets become paramagnetic.
• Microwave-frequency use depends strongly on material losses and conductivity.

Concept / Approach:
Ferromagnets exhibit spontaneous magnetisation below the Curie temperature. Bulk metallic ferromagnets (like iron) have relatively low resistivity compared with insulating ferrites; thus, statement about “very high resistivity compared to iron” is false—iron is a ferromagnet and a good conductor. At microwave frequencies, eddy-current and magnetic losses are substantial in metals, so they are generally unsuitable; instead, high-resistivity ferrites (ceramic ferromagnets) are used.

Step-by-Step Solution:
Identify correct core property: spontaneous magnetisation below Curie temperature → true.Evaluate resistivity claim: false for metallic ferromagnets vs iron.Evaluate microwave usability: metallic ferromagnets suffer high losses; thus false as stated generally.Therefore only option (a) is correct.

Verification / Alternative check:
Magnetization–temperature curves and B–H hysteresis loops confirm spontaneous magnetization below Curie temperature; engineering practice uses ferrites (not metals) in microwave devices.

Why Other Options Are Wrong:
(b) contradicts basic electrical properties; (c) ignores microwave loss mechanisms; (d) cannot be true if (b) and (c) are false; (e) is wrong because (a) is correct.

Common Pitfalls:

• Conflating ferrites (ceramic, high resistivity) with metallic ferromagnets.
• Assuming good low-frequency magnetic behavior translates to microwave suitability.

Final Answer:
They show spontaneous magnetisation below a certain temperature