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Asked in GSEB Board July 2021 · Dia, para and ferromagnetism
Idea: on the hysteresis loop, retentivity (remanence) is the magnetisation left when the magnetising field is switched off, and coercivity is the reverse field needed to destroy that magnetisation.
A permanent magnet must stay strongly magnetised with no current and no external field, so it needs high retentivity.
It must also survive stray fields, heat and rough handling without demagnetising, so it needs high coercivity.
Both together mean a fat hysteresis loop, which is what steel and alnico have.
Soft iron is the opposite case - high retentivity but low coercivity - which suits electromagnets and cores, not permanent magnets.
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