Q.What should be the retentivity and coercivity of a permanent magnet?

The retentivity and coercivity of a ferromagnetic material -- how much magnetization it retains, and how hard it is to demagnetise -- decide which of two very different devices it is suited to build. An electromagnet uses a SOFT ferromagnetic core (large permeability, low retentivity and coercivity -- a thin hysteresis loop) inside a solenoid: passing current through the solenoid magnetises the core, boosting the field a thousand-fold or more, while switching the current off lets the field collapse to near zero, since the soft core retains almost none of its magnetisation on its own. This switchability makes electromagnets useful in electric bells, loudspeakers, circuit breakers, cranes for lifting iron loads, and (as superconducting magnets, for the highest fields) in NMR spectroscopy.

A permanent magnet instead uses a HARD ferromagnetic rod (high retentivity and coercivity -- a wide hysteresis loop): magnetising it once via a solenoid leaves it strongly, durably magnetised even after the current -- and its field -- are switched off completely, because a hard material retains its magnetisation to a far greater extent than a soft one. The opposite requirements of these two devices (low retentivity/coercivity for an electromagnet's switchable core, versus high retentivity/coercivity for a permanent magnet's durable field) are a direct, practical consequence of the hysteresis loop shape of the chosen material.