The $B-H$ curves $S_1$ and $S_2$ in the adjoining figure are associated with:

The figure illustrates how $B$,the flux density,inside a sample of ferromagnetic material varies with external magnetic field $B_0$. For the sample to be suitable for making a permanent magnet,

Answer the following questions:
$(a)$ Explain qualitatively on the basis of the domain picture the irreversibility in the magnetisation curve of a ferromagnet.
$(b)$ The hysteresis loop of a soft iron piece has a much smaller area than that of a carbon steel piece. If the material is to go through repeated cycles of magnetisation,which piece will dissipate greater heat energy?
$(c)$ '$A$ system displaying a hysteresis loop such as a ferromagnet,is a device for storing memory?' Explain the meaning of this statement.
$(d)$ What kind of ferromagnetic material is used for coating magnetic tapes in a cassette player,or for building 'memory stores' in a modern computer?
$(e)$ $A$ certain region of space is to be shielded from magnetic fields. Suggest a method.

$A$ bar magnet has a coercivity of $4 \times 10^3 \text{ A m}^{-1}$. It is placed inside a solenoid of $12 \text{ cm}$ length and $60$ turns. The current that should be passed through the solenoid to demagnetize the bar magnet is: (in $\text{ A}$)

Which of the following is represented by the area enclosed by a hysteresis loop?

The figure illustrates how $B$,the flux density inside a sample of unmagnetised ferromagnetic material,varies with $B_0$,the magnetic flux density in which the sample is kept. For the sample to be suitable for making a permanent magnet: