What is Curie temperature? And what is retentivity?

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.

Which of the following statements is incorrect about hysteresis?

The coercivity of a magnet is $5 \times 10^3 \text{ A/m}$. The amount of current required to be passed in a solenoid of length $30 \text{ cm}$ and the number of turns $150$,so that the magnet gets demagnetized when placed inside the solenoid is ............. $A$.

The energy dissipated per unit volume per cycle in the hysteresis of an iron sample of mass $10 \, kg$ is $200 \, J \, m^{-3} \, cycle^{-1}$. The density of iron is $7500 \, kg \, m^{-3}$. The loss of energy per hour at $50 \, cycle \, s^{-1}$ is: (in $J$)

The coercivity of a small magnet is $4 \times 10^3 \ A/m$. It is placed inside a solenoid of length $1 \ m$ having $500$ turns to demagnetize it. The current that must be passed through the solenoid is .......... $A$.