Question

Which of the following properties truly characterise ferromagnets ?

• A. They are capable of being magnetised very strongly by a magnetic field
• B. They retain a considerable amount of their magnetisation on removal of the applied field
• C. They lose their ferromagnetism and become paramagnetic when the temperature is raised above a certain temperature.
• D. The linear relation between magnetic field induction $\overrightarrow{B}$ and intensity of magnetisation $\overrightarrow{M}$ does not exist.
  

Answer 1

Answer: (A), (B), (C), (D)

The linear relation between magnetic field induction $\overrightarrow{B}$ and intensity of magnetisation $\overrightarrow{M}$ does not exist.


Behaviour of ferromagnetic material in presence of external magnetic field is given by


Hysteresis:
On removing the external magnetic field, a ferromagnetic material doesn’t get demagnetized fully. To bring the material back to zero magnetization, a magnetic field in the opposite direction has to be applied. The property of ferromagnetic materials retaining magnetization after the external field is removed is called hysteresis.
The magnetization of the material measured in terms of magnetic flux density (B) when plotted against the external applied magnetic field intensity (H) will trace out a loop. This is called the hysteresis loop.


Effect of temperature:
Ferromagnetic property depends on temperature. At a high enough temperature, ferromagnetic substances become paramagnetic. The temperature at which this transition occurs is called Curie’s temperature. It is denoted by $T_C$.
Relation between intensity of Magnetisation and magnetic field induction is given by
$\frac{\overrightarrow{B}}{{\mu }_0}=\overrightarrow{H}+\overrightarrow{I}$