Magnetic Properties of Materials

(a) A ferromagnetic substance becomes paramagnetic

Above the Curie temperature, thermal agitation destroys the alignment of the magnetic domains, so a ferromagnetic material (e.g. iron, whose Curie temperature is about $770^\circ$C) loses its strong magnetism and behaves as a paramagnetic substance.

(a) Increases

The susceptibility of a ferromagnetic substance (above its Curie temperature) follows the Curie–Weiss law, $\chi = \dfrac{C}{T - T_C}$. As the temperature decreases, thermal agitation becomes weaker, the magnetic dipoles align more easily, and $\chi$ increases.

(D) Cobalt

Iron, cobalt and nickel (and their alloys) are ferromagnetic. Aluminium is paramagnetic, while gold and copper are diamagnetic.

(A) $\chi + 1$

Since $B = \mu_0(H + M) = \mu_0(1 + \chi)H$ and $B = \mu_0\mu_r H$, it follows that $\mu_r = 1 + \chi$.

(d) AE

Coercivity is the reverse magnetising force $H$ needed to reduce the residual flux density $B$ to zero. On the loop, this is where the curve cuts the $-H$ axis, at E, so it is the length AE. AD (and AG) on the $B$-axis represent the retentivity (residual magnetism).