Q.Explain the term intensity of magnetisation, and state, with reasons, whether its value (and hence the magnetic susceptibility) is positive or negative for a diamagnetic, a paramagnetic, and a ferromagnetic sample placed in the same magnetising field.

Intensity of magnetisation $M$ is the net atomic magnetic moment per unit volume that a sample acquires in field $H$ (Section 1.10); its sign follows directly from how each class's atomic moments respond.

Diamagnetic: the field INDUCES a moment that OPPOSES $H$ (Section 1.14), so the net moment per volume, $M$, points opposite to $H$ -- NEGATIVE $M$, hence negative $\chi=M/H$.

Paramagnetic: the sample's already-existing atomic moments partially align WITH $H$, so $M$ points along $H$ but is small (thermal agitation limits the alignment) -- small POSITIVE $\chi$. …

Trace each material class's atomic-level response to $H$ (Section 1.14) through to the sign and r …

• Assuming $M$ must always be positive -- diamagnetic materials genuinely have negat …