Diamagnetic, Paramagnetic and Ferromagnetic Substances

Every material responds, at least a little, to an external magnetic field, and the response falls into one of three sharply distinct classes, characterised by the sign and rough size of the material's susceptibility $\chi$ (Section 1.9) and by what is physically happening at the atomic level (Table 1 summarises all three side by side).

Diamagnetic substances. In a diamagnetic material, the constituent atoms have NO net magnetic dipole moment of their own in the absence of a field (their electrons' individual orbital moments cancel in pairs). When an external field IS applied, it INDUCES a small magnetic moment in each atom that opposes the applied field (a direct consequence of Lenz's law acting at the atomic scale on the orbiting electrons) -- so diamagnetic substances are WEAKLY REPELLED by a magnet and tend to move from a stronger to a weaker part of a non-uniform field. Their susceptibility $\chi$ is small and NEGATIVE, essentially independent of temperature, and $\mu_r$ is very slightly LESS than 1. Common examples: bismuth, copper, water, gold, antimony. Every material has some diamagnetic contribution, but it is completely masked whenever paramagnetic or ferromagnetic behaviour is also present.

Paramagnetic substances. Here the individual atoms DO possess a small permanent magnetic dipole moment (from unpaired electrons), but in the absence of a field these atomic moments point in completely random directions due to thermal agitation, so their net effect averages to zero. An external field partially aligns these already-existing moments (Section 1.10), so paramagnetic substances are WEAKLY ATTRACTED toward a magnet and move toward the stronger part of a non-uniform field. Their susceptibility $\chi$ is small and POSITIVE, and $\mu_r$ is very slightly GREATER than 1; because thermal agitation actively fights the alignment, $\chi$ FALLS as temperature rises (Curie's law, $\chi \propto 1/T$). Common examples: aluminium, platinum, chromium, and oxygen (in its liquid or gaseous state). …