Intensity of Magnetisation

Definition. When a material is magnetised, the enormous number of atomic current loops within it (Section 1.3) become, on average, partially or wholly aligned, so the material as a whole acquires a net magnetic dipole moment. The intensity of magnetisation $M$ is defined as this net magnetic moment per unit volume of the material:

with SI unit ampere/metre (A/m), the same unit as the magnetising field intensity $H$ -- which is precisely what allows the dimensionless ratio $\chi=M/H$ (Section 1.9) to be defined.

Physical picture. In an UNMAGNETISED sample, the countless atomic dipole moments point in random directions (thermal agitation constantly disturbs any alignment), so their vector sum, and hence $M$, is zero even though each individual atom may carry a permanent moment. When an external field $H$ is applied, it exerts a torque on each atomic dipole (Section 1.7 applied at the atomic scale) that tends to turn it toward alignment with the field; the more of these atomic moments end up aligned, the larger $M$ becomes. …