Retentivity and Coercivity

Sections 1.9-1.10 described a material's magnetic response in general; this section focuses specifically on FERROMAGNETIC materials, whose response does not vanish the instant the applied field is switched off.

The experiment. Take an unmagnetised ("virgin") ferromagnetic sample and increase the magnetising field $H$ steadily from zero. The flux density $B$ inside the sample (or, equivalently, its magnetisation $M$) rises rapidly at first and then levels off as the sample approaches SATURATION, the state in which essentially all its atomic dipoles are aligned and $B$ can rise no further no matter how much $H$ is increased (Section 1.10).

Retentivity (remanence). If $H$ is now reduced back toward zero from this saturated state, $B$ does NOT fall back to zero along the same path it rose -- it decreases more slowly, and when $H$ has been brought all the way back to exactly zero, a SUBSTANTIAL flux density still remains inside the material. This residual value of $B$ (or, equivalently, of $M$), measured at $H=0$ after the sample has been carried to saturation, is called the retentivity (or remanence) of the material. Physically, it means the sample has become, and stays, a PERMANENT magnet even after the external field that created it is completely removed -- this is exactly how permanent bar magnets are manufactured.

Coercivity. To bring this residual magnetism back down to zero, a magnetising field must now be applied in the OPPOSITE (reverse) direction. The magnitude of this reverse field required to reduce $B$ (or $M$) exactly to zero, again starting from the saturated state, is called the coercivity of the material, with SI unit ampere/metre (A/m) (the same unit as $H$, since coercivity IS a value of $H$). …