Ferromagnetism

Ferromagnetic substances are strongly magnetised in a field and are strongly drawn from weak-field to strong-field regions.

Their atoms carry dipole moments, as in a paramagnet, but the moments interact and line up spontaneously over a macroscopic region called a domain (the explanation needs quantum mechanics). A typical domain is about 1 mm across and holds about 10¹¹ atoms.

At first the domains point randomly and there is no net magnetisation. An applied field B₀ turns the domains towards it, and those already along B₀ grow, until they merge into one giant domain. Domains and their motion can be seen under a microscope using a liquid suspension of ferromagnetic powder.

The field lines crowd strongly into a ferromagnet, and in a non-uniform field it moves towards the high-field region.

Hard ferromagnets stay magnetised after the field is switched off, so they make permanent magnets such as compass needles. Examples: lodestone, found in nature, and alnico, whose ingredients are aluminium, nickel, cobalt, copper and iron.

Soft ferromagnets, such as soft iron, lose their magnetisation when the field is removed.

Ferromagnetic elements include iron, cobalt, nickel and gadolinium, with relative permeability above 1000.

Heated enough, a ferromagnet becomes a paramagnet as its domain structure breaks up; the magnetisation fades gradually with temperature.

Substances that keep their ferromagnetism at room temperature for a long time are permanent magnets.