Demagnetization by domains and deperming

Demagnetization is the reduction of the net magnetization of a ferromagnetic material to zero. It happens naturally through the formation of domains, and it can be done deliberately by heating, by a reverse field, or by an alternating field of decreasing amplitude.

Demagnetization through domain formation

A single fully magnetized domain has poles at its ends and a large external field, so its magnetostatic energy is high. The crystal lowers its energy by dividing into domains:

 (a) single     (b) two        (c) many        (d) closure
     domain         domains        domains         domains
 +--------+   +--------+     +--+--+--+--+   +--------+
 | ---->  |   | ---->  |     |^ |v |^ |v |   |\ <--- /|
 | N    S |   | <----  |     |  |  |  |  |   | v    ^ |
 +--------+   +--------+     +--+--+--+--+   |/ ---> \|
 large field  field halved   field small     +--------+
                                             no free poles

1. (a) Single domain: all moments parallel, poles at the ends, large external field and maximum magnetostatic energy $\left(\tfrac{\mu_0}{2}\int H^2 dV\right)$.
2. (b) Two antiparallel domains: part of the flux returns inside the crystal; the magnetostatic energy is roughly halved, at the cost of one domain wall.
3. (c) Many domains: with $N$ domains the energy falls to about $1/N$. Splitting stops when the energy of a new wall equals the magnetostatic energy saved.
4. (d) Closure domains: triangular domains at the ends, magnetized at 90° to the main domains, complete the flux loop inside the crystal. No free poles remain, the external field and magnetostatic energy are almost zero, and the net magnetization is zero: the crystal is demagnetized.

The number and size of domains are fixed by the balance between the saved magnetostatic energy and the energy of the domain walls (plus anisotropy and magnetostrictive energy).

Practical methods of demagnetization

1. Heating above the Curie temperature and cooling in zero field: domains re-form randomly.
2. Applying a reverse field equal to the coercive force $H_c$: brings $B$ to zero (but not stable).
3. AC demagnetization: an alternating field whose amplitude is reduced slowly to zero takes the material round smaller and smaller hysteresis loops until it ends at the origin.

        B
        ^   ___
        |  / _ \
     ---+-/-(o)-\---> H     shrinking loops
        |  \___/            spiral to B = 0

Deperming method

Deperming is a demagnetization process used for large steel structures, mainly ships and submarines, to remove their permanent magnetism (acquired from the earth's field, welding and hammering during building) so that they do not trigger magnetic mines or disturb compasses.

• Large cables are wound round the hull (or the ship is placed in a deperming facility).
• Strong DC pulses of alternating polarity and decreasing magnitude are passed through the cables, e.g. +I, −0.8I, +0.6I, ….
• Each pulse drives the steel round a smaller hysteresis loop, so the permanent magnetization is reduced almost to zero.
• It is repeated periodically. A related method, degaussing, uses permanently installed coils carrying current to cancel the ship's field continuously.