Demagnetization and material for digital storage

Demagnetization is the process by which the net magnetization of a ferromagnetic material is reduced to zero. It occurs naturally when the material divides into magnetic domains.

Demagnetization through domain formation

A single domain has free poles and a large external field, i.e. high magnetostatic energy. The crystal lowers this energy by splitting:

 (a) single   (b) two      (c) closure
 +-------+   +-------+    +-------+
 | --->  |   | --->  |    |\ <-- /|
 |N    S |   | <---  |    |^     v|
 +-------+   +-------+    |/ --> \|
 big field   half field   +-------+  zero field

1. (a) One domain: large magnetostatic energy.
2. (b) Two antiparallel domains: energy about halved; more domains reduce it further, until the wall energy balances the saving.
3. (c) Closure domains at the ends close the flux inside; no free poles, net $M = 0$.

In practice a material is demagnetized by an AC field of slowly decreasing amplitude or by heating above $T_C$; the domains then return to this random, flux-closed arrangement.

Material for storing digital information

Hard (semi-hard) ferrimagnetic or ferromagnetic materials with a square hysteresis loop, e.g. γ-Fe₂O₃, CrO₂, barium ferrite, Co-Cr/Co-Pt thin films (hard disks).

Why: the square loop gives two stable remanent states $+B_r$ and $-B_r$ for 1 and 0; high remanence gives a strong read signal; adequate coercivity keeps data safe from stray fields (non-volatile) yet allows writing by the head; sharp switching and fine grains allow fast, dense storage.