Demagnetizing via B-H curve; data storage material

Demagnetizing a magnetic material using the B–H curve

After a ferromagnetic material has been magnetized to saturation and the field removed, it keeps a remanent flux density $B_r$ (point b). It can be demagnetized in these ways:

(a) By a reverse field equal to the coercive force: applying $H = -H_c$ takes the material from $B_r$ to $B = 0$ (point c). This is not stable; small disturbances leave some residual magnetism.

(b) By an alternating field of decreasing amplitude (AC demagnetization): the material is taken round a series of hysteresis loops, each smaller than the last. As the amplitude falls to zero, the loops shrink and spiral into the origin, where $B = 0$ and $H = 0$. The domains end up randomly oriented, so this method gives true demagnetization.

               B
               ^  a (sat.)
          b Br |   ___
             __|__/  /
            /  | __ /
           / /_|/ //
   -------c-/--o--/-------> H
         -Hc|  |/ / +Hc
           /___|_/
               |   loops shrink to o

(c) By heating above the Curie temperature and cooling without a field: thermal agitation destroys alignment and domains re-form randomly.

Material for electronic storage of digital data

A hard (semi-hard) magnetic material with a square (rectangular) hysteresis loop is chosen, e.g. γ-Fe₂O₃, CrO₂, barium ferrite, or Co-Cr/Co-Pt alloy thin films in hard disks.

Justification

• Two stable states: a square loop has high remanence $+B_r$ and $-B_r$ at $H = 0$, which represent binary 1 and 0.
• High remanence: gives a strong readable signal.
• Sufficient coercivity: stray fields and neighbouring bits cannot erase stored data, so storage is non-volatile; yet coercivity is not so high that the write head cannot switch it.
• Sharp switching: a field just above $H_c$ flips the bit quickly and cleanly.
• Fine particles/grains: allow high storage density.

The read/write heads themselves use soft materials (permalloy, ferrite) so they follow the signal without retaining it.