Deperming and graphite in non-uniform field

Deperming method of demagnetization

Deperming is the process of removing the permanent (remanent) magnetism of large steel bodies, mainly ships and submarines. A steel hull becomes magnetized by the earth's field during construction (welding, riveting, hammering) and in service. This magnetism can trigger magnetic mines and disturb compasses.

Procedure

1. Heavy cables are wrapped round the hull (or the ship enters a deperming station with built-in coils).
2. Large DC current pulses of alternating polarity and decreasing magnitude are passed: +I₁, −I₂, +I₃, … with I₁ > I₂ > I₃ ….
3. Each pulse drives the steel round a smaller hysteresis loop, so the B–H point spirals towards the origin.
4. The magnetic signature is measured and the process is repeated until the remanent field is nearly zero.

   B
   ^  /\      loops shrink
   | /  \  /\
 --+------\/--\/-> H
   |  ending near B = 0

It is the same principle as AC demagnetization with a decaying field, applied with DC pulses to a very large object. (Degaussing, by contrast, uses coils carrying a steady current on board to cancel the remaining field.)

Graphite in a non-uniform magnetic field

Graphite is diamagnetic, and strongly so (one of the largest diamagnetic susceptibilities, $\chi \approx -10^{-4}$, especially pyrolytic graphite).

• In a field it develops a magnetization opposite to $H$.
• Its energy is $U = -\tfrac{\chi V B^2}{2\mu_0}$, which is positive for $\chi < 0$, so it is lowest where $B$ is weakest.
• Hence in a non-uniform field, graphite feels a force $F = \dfrac{\chi V}{2\mu_0}\nabla(B^2)$ directed from the stronger field region towards the weaker region: it is repelled by the magnet's poles.
• A thin piece of pyrolytic graphite can even levitate stably above an array of strong NdFeB magnets.
• In a uniform field it feels no net force, only a weak magnetization opposing the field.