Theory 13/Magnetic Materials
Coercivity Peaks at a Grain Size
The claim
Coercivity is maximised when the grain size approaches the single-domain size, falling for both smaller and larger grains.
01The mechanism
Below single-domain size, thermal fluctuations erase magnetisation; above it, domain walls move easily and demagnetise the grain at low field.
02Predictions
- P1Coercivity should peak at the single-domain size.
- P2Both finer and coarser grains should be softer.
03Refutation
Monotonic coercivity with grain size would remove the optimum.