Ferromagnetic materials consist of magnetic domains. When a small external field is applied, domains parallel to the external field start to grow. In this region, domain walls are moving. They are hindered by material defects. Lord Rayleigh investigated this first  and quantified the magnetization as a linear and quadratic term in the field:
- Rayleigh, Lord (1887). "On the behaviour of iron and steel under the operation of feeble magnetic forces". Phil. Mag. 1 23: 225–248.
- Kronmüller, Helmut; Fähnle, Manfred (2003). Micromagnetism and the microstructure of ferromagnetic solids. Cambridge University Press. p. 148. ISBN 0-521-33135-8.
- Cullity (1972). Introduction to magnetic materials. Addison-Wesley. p. 342.
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