= Magnetic domain wall
{wiki=Domain_wall_(magnetism)}
A <magnetic domain wall> separates differently oriented <magnetic domains>. Its motion changes their <volumes> and average <magnetization>. It is a magnetic <domain wall>.
An original two-<magnetic domain> model makes the collective change explicit. Suppose opposite regions have <magnetizations> $+M_s$ and $-M_s$ along the measuring axis, where $M_s$ is the local <saturation magnetization>. If a <magnetic domain wall> sweeps <volume> $\Delta V$ from the negative region into the positive one, in a specimen of fixed <volume> $V$, then
$$
\Delta m=2M_s\Delta V,\qquad
\Delta M=\frac{2M_s\Delta V}{V}.
$$
The factor two comes from replacing $-M_s$ by $+M_s$. The local magnitude $M_s$ stays fixed while the average <magnetization> changes; the event size depends on the swept <volume>, rather than a universal single-<spin> increment.
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