= Symmetry-broken magnetisation versus a symmetric ensemble
An even double-well <Landau free energy> can have stable homogeneous <phases> at $M=\pm M_0$. In a selected <phase>, for example the thermodynamic limit with an infinitesimal symmetry-breaking field, mean magnetisation approaches one of those values. An exactly symmetry-preserving finite-system ensemble instead averages both signs and can have mean zero. This ensemble average does not make $M=0$ a stable homogeneous minimum. Distinguish a free-energy minimum, a domain mixture and an unrestricted ensemble average when discussing spontaneous magnetisation.
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