= Field conjugate to an order parameter
{title2=$-hm$}
= Conjugate field
{synonym}
A field conjugate to a scalar <order parameter> $m$ contributes $-hm$ to its <free-energy density>. Differentiating the equilibrium <free energy> with respect to $h$ gives $-m$, with density or extensive normalization stated consistently. In a ferromagnet the <conjugate field> is magnetic; for a fluid-density <order parameter> it is related to a chemical-potential difference. It can explicitly break the symmetry whose spontaneous breaking defines the ordered phase.
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