Landau Mean-Field Theory
Free energy landscape F(m) = a·m² + b·m⁴ − h·m for a ferromagnet. Drag temperature and field to see double-well ↔ single-well transition and spontaneous symmetry breaking.
Landau expansion: F(m) = a(T)m² + bm⁴ − hm where a(T)=a₀(T/Tc−1).
T<Tc, h=0: double-well → spontaneous magnetization m*=±√(−a/2b).
T>Tc: single well, m*=0. At h≠0: wells tilt, one minimum disappears at spinodal.