Free energy F(m,T,h) — metastable states, spinodal limits, hysteresis loop
Landau free energy F(m) = a(T-Tc)m² + bm⁴ - hm for a first-order transition (modified with m⁶ term).
At low T, F has two minima — the system follows one metastable branch past the spinodal point,
then jumps discontinuously. Sweeping h up then down traces a hysteresis loop.