Past exam of the theoretical physics I course of the University of Cambridge / 2013 / Paper 1 / 6 / b / Solution

Equilibrium requires $h=am+b{m}^3$. On the critical isotherm $a=0$, soHere $\delta$ is defined by $h\propto \mathrm{sgn}(m)|m{|}^{\delta }$ at the critical temperature, or equivalently $|m|\propto |h{|}^{1/\delta }$. The positive quartic coefficient selects a stable equilibrium, including the flat but stable zero-field thermodynamic critical point.