Solution
= Solution
Equilibrium requires $h=am+bm^3$. On the critical isotherm $a=0$, so
$$
\boxed{h=bm^3,\qquad m=\operatorname{sgn}(h)|h/b|^{1/3},\qquad\delta=3.}
$$
Here $\delta$ is defined by $h\propto\operatorname{sgn}(m)|m|^\delta$ 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>.