= Solution
<Critical exponents> describe singular behavior near a <continuous phase transition>. Define $m(t,0)\sim(-t)^\beta$ below $T_c$, susceptibility $\chi(t,0)=\partial m/\partial h\sim|t|^{-\gamma}$, singular <heat capacity> $C_s\sim|t|^{-\alpha}$, and critical isotherm $m(0,h)\sim\operatorname{sgn}(h)|h|^{1/\delta}$. The ordinary quartic <Landau free energy> gives the equation of state
$$
h=r_t t\,m+um^3.
$$
At zero field its ordered minimum is proportional to $(-t)^{1/2}$. Differentiating the equation of state gives $\chi^{-1}=r_t t+3um^2$, proportional to $|t|$ on either side. At $t=0$, $h=um^3$, and the minimized free energy below the transition is proportional to $-t^2$. Thus
$$
\boxed{\beta=\tfrac12,\qquad\gamma=1,\qquad\delta=3,\qquad\alpha=0.}
$$
Here $\alpha=0$ denotes a finite heat-capacity jump, rather than a power divergence. Including gradients gives the <Landau scalar correlation length> $\xi\propto|t|^{-1/2}$ and correlation-length exponent $\nu=1/2$.
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