= Solution
The equilibrium magnetization is a <global minimum> of the <Landau free energy>. Its stationary values solve the <polynomial equation>
$$
\frac{\partial f}{\partial m}=4a_4m^3+6a_6m^5-B=0,
$$
and a local minimum must satisfy
$$
\frac{\partial^2f}{\partial m^2}=12a_4m^2+30a_6m^4\geq0.
$$
One compares the value of $f$ at every such local minimum and chooses the smallest. Since $a_6>0$, the <polynomial> tends to positive infinity as $|m|$ tends to infinity, so a global minimum exists.
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