= Solution
Ferromagnetic interactions enhance the response above the ordering transition. The <Curie–Weiss law> improves the independent-spin approximation:
$$
\boxed{\chi(T)\simeq\frac{C}{T-\Theta},\qquad T>T_c,}
$$
with positive <Weiss temperature> $\Theta$ and the same high-<temperature> coefficient $C$. In a simple <mean-field theory>, take $\Theta=T_c=70\,\mathrm K$. More generally $\Theta$ is fitted and need not equal $T_c$, and the simple pole does not give exact critical behaviour arbitrarily close to the transition.
At zero <temperature>, a selected ferromagnetic orientation has all magnetic moments aligned. Its <saturation magnetization> is
$$
\boxed{M_s(0)=n\mu=\frac4{a^3}(7\mu_B)=2.077\times10^6\,\mathrm{A\,m^{-1}}.}
$$
Equivalently, there are $28\mu_B$ per conventional cell. The moment points along either allowed easy-axis direction at zero <magnetic field>. This is the local or single-domain spontaneous <magnetization>; a macroscopic multidomain sample can have smaller, even zero, net <magnetization> in the absence of a selecting field.
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