  Recent studies have shown that logarithmic divergence of entanglement entropy
as function of size of a subsystem is a signature of criticality in quantum
models. We demonstrate that the ground state entanglement entropy of $ n$ sites
for ferromagnetic Heisenberg spin-1/2 chain of the length $L$ in a sector with
fixed magnetization $y$ per site grows as ${1/2}\log_{2} \frac{n(L-n)}{L}C(y)$,
where $C(y)=2\pi e({1/4}-y^{2})$
