  We have investigated the ferromagnetic phase transition of elemental Co by
high-resolution neutron backscattering spectroscopy. We monitored the splitting
of the nuclear levels by the hyperfine field at the Co nucleus. The energy of
this hyperfine splitting is identified as the order parameter of the
ferromagnetic phase transition. By measuring the temperature dependence of the
energy we determined the critical exponent $\beta = 0.350 \pm 0.002$ and the
ferromagnetic Curie temperature of $T_{\text{C}} = 1400$~K. The present result
of the critical exponent agrees better with the predicted value (0.367) of the
3-dimensional Heisenberg model than that determined previously by NMR.
