  We present a time-resolved terahertz spectroscopic study of the half-metallic
ferromagnet CrO$_2$. The ultrafast conductivity dynamics excited by an optical
pump displays a very short (several picoseconds) and a very long (several
hundred picoseconds) characteristic time scales. We attribute the former to the
electron-phonon relaxation and the latter to the spin-lattice relaxation. We
use this distinction to quantify the relative contribution of the scattering by
spin fluctuations to the resistivity of CrO$_2$: we find that they contribute
less than one half of all scattering events below room temperature. This
contribution rises to $\sim70$ % as the temperature approaches $T_C$=390 K. The
small effect of spin fluctuations on the resistivity is unexpected in the light
of the proposed double-exchange nature of the electronic and magnetic
properties of CrO$_2$.
