  Harmonic generation in atoms and molecules has reshaped our understanding of
ultrafast phenomena beyond the traditional nonlinear optics and has launched
attosecond physics. Harmonics from solids represent a new frontier, where both
majority and minority spin channels contribute to harmonics.} This is true even
in a ferromagnet whose electronic states are equally available to optical
excitation. Here, we demonstrate that harmonics can be generated {mostly} from
a single spin channel in half metallic chromium dioxide. {An energy gap in the
minority channel greatly reduces the harmonic generation}, so harmonics
predominantly emit from the majority channel, with a small contribution from
the minority channel. However, this is only possible when the incident photon
energy is well below the energy gap in the minority channel, so all the
transitions in the minority channel are virtual. The onset of the photon energy
is determined by the transition energy between the dipole-allowed transition
between the O-$2p$ and Cr-$3d$ states. Harmonics {mainly} from a single spin
channel can be detected, regardless of laser field strength, as far as the
photon energy is below the minority band energy gap. This prediction should be
tested experimentally.
