  We report on the use of parametric excitation to coherently manipulate the
collective spin state of an atomic vapour at room temperature. Signatures of
the parametric excitation are detected in the ground-state spin evolution.
These include the excitation spectrum of the atomic coherences, which contains
resonances at frequencies characteristic of the parametric process. The
amplitudes of the signal quadratures show amplification and attenuation, and
their noise distribution is characterized by a strong asymmetry, similarly to
those observed in mechanical oscillators. The parametric excitation is produced
by periodic modulation of the pumping beam, exploiting a Bell-Bloom-like
technique widely used in atomic magnetometry. Notably, we find that the
noise-squeezing obtained by this technique enhances the signal-to-noise ratio
of the measurements up to a factor of 10, and improves the performance of a
Bell-Bloom magnetometer by a factor of 3.
