  We identify the phase boundary between spiral spin and ferromagnetic phases
in Au$_2$Mn at a critical pressure of 16.4 kbar, as determined by neutron
diffraction, magnetization and magnetoresistance measurements. The
temperature-dependent critical field at a given pressure is accompanied by a
peak in magnetoresistance and a step in magnetization. The critical field
decreases with increasing temperature and pressure. The critical pressure
separating the spiral phase and ferromagnetism coincides with the disappearance
of the magnetroresistance peak, where the critical field goes to zero. The
notable absence of an anomalous Hall effect in the the ferromagnetic phase is
attributable to the high conductivity of this material.
