  The emergence of superconductivity in the iron pnictide and cuprate high
temperature superconductors usually accompanies the suppression of an
antiferromagnetically (AFM) ordered state in a corresponding parent compound
through the use of chemical doping or external pressure1-5. A great deal of
effort has been made to find superconductivity in Mn-based compounds6-14, which
are thought to bridge the gap between the two families of high temperature
superconductors7,15,16, but long-ranged AFM order was not successfully
suppressed via chemical doping in these investigations. Here we report the
first observation of the pressure-induced elimination of long-ranged AFM order
in LaMnPO single crystals that are iso-structural to the LaFeAsO
superconductor15,17. By combining in-situ high pressure resistance and ac
susceptibility measurements, we found that LaMnPO undergoes a crossover from an
AFM insulating to an AFM metallic state at a pressure ~20 GPa and that the
long-ranged AFM order collapses at a higher pressure ~32 GPa. Our findings are
of importance to explore potential superconductivity in Mn-based compounds and
to shed new light on the underlying mechanism of high temperature
superconductivity.
