  The n-type HgCr2Se4 exhibits a sharp semiconductor-to-metal transition (SMT)
in resistivity accompanying the ferromagnetic order at TC = 106 K. Here, we
investigate the effects of pressure and magnetic field on the concomitant SMT
and ferromagnetic order by measuring resistivity, dc and ac magnetic
susceptibility, as well as single-crystal neutron diffraction under various
pressures up to 8 GPa and magnetic fields up to 8 T. Our results demonstrate
that the ferromagnetic metallic ground state of n-type HgCr2Se4 is destabilized
and gradually replaced by an antiferromagnetic, most likely a spiral magnetic,
and insulating ground state upon the application of high pressure. On the other
hand, the application of external magnetic fields can restore the ferromagnetic
metallic state again at high pressures, resulting in a colossal
magnetoresistance (CMR) as high as ~ 3 * 10^11 % under 5 T and 2 K at 4 GPa.
The present study demonstrates that n-type HgCr2Se4 is located at a peculiar
critical point where the balance of competion between ferromagnetic and
antiferromagnetic interactions can be easily tipped by the external stimuli,
providing a new platform for achieving CMR in a single-valent system.
