The $n$-type ${\text{HgCr}}_2{\mathrm{Se}}_4$ exhibits a sharp semiconductor-to-metal transition (SMT) in resistivity accompanying the ferromagnetic order at $T_C=106\mathrm{K}$. 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 ${\text{HgCr}}_2{\mathrm{Se}}_4$ 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 $\sim 3\times {10}^{11}\%$ under 5 T and 2 K at 4 GPa. The present study demonstrates that $n$-type ${\text{HgCr}}_2{\mathrm{Se}}_4$ is located at a peculiar critical point where the balance of competition between ferromagnetic and antiferromagnetic interactions can be easily tipped by external stimuli, providing a new platform for achieving CMR in a single-valent system.

• Received 25 March 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.047201