Pressure dependence of the magnetic ground states in MnP

Abstract

MnP, a superconductor under pressure, exhibits a ferromagnetic order below ${T}_{\mathrm{C}}\ensuremath{\sim}290$ K followed by a helical order with the spins lying in the $ab$ plane and the helical rotation propagating along the $c$ axis below ${T}_{\mathrm{s}}\ensuremath{\sim}50$ K at ambient pressure. We performed single-crystal neutron diffraction experiments to determine the magnetic ground states under pressure. Both ${T}_{\mathrm{C}}$ and ${T}_{\mathrm{s}}$ are gradually suppressed with increasing pressure and the helical order disappears at $\ensuremath{\sim}1.2$ GPa. At intermediate pressures of 1.8 and 2.0 GPa, the ferromagnetic order first develops and changes to a conical or two-phase (ferromagnetic and helical) structure with the propagation along the $b$ axis below a characteristic temperature. At 3.8 GPa, a helical magnetic order appears below 208 K, which hosts the spins in the $ac$ plane and the propagation along the $b$ axis. The period of this $b$ axis modulation is shorter than that at 1.8 GPa. Our results indicate that the magnetic phase in the vicinity of the superconducting phase may have a helical magnetic correlation along the $b$ axis.