Pressure dependence of the magnetic ground states in MnP

MnP, a superconductor under pressure, exhibits a ferromagnetic order below T-C similar to 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-s similar to 50 K at ambient pressure. We performed single-crystal neutron diffraction experiments to determine the magnetic ground states under pressure. Both T-C and T-s are gradually suppressed with increasing pressure and the helical order disappears at similar to 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.