Helical magnetic ground states in the vicinity of the superconducting state of MnP*

MnP, the first Mn-based superconductor under pressure, exhibits superconductivity near the critical pressure of 7.5 GPa. It shows a ferromagnetic order followed by a helical order (helical-c) with the spins lying in the ab plane and the helical rotation propagating along c axis at ambient pressure. We performed high pressure single crystal neutron diffraction study up to 7 GPa, which is in the vicinity of the superconducting phase. Our results indicate that the magnetic phase in the vicinity of the superconducting phase has a helical-b structure with the spins lying in the ac plane and the magnetic propagation vector along b axis. Polarized single crystal neutron diffraction measurements at 1.8 GPa confirmed the helicity in the ac plane. With increasing pressure, incommensurability $\delta $ increases and the magnetic moment decreases. Furthermore, small lattice anomalies were observed at different magnetic transitions, indicating that magnetoelastic coupling works to stabilize the magnetic order.