Pressure dependence of the helical spin structure in MnP

MnP is a metal that shows successive magnetic transitions from paramagnetic to ferromagnetic and helical magnetic phases with decreasing temperature at ambient pressure. With applied pressure, the magnetic transition temperatures decrease and superconductivity appears around 8 GPa where the magnetic order is fully suppressed and the quantum critical behavior is observed [1]. These results suggest that the superconducting pairing mechanism is unconventional and may be relevant to magnetic fluctuations. In order to elucidate the magnetic ground state adjacent to the superconducting phase, high-pressure neutron diffraction measurements have been performed. The helical magnetic structure with the propagation vector along the b axis, reported previously at 3.8 GPa [2], was found to be robust up to 7.6 GPa. Pressure dependences of the magnetic propagation vector, the magnetic transition temperature, and the magnetic moment in the helical phase were elucidated. Furthermore, we performed theoretical calculations to evaluate the frustrated magnetic interactions as a function of pressure and understand how the helical structure is stabilized with applied pressure.

[1] J.-G. Cheng et al., Phys. Rev. Lett. 114, 117001 (2015); [2] M. Matsuda et al., Phys. Rev. B 93, 100405 (2016).