Helical magnetic state in the vicinity of the pressure-induced superconducting phase in MnP

TL;DR

This study investigates the magnetic structure of MnP under high pressure, revealing a robust helical magnetic phase near the superconducting transition, and combines experimental and theoretical approaches to understand magnetic interactions relevant to unconventional superconductivity.

Contribution

The paper provides detailed high-pressure neutron diffraction data and theoretical modeling to elucidate the magnetic ground state and exchange interactions near the superconducting phase in MnP.

Findings

Helical magnetic structure persists up to 7.5 GPa.

Exchange interactions change significantly with pressure.

Magnetic fluctuations may influence superconducting pairing.

Abstract

MnP is a metal that shows successive magnetic transitions from paramagnetic to ferromagnetic and helical magnetic phases at ambient pressure with decreasing temperature. 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. These results suggest that MnP is an unconventional superconductor in which magnetic fluctuations may be relevant to the superconducting pairing mechanism. In order to elucidate the magnetic ground state adjacent to the superconducting phase first discovered in Mn-based materials, high-pressure neutron diffraction measurements have been performed in hydrostatic pressure up to 7.5 GPa. The helical magnetic structure with the propagation vector along the $b$ axis, reported previously at 3.8 GPa, was found to be robust up to 7.5 GPa. First principles and classical Monte Carlo calculations have also been performed to understand how the pressure-driven magnetic phase transitions are coupled with change of the exchange interactions. The calculations, which qualitatively reproduce the magnetic structures as a function of pressure, suggest that the exchange interactions change drastically with applied pressure and the further-neighbor interactions become more influential at high pressures. Combining the experimental and theoretical results, we describe the detail of exchange interactions in the vicinity of the superconducting phase which is critical to understand the pairing mechanism of the unconventional superconductivity in MnP.