Magnetic states of MnP: muon-spin rotation studies

Muon-spin rotation data collected at ambient pressure ($p$) and at $p=2.42$ GPa in MnP were analyzed to check their consistency with various low- and high-pressure magnetic structures reported in the literature. Our analysis confirms that in MnP the low-temperature and low-pressure helimagnetic phase is characterised by an increased value of the average magnetic moment compared to the high-temperature ferromagnetic phase. An elliptical double-helical structure with a propagation vector ${\bf Q}=(0,0,0.117)$, an $a-$axis moment elongated by approximately 18% and an additional tilt of the rotation plane towards $c-$direction by $\simeq 4-8^{\rm o}$ leads to a good agreement between the theory and the experiment. The analysis of the high-pressure $\mu$SR data reveals that the new magnetic order appearing for pressures exceeding $1.5$ GPa can not be described by keeping the propagation vector ${\bf Q} \parallel c$. Even the extreme case -- decoupling the double-helical structure into four individual helices -- remains inconsistent with the experiment. It is shown that the high-pressure magnetic phase which is a precursor of superconductivity is an incommensurate helical state with ${\bf Q} \parallel b$.