  Most unconventional superconductors, including cuprates and iron-based
superconductors, are derived from chemical doping or application of pressure on
their collinearly magnetic-ordered parent compounds[1-5]. The recently
discovered pressure-induced superconductor CrAs, as a rare example of a
non-collinear helimagnetic superconductor, has therefore generated great
interest in understanding microscopic magnetic properties and their interplay
with superconductivity [6-8]. Unlike cuprates and iron based superconductors
where the magnetic moment direction barely changes upon doping, here we show
that CrAs exhibits a spin reorientation from the ab plane to the ac plane,
along with an abrupt drop of the magnetic propagation vector at a critical
pressure (Pc~0.6 GPa). This magnetic phase transition coincides with the
emergence of bulk superconductivity, indicating a direct connection between
magnetism and superconductivity. With further increasing pressure, the magnetic
order completely disappears near the optimal Tc regime (P~0.94 GPa). Moreover,
the Cr magnetic moments between nearest neighbors tend to be aligned
antiparallel with increasing pressure toward the optimal superconductivity
regime. Our findings suggest that the non-collinear helimagnetic order is
strongly coupled to structural and electronic degrees of freedom, and that
antiferromagnetic correlations associated with the low magnetic vector phase
are crucial for superconductivity.
