Structural and magnetic phase diagram of CrAs and its relationship with pressure-induced superconductivity

We use neutron diffraction to study the structure and magnetic phase diagram of the newly discovered pressure-induced superconductor CrAs. Unlike most magnetic unconventional 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, accompanied by a lattice anomaly, coincides with the emergence of bulk superconductivity. With further increasing pressure, the magnetic order completely disappears near the optimal Tc regime (P≈0.94 GPa). Moreover, the Cr magnetic moments tend to be aligned antiparallel between nearest neighbors with increasing pressure toward the optimal superconductivity regime. Our findings suggest that the noncollinear helimagnetic order is strongly coupled to structural and electronic degrees of freedom, and that the antiferromagnetic correlations between nearest neighbors might be essential for superconductivity.