  Superconducting FeSe single crystals of (001) orientation are synthesized via
a hydrothermal ion-release route. An Ising spin-nematic order is identified by
our systematic measurements of in-plane angular-dependent magnetoresistance
(AMR) and static magnetization. The turn-on temperature of anisotropic AMR
signifies the Ising spin-nematic ordering temperature Tsn, below which a
two-fold rotational symmetry is observed in the iron plane. A downward
curvature appears below Tsn in the temperature dependence of static
magnetization for the weak in-plane magnetic field as reported previously.
Remarkably, we find a universal linear relationship between Tc and Tsn among
various superconducting samples, indicating that the spin nematicity and the
superconductivity in FeSe have a common microscopic origin.
