  To elucidate the magnetic structure and the origin of the nematicity in FeSe,
we perform a high-pressure $^{77}$Se NMR study on FeSe single crystals. We find
a suppression of the structural transition temperature with pressure up to
about 2 GPa from the anisotropy of the Knight shift. Above 2 GPa, a
stripe-order antiferromagnetism that breaks the spatial four-fold rotational
symmetry is determined by the NMR spectra under different field orientations
and with temperatures down to 50 mK. The magnetic phase transition is revealed
to be first-order type, implying the existence of a concomitant structural
transition via a spin-lattice coupling. Stripe-type spin fluctuations are
observed at high temperatures, and remain strong with pressure. These results
provide clear evidences for strong coupling between nematicity and magnetism in
FeSe, and therefore support a universal scenario of magnetic driven nematicity
in iron-based superconductors.
