  Here we present the tetragonal stoichiometric Heusler compound Pt$_2$MnGa
with the noncollinear AFM order stable up to 350 K. It is resolved by the
neutron diffraction as a helical spiral propagating along the tetragonal axis.
Ab-initio calculations suggest a pure exchange origin of the spiral and explain
its helical character being stabilized by a large basal plane
magnetocrystalline anisotropy (MCA). Together with the inversion-symmetric
crystal structure, this provides a bi-stability of a spiral with respect to the
right- and left-handed magnetic helices. Despite the large MCA, the long period
of a helix might greatly facilitate the switch of the helicity by the
precessional reorientation, suggesting Pt$_2$MnGa as a potential candidate for
the vector-helicity based non-volatile magnetic memory.
