  NiO is a prototypical antiferromagnet with a characteristic resonance
frequency in the THz range. From atomistic spin dynamics simulations that take
into account the crystallographic structure of NiO, and in particular a
magnetic anisotropy respecting its symmetry, we describe antiferromagnetic
switching at THz frequency by a spin transfer torque mechanism. Sub-picosecond
S-state switching between the six allowed stable spin directions is found for
reasonably achievable spin currents, like those generated by laser induced
ultrafast demagnetization. A simple procedure for picosecond writing of a
six-state memory is described, thus opening the possibility to speed up current
logic of electronic devices by several orders of magnitude.
