  Recent advances in the understanding of spin orbital effects in ultrathin
magnetic heterostructures have opened new paradigms to control magnetic moments
electrically. The Dzyaloshinskii-Moriya interaction (DMI) is said to play a key
role in forming a Neel-type domain wall that can be driven by the spin Hall
torque, a torque resulting from the spin current generated in a neighboring
non-magnetic layer via the spin Hall effect. Here we show that the sign of the
DMI, which determines the direction to which a domain wall moves with current,
can be changed by modifying the adjacent non-magnetic layer. We find that the
sense of rotation of a domain wall spiral is reversed when the Ta underlayer is
doped with nitrogen in Ta|CoFeB|MgO heterostructures. The spin Hall angle of
the Ta and nitrogen doped Ta underlayers carry the same sign, suggesting that
the sign of the DMI is defined at the interface. Depending on the sense of
rotation, spin transfer torque and spin Hall torque can either compete or
assist each other, thus influencing the efficiency of moving domain walls with
current.
