  Skyrmions are magnetic bubbles with nontrivial topology envisioned as data
bits for ultrafast and power-efficient spintronic memory and logic devices.
They may be stabilized in heavy-metal/ferromagnetic/oxide trilayer systems. The
skyrmion chirality is then determined by the sign of the interfacial
Dzyaloshinskii-Moriya interaction (DMI). Nevertheless, for apparently identical
systems, there is some controversy about the DMI sign. Here, we show that the
degree of oxidation of the top interface and the thickness of the ferromagnetic
layer play a major role. Using Brillouin light-scattering measurements in
Ta/Fe-Co-B/TaOx trilayers, we demonstrate a sign change of the DMI with the
degree of oxidation of the Fe-Co-B/TaOx interface. Using polar magneto-optical
Kerr effect microscopy, we consistently observe a reversal of the direction of
current-induced motion of skyrmions with the oxidation level of TaOx; this is
attributed to their chirality reversal. In addition, a second chirality
reversal is observed when changing the Fe-Co-B thickness, probably due to the
proximity of the two Fe-Co-B interfaces in the ultrathin case. By properly
tuning the chirality of the skyrmion, spin-transfer and spin-orbit torques
combine constructively to enhance the skyrmion velocity. These observations
thus allow us to envision an optimization of the material parameters to produce
highly mobile skyrmions. Moreover, this chirality control enables a versatile
manipulation of skyrmions and paves the way towards multidirectional devices.
