  We study deterministic magnetic reversal of a perpendicularly magnetized Co
layer in a Co/MgO/Ta nano-square driven by spin Hall torque from an in-plane
current flowing in an underlying Pt layer. The rate-limiting step of the
switching process is domain-wall (DW) depinning by spin Hall torque via a
thermally-assisted mechanism that eventually produces full reversal by domain
expansion. An in-plane applied magnetic field collinear with the current is
required, with the necessary field scale set by the need to overcome DW
chirality imposed by the Dzyaloshinskii-Moriya interaction. Once Joule heating
is taken into account the switching current density is quantitatively
consistent with a spin Hall angle {\theta}$_{SH}$ ${\approx}$ 0.07 for 4 nm of
Pt.
