  In this work, we study magnetization switching induced by spin-orbit torque
in W(Pt)/Co/NiO heterostructures with variable thickness of heavy-metal layers
W and Pt, perpendicularly magnetized Co layer and an antiferromagnetic NiO
layer. Using current-driven switching, magnetoresistance and anomalous Hall
effect measurements, perpendicular and in-plane exchange bias field were
determined. Several Hall-bar devices possessing in-plane exchange bias from
both systems were selected and analyzed in relation to our analytical switching
model of critical current density as a function of Pt and W thickness,
resulting in estimation of effective spin Hall angle and perpendicular
effective magnetic anisotropy. We demonstrate in both the Pt/Co/NiO and the
W/Co/NiO systems the deterministic Co magnetization switching without external
magnetic field which was replaced by in-plane exchange bias field. Moreover, we
show that due to a higher effective spin Hall angle in W than in Pt-systems the
relative difference between the resistance states in the magnetization current
switching to difference between the resistance states in magnetic field
switching determined by anomalous Hall effect ($\Delta R/\Delta
R_{\text{AHE}}$) is about twice higher in W than Pt, while critical switching
current density in W is one order lower than in Pt-devices. The current
switching stability and training process is discussed in detail.
