  We use three-terminal magnetic tunnel junctions (MTJs) designed for
field-free switching by spin-orbit torques (SOTs) to systematically study the
impact of dual voltage pulses on the switching performances. We show that the
concurrent action of an SOT pulse and an MTJ bias pulse allows for reducing the
critical switching energy below the level typical of spin transfer torque while
preserving the ability to switch the MTJ on the sub-ns time scale. By
performing dc and real-time electrical measurements, we discriminate and
quantify three effects arising from the MTJ bias: the voltage-controlled change
of the perpendicular magnetic anisotropy, current-induced heating, and the spin
transfer torque. The experimental results are supported by micromagnetic
modeling. We observe that, depending on the pulse duration and the MTJ
diameter, different effects take a lead in assisting the SOTs in the
magnetization reversal process. Finally, we present a compact model that allows
for evaluating the impact of each effect due to the MTJ bias on the critical
switching parameters. Our results provide input to optimize the switching of
three-terminal devices as a function of time, size, and material parameters.
