  The flow of in-plane current through ultrathin magnetic heterostructures can
cause magnetization switching or domain wall nucleation owing to bulk and
interfacial effects. Within the magnetic layer, the current can create magnetic
instabilities via spin transfer torques (STT). At interface(s), spin current
generated from the spin Hall effect in a neighboring layer can exert torques,
referred to as the spin Hall torques, on the magnetic moments. Here, we study
current induced magnetization switching in perpendicularly magnetized CoFeB/MgO
heterostructures with a heavy metal (HM) underlayer. Depending on the thickness
of the HM underlayer, we find distinct differences in the inplane field
dependence of the threshold switching current. The STT is likely responsible
for the magnetization reversal for the thinner underlayer films whereas the
spin Hall torques cause the switching for thicker underlayer films. For the
latter, we find differences in the switching current for positive and negative
currents and initial magnetization directions. We find that the growth process
during the film deposition introduces an anisotropy that breaks the symmetry of
the system and causes the asymmetric switching. The presence of such symmetry
breaking anisotropy enables deterministic magnetization switching at zero
external fields.
