  The origin of spin-orbit torques generated from the conversion of
charge-to-spin currents is of considerable debate. Solid understanding of the
physics behind is key to the development of current and voltage controlled
switching dynamics in ultrathin heterostuctures. The field free switching
observed recently (Phys. Rev. Lett. 120, 117703 (2018)) in a Pt/W/CoFeB
structure has intensified such a debate. Here we derive a formula to evaluate a
perpendicular effective field generated when the current flows through the
heterostructure, considering the large resistivity difference between the two
normal metal layers and the chemical potential gradient created at the
interface. Together with recent X-ray photoelectron spectroscopy findings at
the interface of a Pd/CoFeB structure, we conclude that a new torque generated
may play a key role in the field free switching. The model and mechanism
proposed agrees with previous reports on spin current injection and field free
switching using different interface engineering methods.
