Asymmetrical magnetic domain wall motion in symmetrical heavy metal/ferromagnet multilayers

In a heavy metal (HM)/ferromagnet (FM)/HM multilayer with a symmetrical sandwich structure, the absence of an interfacial Dzyaloshinskii-Moriya interaction (iDMI) is generally expected. A zero iDMI constant is usually verified by the motion of a magnetic domain wall (DW) with a velocity that is symmetrical to the longitudinal magnetic field (symmetrical DW motion). However, asymmetrical DW motion was still experimentally observed in a symmetrical HM/FM/HM multilayer, and its mechanism is unclear. In this letter, we theoretically prove that even in a perfectly symmetrical HM/FM/HM multilayer with a zero total iDMI constant, asymmetrical DW motion is still possible owing to the exchange coupling between the neighboring FM atomic layers in the FM medium. This work paves a way to unraveling unique DW motion in HM/FM multilayers from a deep perspective of the subtle magnetic interactions within a FM medium.