Role of interfacial contribution in the bulklike enhanced Dzyaloshinskii-Moriya interaction of [Co/Tb]N multilayers

Recently, magnetic multilayers composed of repetitive heavy metal/ferromagnetic bilayers have been extensively studied due to their large Dyzaloshinskii-Moriya interaction (DMI) and perpendicular magnetic anisotropy, which are considered key parameters to stabilize skyrmions at room temperature. However, the limitations of characterizing the ultrathin interfacial properties hinder the understanding of the microscopic origin of DMI. Here we report a bulklike enhancement of DMI strength ($D$) through the number of stacking layers ($N$) in ${[\mathrm{Co}/\mathrm{Tb}]}_N$ multilayers without introducing heavy metals and observe the value $1.38\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{m}}²$ of $D$ at $N=8$. Using a phenomenological model, we attribute this bulklike behavior of $D$ mainly to the increase in net interfacial DMI (iDMI) with $N$. The increase of iDMI is strongly correlated with the growth of the gradient in crystallographic asymmetry for adjacent interfaces during the stacking process. This structural phase transition concerning iDMI is demonstrated by the energy dispersive x-ray spectroscopy measurements and the $N$ dependence of the longitudinal resistivity. We suggest that the asymmetrical distribution of Tb atoms breaks interfacial symmetry, leading to enhanced deviations in the crystallographic qualities for the top Tb/Co and bottom Co/Tb interfaces, resulting in an increase of the iDMI as $N$ grows. Our findings suggest the potential of rare-earth transition-metal based magnetic multilayers for skyrmion applications due to their tunable magnetic properties regarding magnetic chiral textures.