Stacking-Dependent Spin Interactions in Pd / Fe Bilayers on Re(0001)

Using spin-polarized scanning tunneling microscopy and density functional theory, we have studied the magnetic properties of $\mathrm{Pd}/\mathrm{Fe}$ atomic bilayers on Re(0001). Two kinds of magnetic ground states are discovered due to different types of stacking of the Pd adlayer on $\mathrm{Fe}/\mathrm{Re}(0001)$. For fcc stacking of Pd on $\mathrm{Fe}/\mathrm{Re}(0001)$, it is a spin spiral propagating along the close-packed ($\overline{\mathrm{\ensuremath{\Gamma}}K}$) direction with a period of about 0.9 nm, driven by frustrated exchange and Dzyaloshinskii-Moriya interactions. For the hcp stacking, the four-site four-spin interaction stabilizes an up-up-down-down state propagating perpendicular to the close-packed direction (along $\overline{\mathrm{\ensuremath{\Gamma}}M}$) with a period of about 1.0 nm. Our work shows how higher-order exchange interactions can be tuned at interfaces.