Influence of the Fe-Co ratio on the exchange coupling in TbFeCo/[Co/Pt] heterostructures

We report on a systematic study of exchange coupled heterostructures, consisting of ferromagnetic [Co/Pt] multilayers and ferrimagnetic (FI) $\mathrm{T}{\mathrm{b}}_{x}\mathrm{F}{\mathrm{e}}_{100\ensuremath{-}x\ensuremath{-}y}\mathrm{C}{\mathrm{o}}_{y}$ (20 nm) alloy thin films with varying composition exhibiting strong perpendicular magnetic anisotropy. In particular, the impact of the Tb content and ratio of Fe and Co of the amorphous FI alloy on the exchange interaction at the interface was investigated. In this paper, the magnetic properties of single ternary TbFeCo thin films were analyzed in a broad composition range and compared to coupled TbFeCo/[Co/Pt] heterostructures. While a rather linear dependence of the exchange coupling strength was observed for Fe/Co-dominated ferrimagnets with increasing amount of Co, a nonlinear behavior is observed for Tb-dominated alloys. The latter behavior is governed by the variation of the exchange stiffness of the ferrimagnet. Additionally, by using element-specific x-ray magnetic circular dichroism measurements, the thickness of the interface domain wall (IDW) in the ferrimagnet, which is formed during the reversal of the ferromagnet, can be extracted. An inverse correlation between the IDW thickness and the exchange coupling strength at the interface was deduced.