The effect of RKKY exchange coupling through heavy metal interlayers on future skyrmionic devices

Abstract

Room temperature skyrmions have been the focus of research for development of skyrmionic devices based on thin film multilayers (MLs) with perpendicular anisotropy (PMA) and interfacial Dzyaloshinskii-Moriya interaction (iDMI)1. The skyrmion spin texture is governed by several competing energies: iDMI, exchange energy, anisotropy energy, and magnetostatic energy. It has been shown that the skyrmion wall type and chirality determine the strength of the stray field above and below the thin film structure2. Moreover, knowing the type and strength of the interlayer coupling is a prerequisite for designing future skyrmionic devices3.Recent studies4 showed that the spin texture of the skyrmion in each ferromagnetic (F) layer depends on its position within the ferromagnet/heavy metal ML. The dependence of the skyrmion wall on the vertical position inside the multilayer was attributed to the minimization of the total energy, which included the formation of a flux-closure structure to reduce the magnetostatic energy. However, the existence of even a weak Ruderman–Kittel–Kasuya–Yosida (RKKY) interlayer exchange could significantly alter this scenario by dominating magnetostatic energy contributions. Neglecting the RKKY exchange in modelling work stems from the fact that it has not been thoroughly studied in structures supporting skyrmion formation. Therefore, it is important to investigate the role of RKKY coupling in such multilayered structures in combination with the other energies present in these systems.Here, we report on multilayered systems that allow the assessment of interlayer RKKY interaction relevant for systems with iDMI, thus providing the required input for the design of multilayer systems with well-defined uniform skyrmion spin textures throughout the multilayer system. Note that a similar concept has been already used to measure RKKY interactions5, but for systems with a magnetic in-plane anisotropy without iDMI. Also, because the coercivity of the pinned layer was very small, only antiferromagnetic (AF) exchange coupling could be measured. Here we are interested in systems with PMA with iDMI supporting skyrmions, with ferromagnetic layers that can be coupled either by ferromagnetic or antiferromagnetic RKKY exchange.To assess RKKY interactions in such systems, a rare-earth ferrimagnet based multilayered system is proposed. The magnetically hard rare-earth amorphous ferrimagnet consisting of a TbxFe1-x alloy is exchange-coupled to a thin Co layer. The magnetization of the latter remains pinned up to reasonably high fields by the magnetically hard ferrimagnet layer. This bi-layer system is then separated from a “free” magnetic layer by an interlayer consisting of the typical noble metal used in systems with iDMI. In the simplest case the “free” layer is a 1.5nm-thick Co layer deposited on a Pt substrate to obtain PMA.The net magnetic moment of the rare-earth based ferrimagnet layer is governed by the composition of rare-earth and transition metal (TbxFe1-x with 15 < x < 30), such that either the magnetic moment of the rare-earth or transition metal element aligns parallel to the direction of an applied test-field Hex. Consequently, the adjacent Co layer aligns antiparallel or parallel to Hex, which permits the measurement of a ferromagnetic or antiferromagnetic RKKY exchange coupling, respectively, occurring between the “pinned” and “free” Co layers through the interlayer. The RKKY coupling energy density can be obtained from magnetometry using E=Mfree layer.t.Hex.The situation however becomes more challenging if the “free” Co layer has asymmetric interfaces, hence the material at the top (e.g. Ir) is different from that at the bottom (Pt). In such a case the remanence of the “free” layer is suppressed by the formation of skyrmions or chiral domains arising from the net DMI generated by the asymmetric interfaces. To overcome this problem, we designed the free layer as a multilayer system with symmetric interfaces suppressing the net DMI.The proposed multilayer design permits to reliably measure the RKKY interactions occurring in all typical magnetic multilayer systems exhibiting iDMI, which provides the so-far missing input for micromagnetic simulations of such systems relevant for the design of future skyrmionic devices. **