In nanoscale magnetic multilayers, capping layers are often used to protect the underlying magnetic layers from oxidation. However, little research has investigated possible long-range coupling interactions between nonmagnetic transition metal (TM) capping layers and neighboring magnetic layers. In this paper, the temperature (T) dependence of the magnetic moment of different thicknesses of cobalt (Co) was studied in a tantalum (Ta)/Co/TM trilayer structure with four TM capping layers, where the TMs were Ta, Chromium (Cr), titanium (Ti), and zirconium (Zr), respectively. It was found that the capping layer had a large effect on the phase-transition behavior and thermal stability of the Co layer. In the Ta and Cr layers, the T-dependence of Co magnetic moment showed nonmonotonic behavior, and in the Ti and Zr layers, the Co M-T curve exhibited very few effects of the capping layer. We attribute this phenomenon to the long-range coupling between the Co and TM layers. Furthermore, the coupling mechanism was linked to the indirect magnetic exchange coupling in Co/TM multilayers, similar to the Ruderman-Kittel-Kasuya-Yoshida coupling. The results of this work will support further development of the understanding of the coupling between the 3d ferromagnetic (FM) metal and nonmagnetic TM at nanoscales. Relative to potential applications, it will inspire us to rediscover the role of both the TM capping layer and buffer nonmagnetic layer in FM/TM multilayers, especially for nanoscale magnetic multilayers with spin-dependent effects, such as spin valves, spin halls, spin transfer torque, and spin–orbit coupling, which are in widespread use in the manufacture of various spintronics devices.

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