Tuning of Room Temperature Skyrmions in Pt/Co/Ta Multilayers

Magnetic skyrmions are nanoscale topological objects which are promising for next-generation information storage technologies and computing. [1,2] In magnetic multilayers, they can be stabilized at room temperature. [3-5]. Skyrmions emerge due to an interplay between several magnetic contributions. Among them the interfacial Dzyaloshinskii-Moriya Interaction (DMI) drives the spins into non-collinear orientation, while the perpendicular magnetic anisotropy (PMA) favours the out-of-plane orientation and the shape anisotropy prefers in-plane spin orientation. To study this competition of energies and the appearance of skyrmions, we have varied the Co film thickness as well as the number of repetitions in [Pt/Co(x)/Ta]$_N$ multilayers. This multilayer system was chosen because it is an established multilayer system for skyrmions and results can be compared with existing investigations, like e.g. [3,6].Polycrystalline [Pt(40 Å)/Co(x)/Ta(19 Å)]$_N$ multilayers were fabricated in a molecular beam epitaxy setup by thermal deposition on oxidized Si(001) substrates with a buffer layer of 47 Å Ta and a 30 Å Pt cap layer. The Co film thickness was varied between 5 Å and 21 Å, the number of repetitions varied between 8 and 10.Magnetic force microscopy measurements reveal the existence of skyrmions at a Co thickness between 9Å and 17 Å. The figure below gives examples with varying thickness and number of repetitions in indicated magnetic field. The density of skyrmions as well as their size varies. In remanence, stable skyrmions form only for the 17 Å Co sample with N=10 otherwise worm domains develop. Topological Hall effect measurements confirm these observations. The relationship between these findings is discussed in this contribution.References [1] A. Fert, V. Cros, and J. Sampaio, Nature Nanotech 8 (2013) 152. [2] K. Raab, M.A. Brems, G. Beneke, et al., Nat Commun 13 (2022) 6982. [3] S. Woo, K. Litzius, B. Krüger, M.-Y. Im, L. Caretta, K. Richter et al., Nat. Mat. 15 (2016) 501 [4] A. Soumyanarayanan, M. Raju, A. Gonzalez Oyarce, et al., Nature Mater 16 (2017) 898. [5] T.Dohi, R.M.Reeve and M. Kläui, Annu Rev. Condens. Matter Phys. 13 (2022) 73. [6] S.Zhang, J.Zhang, Y.Wen, E.M.Chudnovsky, and Y.Zhang, Comms. Phys. 36 (2018) 1