ROOM TEMPERATURE INVESTIGATION OF SKYRMION- HOSTING PT/CO/TA MULTILAYERS

Multilayers composed of heavy metals and ferromagnetic materials with strong perpendicular anisotropy are potential candidates for magnetic memory applications [1,2]. In particular, magnetic skyrmions may enable ultra-dense storage devices due to the extremely low spin currents needed to move/manipulated them [2]. Skyrmions emerge from the competition between the Dzyaloshinskii–Moriya interaction and exchange interactions generated at the interface of thin ferromagnetic layers and heavy metals with large spin-orbit coupling [3]. Pt/Co-based multilayers generally exhibit worm domains, which can nucleate into skyrmions through breaking/nucleation processes [4]. Recent studies have demonstrated the nucleation of skyrmions by varying external magnetic field, temperature, and current in sputtered Pt/Co/Ta multilayers [4,5].In this work, [Pt/Co/Ta]x multilayers with perpendicular magnetic anisotropy were grown by molecular beam epitaxy. We have demonstrated the feasibility of manipulating magnetic domains in our multilayers by changing the number of repetitions x and the Co layer thickness between 5 Å to 21 Å. Using magnetic force microscopy (MFM), we observed worm domains or stripe domains. These domains can be broken into skyrmions, by applying an out- of-plane field or into stripe domains by applying in-plane fields. We achieved partially ordered skyrmions at a low external field of ~38 mT for the multilayer with a cobalt thickness of 17 Å (see Figure 1). Furthermore, isolated skyrmions in this multilayer remain even after the external magnetic field has been removed.References[1] A. Fert and V. Sampai (2013) Nat. Nanotechnol. 8, 152–156[2] C Wang C, Seinige H. and Tsoi M. (2013), J. Phys. D: Appl. Phys. 46, 285001[3] Xichao Zhang X., Zhou Y., Song K.M., Park T.E., Xia J., Ezawa M., Liu X., Zhao W., Zhao G. and Woo S. (2020), J. Phys. Condens. Matter 32, 143001[4] Ma M., Ang C., Li Y., Pan Z., Gan W., Lew W.S. and Ma F. (2020), J. Appl. Phys. 127, 223901[5] Brandao J., Dugato D.A., Puydinger dos Santos M.V., Berón F. and Cesar J.C. (2022), Appl. Surf. Sci. 585, 15259

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

Room Temperature Skyrmions in Pt/Co/Ta multilayers

Magnetic skyrmions are topologically stabilized spin configurations on the nanoscale which makes them promising for next-generation information storage technologies and computing. [1,2] In magnetic multilay- ers, they can be stabilized at room temperature [3]. 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. Polycrystalline[Pt(40Å)/Co(x)/Ta(19Å)]N multilayerswerefabricatedinamolecularbeamepitaxysetupby 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. Mag- netic force microscopy measurements reveal the existence of skyrmions at a Co thickness between 9 Å and 17 Å. We discuss results obtained from magnetic hysteresis, transport and neutron reflectometry measurements. The latter have been performed with the neutron reflectometer Platypus at ANSTO, Australia.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) 50