Tuning the Density of Incomplete and Tubular Skyrmions in Ferro/ferri/ferromagnetic Trilayers

Abstract

Room temperature magnetic skyrmions have the potential of enabling compact and energetically-efficient storage such as racetrack memories, albeit with two fundamental challenges: 1) Not being able to restrict their motion along straight paths, leading to skyrmions being lost at the device edge; one possible solution is the use of antiferromagnetically-coupled skyrmions [1]; and 2) Not having stable inter-skyrmion distances, leading to fluctuating distances among bits; a potential solution was revealed with the experimental observation of coexisting skyrmions and chiral bobbers [2], opening the possibility that a chain of binary data bits could be encoded by the two states. However, these observations were in B20-type crystalline materials and at low temperatures.In our previous work [3], we developed a ferro/ferri/ferromagnetic (FM/FI/FM) trilayer system that can host two distinct skyrmions at room temperature and can serve as a solution to the second challenge. The two phases represent a tubular skyrmion (running through the entire trilayer) and an incomplete skyrmion (existing in the FM layers only), as revealed from magnetic force microscopy (MFM) data and micromagnetic simulations.Such a two-skyrmion phase system exists for specific magnetic parameters of the individual sublayers. By exploring this parameter space, in this new study we reveal that the densities of the two skyrmions can be tuned via different routes: either by changing the magnetic properties of the FI or those of the FM. More specifically, we investigate the trilayer system when changing the thickness of the FM multilayers by using a combination of MFM, vibrating sample magnetometry and Brillouin light scattering experiments together with micromagnetic simulations. We show how the competition between different energies allows for the stabilization of either incomplete-only skyrmions, combination of both or mainly tubular skyrmions (Figure 1). Having a good understanding of the interplay between different magnetic properties can lead to better-tailored systems and possibly bring skyrmions a step closer to applications.  Fig. 1: Sample schematics and MFM images of three FM/FI/FM samples with varying Fe-layer thickness.