Chiral phenomena in magnetic multilayers

Abstract

This thesis focused on chiral phenomena in thin magnetic layers. The three main points of the thesis are: Thin magnetic films with Dzyaloshinskii-Moriya interactions are known to host skyrmion crystals, which typically have a hexagonal lattice structure. We investigate skyrmion-lattice configurations in synthetic antiferromagnets, i.e., a bilayer of thin magnetic films that is coupled antiferromagnetically. By means of Monte Carlo simulations, we find that by tuning the interlayer coupling the skyrmion lattice structure can be tuned from square to hexagonal. We give a simple interpretation for the existence of this transition based on the fact that for synthetic antiferromagnetic coupling the skyrmions in different layers repel each other and form each others' dual lattice. Our findings may be useful to experimentally switch between two lattice configurations to, for example, modify spin-wave propagation. Magnets with broken local inversion symmetries are interesting candidates for chiral magnetic textures, such as skyrmions and spin spirals. The property of these magnets is that each subsequent layer can possess a different Dzyaloshniskii-Moriya interaction (DMI) originating from the local inversion symmetry breaking. Given that new candidates of such systems are emerging, with the Van der Waals crystals and magnetic multilayer systems, it is interesting to investigate how the chiral magnetic textures depend on the number of layers and the coupling between them. In this article, we model the magnetic layers with a classical Heisenberg spin model where the sign of the DMI alternates for each consecutive layer. We use Monte Carlo simulations to examine chiral magnetic textures and show that the pitch of magnetic spirals is influenced by the interlayer coupling and the number of layers. We observe even-odd effects in the number of layers where we observe a suppression of the spin spirals for even layer numbers. We give an explanation for our findings by proposing a net DMI in systems with strongly coupled layers. Our results can be used to determine the DMI in systems with a known number of layers and for new technologies based on the tunability of the spiral wavelength. Synthetic antiferromagnets have been shown to have beneficial properties for hosting skyrmions such as bringing extra stability and minimising the Magnus effect. In this chapter we calculate how to detect skyrmions using a ferromagnetic resonance (FMR) spectrometer. This is done by looking at the collective modes of skyrmions in two coupled layers. We approached the system analytically using the principle of least action and numerically where we calculated the excited states of ground states found using Monte Carlo simulations. Both methods gave corresponding results, and resulted a profile which can be used to detect skyrmions.