Thermally excited Skyrmions: from individual to collective dynamics

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In this project we will use a combined theoretical and experimental approach to understand the statics and dynamics of thermally excited skyrmions. We will go beyond currently studied ferromagnetic systems and explore systems with multiple magnetic sublattices that are coupled antiparallelly, including synthetic as well as intrinsic ferrimagnetic and antiferromagnetic skyrmions with complex properties resulting from their multiple sub-lattices.The first step is to ascertain the thermal dynamics of individual skyrmions based on our joint work on ferromagnetic skyrmions and first observations of diffusion in ferrimagnetic and synthetic antiferromagnetic skyrmion systems. With exciting predictions of strongly enhanced diffusive dynamics for low net magnetic moments, this bodes well for diffusion-based non-conventional computing. Thus, we will probe the skyrmion diffusion from the regime of large ferromagnetic moments down to zero net moment where the sub-lattices fully compensate each other. We will vary the net magnetic moment in ferrimagnets by tuning the temperature and in synthetic antiferromagnets by choosing appropriate thicknesses of the layers. In the second step, we will probe the collective dynamics of skyrmion ensembles that can form different phases, such as the hexatic phase and topological phase transitions unique to 2D systems. We start by statistically analysing the skyrmion positions in a sample, which allows us to quantify the potential landscape that the skyrmions feel, including probing predictions of enhanced attempt frequencies for antiferromagnetic skyrmions and the skyrmion interactions that govern the resulting phase transitions. The phases and phase transitions will then be probed by analysing their local orientational order parameter in comparison to numerical simulations. Finally, we will use the unique dynamic tunability of skyrmion sizes and shapes to study the dynamics of the phase formation and phase transition. By abruptly varying the skyrmion size and shape, we can study the time-resolved equilibration process to identify key processes driving phase transitions.We will implement the work using a range of selected experimental magnetic techniques that operate at variable temperature in combinations with variable applied magnetic fields. We will use primarily real-time magnetic microscopy for direct imaging complemented by magnetic scattering techniques. Theoretically we use a unique combination of numerical simulation techniques that allow us to span from the electronic to the mesoscopic scale. From atomistic spin simulations we will ascertain the intrinsic thermal dynamics of antiferromagnetic skyrmions, which then allows us to extract realistic parameters to model experimentally accessible skyrmion sizes using micromagnetic approaches, finally enabling molecular dynamics simulations of as large ensembles of skyrmions as necessary to model phases and phase transitions.