Studying magnetic dynamics in artificial spin ices in the presence of topological defects*

Artificial spin ices (ASIs) are composed of arrays of nanomagnets whose size and spacing can be used to tailor their magnetic interactions. This provides a method to construct artificial versions of spin ice systems that may be difficult to measure otherwise, and one can study effects such as magnetic order and dynamics in the presence of topological defects. Here, we use resonant x-ray scattering and x-ray photon correlation spectroscopy (XPCS) to study the magnetic fluctuations in ASIs containing topological defects with different charges Z (Z = 0, 1, 2, 3). Due to the non-equilibrium nature of the fluctuations, we use two-time correlations to understand the magnetization dynamics. We also obtained a real-space picture of the fluctuations using photoemission electron microcopy (PEEM) with x-ray magnetic circular dichroism. With XPCS and PEEM, we show that ASIs with Z = 0 or 2 are relatively stable, with fluctuations mainly occurring at the edges of the ASI. On the other hand, odd topological charges with Z = 1 or 3 lead to a frustrated magnetic system with fluctuations concentrated around a domain wall.