Peculiar corner states in magnetic fractals

Abstract: Topological excitations in periodic magnetic crystals have received significant recent attention. However, it is an open question on their fate once the lattice periodicity is broken. In this work, we theoretically study the topological properties embedded in the collective dynamics of magnetic texture array arranged into a Sierpi\'nski carpet structure with effective Hausdorff dimensionality $d_{f}=1.893$. By evaluating the quantized real-space quadrupole moment, we obtain the phase diagram supporting peculiar corner states that are absent in conventional square lattices. We identify three different higher-order topological states, i.e., outer corner state, type I and type II inner corner states. We further show that all these corner states are topologically protected and are robust against moderate disorder. Full micromagnetic simulations are performed to verify theoretical predictions with good agreement. Our results pave the way to investigating topological phases of magnetic texture based fractals and bridging the gap between magnetic topology and fractality.