Bilayer Square Lattice Tb₂SrAl₂O₇with Structural Z₈Vortices and Magnetic Frustration

The bilayer perovskites' family A3B2O7 holds rich structural complexity. When magnetism freedom is added in, great opportunities appear for new physics. Magnetic Tb ions in Tb2SrAl2O7 are known to crystallize in a bilayer square lattice. The results of our comprehensive neutron, X-ray diffraction, and transmission electron microscopy (TEM) experiments reveal a crystallographic P42/mnm symmetry accompanied with room-temperature topological type-II Z8 vortex domains, whose density is controllable by cooling rates, as described by the Kibble-Zurek mechanism. The DC magnetic susceptibility without long-range order down to 1.8 K suggests frustrated magnetism, while a spin freezing <2.5 K is observed in the AC susceptibility, which is likely due to the antisite disorder and another possible cause of no long-range ordering. Strong temperature evolution of magnetic anisotropy indicates an interplay of low-lying crystal electric field levels and anisotropic exchange interactions. Our results suggest that Tb2SrAl2O7 is a promising candidate of magnetic frustration in a bilayer square lattice, and this system can be a new playground for exploring exotic magnetic states in bilayer square lattices, topological magnetic edge states at coherent crystallographic domain walls, and multifunctional applications.