How topological defects, unavoidable at symmetry-breaking phase transitions in a wide range of systems, evolve through consecutive phase transitions with different broken symmetries remains unexplored. ${\mathrm{Nd}}_2\mathrm{Sr}{\mathrm{Fe}}_2{\mathrm{O}}_7$, a bilayer ferrite, exhibits two intriguing structural phase transitions and dense networks of the so-called type II ${\mathrm{Z}}_8$ structural vortices at room temperature, so it is an ideal system to explore the topological defect evolution. From our extensive experimental investigation, we demonstrate that the cooling rate at the second-order transition (1290°C) plays a decisive role in determining the vortex density at room temperature, following the universal Kibble-Zurek mechanism. In addition, we discovered a transformation between topologically distinct vortices (${\mathrm{Z}}_8$ to ${\mathrm{Z}}_4$ vortices) at the first-order transition (550°C), which conserves the number of vortex cores. Remarkably, the ${\mathrm{Z}}_4$ vortices consist of two phases with an identical symmetry but two distinct magnitudes of an order parameter. Furthermore, when lattice distortion is enhanced by chemical doping, an alternative type of topological defects emerges: loop domain walls with orthorhombic distortions in the tetragonal background, resulting in unique pseudo-orthorhombic twins. Our findings open an avenue to explore the evolution of topological defects through multiple phase transitions.

• Received 8 August 2020
• Revised 4 March 2021
• Accepted 13 May 2021

DOI:https://doi.org/10.1103/PhysRevResearch.3.023216

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Published by the American Physical Society