Evolution of topological defects at two sequential phase transitions of Nd2SrFe2O7

TL;DR

This study investigates how topological defects evolve through sequential phase transitions in Nd2SrFe2O7, revealing the influence of cooling rate, vortex transformations, and defect emergence due to doping, advancing understanding of defect dynamics in complex materials.

Contribution

It provides experimental evidence of topological defect evolution across multiple phase transitions, including vortex transformations and defect formation mechanisms in Nd2SrFe2O7.

Findings

Cooling rate affects vortex density via Kibble-Zurek mechanism.

Transformation from Z8 to Z4 vortices conserves vortex cores.

Doping induces loop domain walls and pseudo-orthorhombic twins.

Abstract

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. Nd2SrFe2O7, a bilayer ferrite, exhibits two intriguing structural phase transitions and dense networks of the so-called type-II Z8 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 (1290oC) 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 (Z8 to Z4 vortices) at the first-order transition (550oC), which conserves the number of vortex cores. Remarkably, the Z4 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, a new type of topological defects emerges: loop domain walls with orthorhombic distortions in the tetragonal background, resulting in unique pseudo-orthorhombic twins. Our findings open a new avenue to explore the evolution of topological defects through multiple phase transitions.