Observation of a topological transition in theA3vortices

Vortices in ${}_{}{}^3{}_{}{}^{}\mathit{A}$ in low magnetic fields (H≤20 mT) have been studied by ultrasonics. A first-order phase change between two vortex types, with dissimilar sound absorption, has been found. The magnetic field at the transition is on the order of the dipolar field ${\mathit{H}}_{\mathit{d}}$. The observed change occurs between continuous vortex textures with different topological charges in the d^ field (where d^ is the magnetic anisotropy vector).