Reentrant multiple-q magnetic order and a “spin meta-cholesteric” phase in Sr3Fe2O7

Abstract Topologically nontrivial magnetic structures such as skyrmion lattices are well known in materials lacking lattice inversion symmetry, where antisymmetric exchange interactions are allowed. Only recently, topological multi- q magnetic textures that spontaneously break the chiral symmetry, for example, three-dimensional hedgehog lattices, were discovered in centrosymmetric compounds, where they are instead driven by frustrated interactions. Here we show that the bilayer perovskite Sr 3 Fe 2 O 7 , previously believed to adopt a simple single- q spin-helical order, hosts two distinct types of multi- q spin textures. Its ground state represents a novel multi- q spin texture with unequally intense spin modulations at the two ordering vectors. This is followed in temperature by a new “spin meta-cholesteric” phase, in which the chiral symmetry is spontaneously broken along one of the crystal directions, but the weaker orthogonal modulation melts, giving rise to intense short-range dynamical fluctuations. Shortly before the transition to the paramagnetic state, vortex-crystal order spanned by two equivalent q vectors emerges. This renders Sr 3 Fe 2 O 7 an ideal material to study transitions among multiple- q spin textures in a centrosymmetric host.