Reentrant multiple-$\mathbf{q}$ magnetic order and a "spin-cholesteric" phase in Sr$_3$Fe$_2$O$_7$

TL;DR

This paper reports the discovery of a novel 'spin-cholesteric' phase in Sr$_3$Fe$_2$O$_7$, a magnetic material exhibiting complex multi-q spin textures and symmetry-breaking phases analogous to liquid crystal cholesterics.

Contribution

It reveals the first magnetic analog of cholesteric order, demonstrating complex multi-q spin textures and a new 'spin-cholesteric' phase in a centrosymmetric material.

Findings

Identification of multi-q spin textures in Sr$_3$Fe$_2$O$_7$

Observation of a 'spin-cholesteric' phase with broken chiral symmetry

Emergence of vortex-crystal order near the paramagnetic transition

Abstract

Spin-nematic and spin-smectic phases have been reported in magnetic materials, which break rotational symmetry while preserving translational symmetry along certain directions. However, until now the analogy to liquid crystals remained incomplete because no magnetic analog of cholesteric order was known. Here we show that the bilayer perovskite Sr$_3$Fe$_2$O$_7$, previously believed to adopt a simple single-$\mathbf{q}$ spin-helical order, hosts two distinct types of multi-$\mathbf{q}$ spin textures and the first "spin-cholesteric". Its ground state represents a novel multi-$\mathbf{q}$ spin texture with unequally intense spin modulations at the two ordering vectors. This is followed in temperature by the new "spin-cholesteric" phase with spontaneously broken chiral symmetry, in which the translational symmetry is broken only along one of the crystal directions while 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 $\mathbf{q}$ vectors emerges. The "spin-cholesteric" phase completes the spin analogy with liquid crystals and renders Sr$_3$Fe$_2$O$_7$ a touchstone for studying transitions among multiple-$\mathbf{q}$ spin textures in a centrosymmetric host.