The low ordering temperature of most non-collinear spiral magnets critically limits their implementation in devices. The layered perovskites RBaCuFeO₅ are a rare case of frustrated oxide family that has raised great expectations as promising high-temperature spiral magnets and spin-driven multiferroic candidates. Though a non-conventional mechanism of ‘spiral order by disorder’ could account for the extraordinary thermal stability of their presumed spiral order, such order was alleged on the basis of non-conclusive neutron data on powder samples. Thus far, it has not yet received support from single-crystal studies able to lift the ambiguities of powder data. Here, a YBaCuFeO₅ crystal has been grown with enough Cu/Fe disorder to stabilize the incommensurate magnetic phase up to T_S≈ 200 K.

Utilizing spherical neutron polarimetry and single-crystal neutron diffraction, we unveil the features of its magnetic structures, demonstrating the non-collinear chiral nature of the magnetic domains in the singular incommensurate phase. It is thus finally proved that such phase is spiral in our crystal, and therefore also in those compositions of this perovskite family where T_S values well above room temperature have been reported. Yet, this study also illustrates critical features of relevance to the search for high-temperature magnetoelectric response induced by the spiral phase.