Spin-reorientation transitions in the Cairo pentagonal magnet Bi4Fe5O13F

We show that interlayer spins play a dual role in the Cairo pentagonal magnet ${\mathrm{Bi}}₄{\mathrm{Fe}}₅{\mathrm{O}}₁₃\mathrm{F}$, on one hand mediating the three-dimensional magnetic order, and on the other driving spin-reorientation transitions both within and between the planes. The corresponding sequence of magnetic orders unraveled by neutron diffraction and M\"ossbauer spectroscopy features two orthogonal magnetic structures described by opposite local vector chiralities, and an intermediate, partly disordered phase with nearly collinear spins. A similar collinear phase has been predicted theoretically to be stabilized by quantum fluctuations, but ${\mathrm{Bi}}₄{\mathrm{Fe}}₅{\mathrm{O}}₁₃\mathrm{F}$ is very far from the relevant parameter regime. While the observed in-plane reorientation cannot be explained by any standard frustration mechanism, our ab initio band-structure calculations reveal strong single-ion anisotropy of the interlayer ${\mathrm{Fe}}³⁺$ spins that turns out to be instrumental in controlling the local vector chirality and the associated interlayer order.