Successive spin reorientations and multi-k magnetism in A-site ordered perovskites RBaFe2O6

The A-site layered double perovskite R BaFe 2 O 6 ( R = Pr, Nd, Sm) containing the unusually high-valent Fe 3.5+ state exhibits a cascade of charge-disproportionation (CD) transitions on cooling driven by competing electronic instabilities: 2Fe 3.5+ → Fe 3+ + Fe 4+ → 1.5Fe 3+ + 0.5Fe 5+ . Using neutron powder diffraction and magnetisation measurements, we determine the evolution of magnetic structures across these successive CD regimes. NdBaFe 2 O 6 and SmBaFe 2 O 6 exhibit two successive spin reorientation transitions, with the ordered moments reorienting between crystallographic directions before stabilising in a low-temperature multi- k ( Γ + Y ) magnetic state. In contrast, PrBaFe 2 O 6 undergoes a direct spin reorientation without an intermediate phase, while a weak Y component is already present near the ordering temperature. These results show that the magnetic evolution closely follows the CD progression and is consistent with changes in exchange topology and magnetocrystalline anisotropy, leading to spin reorientation and multi- k magnetism. R BaFe 2 O 6 thus provides a model system in which charge redistribution directly controls complex magnetic order in high-valent Fe oxides.