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Nanosized helical magnetic domains in strongly frustrated Fe3PO4O3

Fe3PO4O3 forms a non-centrosymmetric lattice structure (space group R3m) comprising triangular motifs of Fe^{3+} coupled by strong antiferromagnetic interactions (|Θ_{CW}| > 900 K). Neutron diffraction from polycrystalline samples shows that strong frustration eventually gives way to an ordered helical incommensurate structure below T_N = 163 K, with the helical axis in the hexagonal ab plane and a modulation length of ~ 86 Å. The magnetic structure consists of an unusual needle-like correlation volume that extends past 900 Åalong the hexagonal c-axis but is limited to ~ 70 Åin the ab plane, despite the three-dimensional nature of the magnetic sublattice topology. The small in-plane correlation length, which persists to at least T = T_N/40, indicates a robust blocking of long-range order of the helical magnetic structure, and therefore stable domain walls, or other defect spin textures, must be abundant in Fe3PO7. Temperature dependent neutron powder diffraction reveals small negative thermal expansion below T_N. No change in lattice symmetry is observed on cooling through T_N, as revealed by high resolution synchrotron X-ray diffraction. The previously reported reduced moment of the Fe^{3+} ions (S=5/2), μ~ 4.2 μ_B, is confirmed here through magnetization studies of a magnetically diluted solid solution series of compounds, Fe3-xGaxPO4O3, and is consistent with the refined magnetic moment from neutron diffraction 4.14(2) μ_B. We attribute the reduced moment to a modified spin density distribution arising from ligand charge transfer in this insulating oxide.