Partial antiferromagnetic helical order in single-crystal Fe3PO4O3

Magnetic frustration in Fe3PO4O3 produces an unusual magnetic state below TN=163K, where incommensurate antiferromagnetic order is restricted to nanosized needle-like domains, as inferred from neutron powder diffraction. Here we show using single-crystal neutron diffraction that Fe3PO4O3 does not exhibit a preferred ordering wave vector direction in the ab plane despite having a well-defined ordering wave vector length. This results in the observation of continuous rings of scattering rather than satellite Bragg peaks. The lack of a preferred incommensurate ordering wave vector direction can be understood in terms of an antiferromagnetic Heisenberg model with nearest-neighbor (J1) and second-neighbor (J2) interactions, which produce a quasidegenerate manifold of ordering wave vectors. This state appears to be similar to the partially ordered phase of MnSi, but in Fe3PO4O3 arises in a frustrated antiferromagnet rather than a chiral ferromagnet.