Partial Antiferromagnetic Helical Order in Fe$_3$PO$_4$O$_3$

TL;DR

This study reveals that Fe$_3$PO$_4$O$_3$ exhibits a unique magnetic state characterized by a ring of scattering in neutron diffraction, indicating a degenerate set of incommensurate antiferromagnetic orderings due to magnetic frustration.

Contribution

The paper demonstrates, through single-crystal neutron diffraction, that Fe$_3$PO$_4$O$_3$ has a degenerate incommensurate magnetic state with no preferred wavevector direction, explained by a Heisenberg model with competing interactions.

Findings

Continuous rings of scattering observed in neutron diffraction.

Absence of a preferred incommensurate wavevector direction.

Degenerate manifold of magnetic ordering wavevectors due to frustration.

Abstract

Magnetic frustration in Fe$_3$PO$_4$O$_3$ has been shown to produce to an unusual magnetic state below T$_N = 163$ K, 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 Fe$_3$PO$_4$O$_3$ does not exhibit a preferred ordering wavevector direction in the $ab$ plane despite having a well-defined ordering wavevector length. This results in the observation of continuous rings of scattering rather than satellite Bragg peaks. The lack of a preferred incommensurate ordering wavevector direction can be understood in terms of an antiferromagnetic Heisenberg model with nearest-neighbor ($J_1$) and second-neighbor ($J_2$) interactions, which produces a quasi-degenerate manifold of ordering wavevectors. This state appears to be similar to the partially ordered phase of MnSi, but in Fe$_3$PO$_4$O$_3$ arises in a frustrated antiferromagnet rather than a chiral ferromagnet.