Sequential magnetic anisotropies in LaFeAs_1-xP_xO uncover the hidden universality of iron-pnictide superconductors

Iron-pnictides have been at the center extensive research motivated by the unusual competition between exotic magnetic orders and unconventional superconductivity. Several magnetic states have been sporadically observed in different systems for which their microscopic origins remain the subject of intense theoretical modeling. A puzzling conundrum was understanding the role played by competing electronic and structural instabilities that lead to seemingly unrelated magnetic states. For example, a collinear spin-density-wave (SSDW) is observed in all the iron-based arsenide series, however, only hole-doped 122-type materials enable the atabilization of a rare double-Q magnetic state (charge spin density wave – CSDW) in a narrow region of the phase diagram just before the onset of superconductivity, whereas their electron-doped 122 counterparts do not! To add intrigue to an already perplexing system, a third magnetic state (spin vortex crystal – SVC) was reported to coexist with or without the SDW magnetic order in another class of hole-doped pnictides known as 1144 (CaKFe₄As₄). Curiously and to the best of our knowledge, not a single pnictide system was previously found to display all the three ground states. Using neutron diffraction, we have recently shown the existence of all three magnetic orders in a single-phase diagram for P-substituted LaFeAsO (1111), all of which compete strongly with superconductivity. Our observations of SDW, CSDW and SVC in the 1111, 122 and 1144 compounds uncovers a long-sought underlying generality of magnetic ordering in the iron-pnictides and the role it plays in relation to superconductivity.