Magnetism is widely considered to be a key ingredient of unconventional superconductivity.In contrast to cuprate high-temperature superconductors, antiferromagnetism in Fe-based superconductors (FeSCs) is characterized by a pair of magnetic propagation vectors 1, 2 . Consequently, three different types of magnetic order are possible. Of theses, only stripe-type spin-density wave (SSDW) and spin-charge-density wave (SCDW) orders have been observed 2-4 . A realization of the proposed spin-vortex crystal (SVC) order is noticeably absent. We report a magnetic phase consistent with the hedgehog variation of SVC order in Ni-and Co-doped CaKFe 4 As 4 based on thermodynamic, transport, structural and local magnetic probes combined with symmetry analysis. The exotic SVC phase is stabilized by the reduced symmetry of the CaKFe 4 As 4 structure.Our results suggest that the possible magnetic ground states in FeSCs have very similar energies, providing an enlarged configuration space for magnetic fluctuations to promote high-temperature superconductivity.

Recently, another consequence of the size of the Caions has been discovered. Iyo et al. 15 have found that a family of ordered CaAFe 4 As 4 (1144) compounds can be formed for A = K, Rb, Cs where the key to the formation is the difference in ionic size between the Ca and the A ion. This family is not a (Ca 1−x A x )Fe 2 As 2 solid-solution, where the Ca and A ions randomly occupy a single crystallographic site, 16 but rather is a distinct, quaternary, line compound in which the Ca and A sites form alternating planes along the crystallographic c-axis, separated by FeAs slabs 15 . In essence, the CaAFe 4 As 4 structure is identical to the CaFe 2 As 2 structure, just with layer by layer segregation of the Ca and A ions. The 1144 structure was also found for SrAFe 4 As 4 (A = Rb, Cs). Solid-solutions of Ca (Sr) 122 structures were found for arXiv:1605.05617v2 [cond-mat.supr-con]

A hallmark of the iron-based superconductors is the strong coupling between magnetic, structural and electronic degrees of freedom. However, a universal picture of the normal state properties of these compounds has been confounded by recent investigations of FeSe where the nematic (structural) and magnetic transitions appear to be decoupled. Here, using synchrotron-based high-energy x-ray diffraction and time-domain Mössbauer spectroscopy, we show that nematicity and magnetism in FeSe under applied pressure are indeed strongly coupled. Distinct structural and magnetic transitions are observed for pressures between 1.0 and 1.7 GPa and merge into a single first-order transition for pressures ≳1.7 GPa, reminiscent of what has been found for the evolution of these transitions in the prototypical system Ba(Fe1−xCox)2As2. Our results are consistent with a spin-driven mechanism for nematic order in FeSe and provide an important step towards a universal description of the normal state properties of the iron-based superconductors.