Fig. 1
From: Hedgehog spin-vortex crystal stabilized in a hole-doped iron-based superconductor
Schematics of possible magnetic order types in CaKFe4As4. a–d Sketches of four magnetic moment patterns on an Fe–As layer in the CaKFe4As4 structure associated with Q 1 = (π, 0), Q 2 = (0, π) magnetic propagation vectors. The upper yellow square in a, e represents the projection of the CaKFe4As4 unit cell. The magnetic unit cells are represented by the central yellow squares in a–d. The brown arrows represent the magnetic moments at the Fe sites and the blue and green arrows the hyperfine field (Hhf) at the inequivalent As1 and As2 sites. In the orthorhombic stripe spin-density wave (SSDW) in a all arsenic atoms experience Hhf normal to the plane. In the spin-charge-density wave (SCDW), “C4 phase15”, in b every other Fe site supports a moment normal to the plane and the As sites experience an in-plane Hhf. In the hedgehog spin-vortex crystal (SVC) phase in c, the Fe moments display an “all in” or “all out” arrangement centered around the As1 sites generating an out-of-plane Hhf ≠ 0 at these sites and Hhf = 0 at the As2 sites. In the loops SVC in d the magnetic moments form loops around As1 and neither As site experiences a Hhf. e The chemical structure of CaKFe4As4. Note the inequivalent As1 and As2 sites adjacent to K and Ca planes, respectively. f Section of the Fe–As sheet with a hedgehog SVC moment arrangement. Spin up currents between the iron atoms, Js (yellow arrows), generate an electric field, E (red arrows), which couples to asymmetric shifts of the two arsenic sites. Unlike in the CaFe2As2 structure, an asymmetric arrangement of arsenic atoms is imposed by the crystallographic symmetry in CaKFe4As4 providing a symmetry-breaking field that favors the SVC-type phases
