Incommensurate Magnetism in FeAs Strips: Neutron Scattering from CaFe₄As₃

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Magnetism in the orthorhombic metal CaFe₄As₃ was examined through neutron diffraction for powder and single crystalline samples. Incommensurate (q m≈ (0.37-0.39)× b) and predominantly longitudinally (∥ b) modulated order develops through a 2nd order phase transition at T N=89.63(6) K with a 3D Heisenberg-like critical exponent β=0.365(6). A 1st order transition at T₂=25.6(9) K is associated with the development of a transverse component, locking q m to 0.375(2) b, and increasing the moments from 2.1(1) to 2.2(3) μ B for Fe²⁺ and from 1.3(3) to 2.4(4) μ B for Fe+. The ab-initio Fermi surface is consistent with a nesting instability in cross-linked FeAs strips.