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Most iron-based superconductors exhibit stripe-type magnetism, characterized by the ordering vector Q=(12,12). In contrast, Fe1+yTe, the parent compound of the Fe1+yTe1-xSex superconductors, exhibits double-stripe magnetic order associated with the ordering vector Q=(12,0). Here, we use elastic neutron scattering to investigate heavily Cu-substituted (Fe1-xCux)1+yTe compounds and reveal that (1) for x0.4, short-range magnetic order emerges around the stripe-type vector at Q=(12±δ,12±δ,12) with δ≈0.05; (2) the short-range magnetic order is associated with a superstructure modulation at Q=(13,13,12), with the magnetic correlation length shorter than that for the superstructure; and (3) for x0.55, we observe an additional intergrown phase with higher Cu content, characterized by a superstructure modulation vector Q=(13,13,0) and magnetic peaks at Q=(23,13,12)/(13,23,12). The positions of superstructure peaks suggest that relative to the tetragonal unit cell of Fe1+yTe, heavy Cu substitution leads to Fe-Cu orderings that expand the unit cell by 2×32 times in the ab plane, corroborated by first-principles calculations that suggest the formation of spin chains and spin ladders. Our findings show that stripe-type magnetism is common in magnetically diluted iron pnictides and chalcogenides, despite the varying associated atomic orderings.

The parent compounds of iron-based superconductors are magnetically ordered bad metals, with superconductivity appearing near a putative magnetic quantum critical point. The presence of both Hubbard repulsion and Hund's coupling leads to rich physics in these multiorbital systems, and motivated descriptions of magnetism in terms of itinerant electrons or localized spins. The NaFe1-xCuxAs series consists of magnetically ordered bad metal (x=0), superconducting (x≈0.02) and magnetically ordered semiconducing/insulating (x≈0.5) phases, providing a platform to investigate the connection between superconductivity, magnetism and electronic correlations. Here we use x-ray absorption spectroscopy and resonant inelastic x-ray scattering to study the valence state of Fe and spin dynamics in two NaFe1-xCuxAs compounds (x=0 and 0.47). We find that magnetism in both compounds arises from Fe2+ atoms, and exhibits underdamped dispersive spin waves in their respective ordered states. The dispersion of spin excitations in NaFe0.53Cu0.47As is consistent with being quasi-one-dimensional. Compared to NaFeAs, the band top of spin waves in NaFe0.53Cu0.47As is slightly softened with significantly more spectral weight of the spin excitations. Our results indicate the spin dynamics in NaFe0.53Cu0.47As arise from localized magnetic moments and suggest the iron-based superconductors are proximate to a correlated insulating state with localized iron moments.

A small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe2As2, which exhibits twin-domains without uniaxial strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature Ts. Although it is generally assumed that such a pressure will not affect the intrinsic electronic/magnetic properties of the system, it is known to enhance the antiferromagnetic (AF) ordering temperature TN (

Spin nematics break spin-rotational symmetry while maintaining time-reversal symmetry, analogous to liquid crystal nematics that break spatial rotational symmetry while maintaining translational symmetry. Although several candidate spin nematics have been proposed, the identification and characterization of such a state remain challenging because the spin-nematic order parameter does not couple directly to experimental probes. KFe_{0.8}Ag_{1.2}Te_{2} (K_{5}Fe_{4}Ag_{6}Te_{10}, KFAT) is a local-moment magnet consisting of well-separated 2×2 Fe clusters, and in its ground state the clusters order magnetically, breaking both spin-rotational and time-reversal symmetries. Using uniform magnetic susceptibility and neutron scattering measurements, we find a small strain induces sizable spin anisotropy in the paramagnetic state of KFAT, manifestly breaking spin-rotational symmetry while retaining time-reversal symmetry, resulting in a strain-induced spin-nematic state in which the 2×2 clusters act as the spin analog of molecules in a liquid crystal nematic. The strain-induced spin anisotropy in KFAT allows us to probe its nematic susceptibility, revealing a divergentlike increase upon cooling, indicating the ordered ground state is driven by a spin-orbital entangled nematic order parameter.