We study the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron scattering (INS) measurements on BaFe 1.85 Co 0.15 As 2 and BaFe 1.91 Ni 0.09 As 2 samples that exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum lacks the threedimensional (3D) 4 2 /m screw symmetry around the ( 1 2 1 2 L) axis that is implied by the I4/mmm space group. This is manifest both in the in-plane anisotropy of the normal-and SC-state spin dynamics and in the out-ofplane dispersion of the spin-resonance mode. We show that this effect originates from the higher symmetry of the magnetic Fe-sublattice with respect to the crystal itself, hence the INS signal inherits the symmetry of the unfolded Brillouin zone (BZ) of the Fe-sublattice. The in-plane anisotropy is temperature-independent and can be qualitatively reproduced in normal-state density-functional-theory calculations without invoking a symmetry-broken ("nematic") ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode ω res , as well as its intensity and the SC spin gap inherit the normal-state intensity modulation along the out-of-plane direction L with a period twice larger than expected from the body-centered-tetragonal BZ symmetry. The amplitude of this modulation decreases at higher doping, providing an analogy to the splitting between even and odd resonant modes in bilayer cuprates. Combining our and previous data, we show that at odd L a universal linear relationship ħ hω res ≈ 4.3 k B T c holds for all the studied Fe-based superconductors, independent of their carrier type. Its validity down to the lowest doping levels is consistent with weaker electron correlations in ferropnictides as compared to the underdoped cuprates.

We have investigated the magnetic structure in a polycrystalline sample of the B20-type MnGe by means of small-angle neutron scattering. On the projected diffraction plane normal to the incoming neutron beam, a Debye-ring-like pattern appears due to the random orientation of the spin helix q vectors (100). When an external magnetic field is applied normal to the incoming neutron beam, an intense peak with wave vector (q) perpendicular to the applied magnetic field is observed as the hallmark of the formation of a skyrmion lattice with a multiple-q helix in a wide temperature-magnetic-field region. This scattering intensity remains even after removing the magnetic field, which indicates that a skyrmion lattice is stabilized as the ground state. A different form of skyrmion lattice, either square or cubic, is proposed, which is also shown to be in good agreement with previous high-angle neutron diffraction results. Calculations based on such structures also describe the magnetic-field profile of the topological Hall resistivity.

We have studied the low-energy spin-excitation spectrum of the single-crystalline Rb 2 Fe 4 Se 5 superconductor (T c = 32 K) by means of inelastic neutron scattering. In the superconducting state, we observe a magnetic resonant mode centered at an energy of hω res = 14 meV and at the (0.5 0.25 0.5) wave vector (unfolded Fe-sublattice notation), which differs from the ones characterizing magnetic resonant modes in other iron-based superconductors. Our finding suggests that the 245-iron-selenides are unconventional superconductors with a sign-changing order parameter, in which bulk superconductivity coexists with the 5 × 5 magnetic superstructure. The estimated ratios of hω res /k B T c ≈ 5.1 ± 0.4 and hω res /2∆ ≈ 0.7 ± 0.1, where ∆ is the superconducting gap, indicate moderate pairing strength in this compound, similar to that in optimally doped 1111-and 122-pnictides. PACS numbers: 74.70.Xa, 74.25.Ha, 78.70.Nx, 74.20.Rp Soon after the discovery of arsenic-free iron-selenide superconductors A 2 Fe 4 Se 5 (A = K, Rb, Cs), also known as 245-compounds [1], their unprecedented physical properties came to light, such as the coexistence of high-T c superconductivity with strong antiferromagnetism [2,3]. The pairing mechanism and the symmetry of the superconducting (SC) order parameter in this family of compounds still remain among the major open questions. In the majority of other Fe-based superconductors, it is widely accepted that the strong nesting between the holelike Fermi surface (FS) at the Brilliouin zone (BZ) center and electronlike FS at the BZ boundary leads to the sign-changing s-wave (s ± -wave) pairing symmetry [4]. This scenario has been supported by different experimental methods, such as angle-resolved photoemission spectroscopy (ARPES) [5], quasi-particle interference [6], and inelastic neutron scattering (INS) [7,8].On the other hand, recent theoretical calculations [9] and ARPES experiments [10, 11] on the 245-system revealed the absence of holelike FS at the BZ center in the electronic structure, implying that the nesting between the hole-and electronlike FS sheets is no longer present. Hence, several theoretical studies proposed alternative pairing instabilities, such as d-wave or another type of s ± -wave symmetry with sign-changing order parameter between bonding and antibonding states [12][13][14]. As a hallmark of sign-changing SC order parameter, several authors theoretically predicted a resonant mode in the magnetic excitation spectrum below the SC transition, yet its precise position in momentum space still remains controversial [12,13].A major complication in treating the 245-compounds theoretically arises from the presence of a crystallographic superstructure of Fe vacancies that has been consistently reported both from x-ray and neutron diffraction experiments [15]. This 5 × 5 superstructure is closely related to the static antiferromagnetic (AFM) order persisting up to the Néel temperature, T N ≈ 540 K [16]. Although most of the existing band structure calculations have so far negle...

The skyrmion crystal (SkX) characterized by a multiple-q helical spin modulation has been reported as a unique topological state that competes with the single-q helimagnetic order in noncentrosymmetric materials. Here we report the discovery of a rich variety of multiple-q helimagnetic spin structures in the centrosymmetric cubic perovskite SrFeO 3 . On the basis of neutron diffraction measurements, we have identified two types of robust multiple-q topological spin structures that appear in the absence of external magnetic fields: an anisotropic double-q spin spiral and an isotropic quadruple-q spiral hosting a three-dimensional lattice of hedgehog singularities. The present system not only diversifies the family of SkX host materials, but furthermore provides an experimental missing link between centrosymmetric lattices and topological helimagnetic order. It also offers perspectives for integration of SkXs into oxide electronic devices.

We have investigated the magnetization profiles in superlattices composed of the two ferromagnets La 0.7 Sr 0.3 MnO 3 and SrRuO 3 using spin-polarized neutron reflectometry. In combination with magnetometry, the neutron data indicate a noncollinear spin configuration where orientation of the Ru moments changes from in plane at the interface to out of plane deep inside the SrRuO 3 layers. The spin structure originates in a competition between antiferromagnetic exchange interactions of Mn and Ru moments across the interface, and the magnetocrystalline anisotropy of the Ru moments, and it is closely related to the "exchange spring" structures previously observed in multilayers composed of ferromagnetic elements and alloys.The magnetotransport properties of metallic ferromagnetic oxides such as La 0.7 (Sr,Ca) 0.3 MnO 3 and SrRuO 3 have been the subject of a large-scale research effort. [1][2][3][4][5] Recent advances in the synthesis of oxide-based heterostructures have now opened up perspectives for the controlled manipulation of the magnetic properties of these compounds, and for the transfer of ferromagnetic spin polarization into other materials. Ferromagnetic oxides have thus been incorporated in heterostructures with a large variety of materials including high-temperature superconductors, 6-8 multiferroics, 9,10 and carbon nanotubes.11 The electronic properties of such heterostructures are determined by the complex interplay between a multitude of parameters including the charge-carrier density, spin and orbital polarization, lattice distortions, and disorder, all of which are in general different from those in the constituent bulk materials. Since these parameters are difficult to probe in an interface-specific manner, current research efforts aimed at a quantitative description of the interfacial electronic properties require simple model systems in which the influence of individual parameters can be determined separately.Recent experiments have identified epitaxial superlattices (SLs) composed of the two ferromagnets La 0.7 Sr 0. 14,15 Both materials share the same pseudocubic lattice structure, and they are metallic and orbitally degenerate at all temperatures. One therefore does not expect modulations of the charge-carrier density or interfacial orbital or lattice polarization to play major roles. Magnetometric measurements on LSMO/SRO SLs with 2-nm-thick LSMO and 5-nm-thick SRO layers (henceforth SL2/5) revealed an isotropically reduced net magnetization for T < T SRO C , suggesting antiferromagnetic (AFM) interlayer exchange coupling (Fig. 1). The out-of-plane magnetization at low temperatures increases with increasing thickness of the SRO layers. Data on SL2/8 samples showed a suppressed magnetization for magnetic fields applied in the SL plane only, whereas the magnetization in a perpendicular field increases upon cooling below T SRO C . The thickness-dependent magnetic properties have been interpreted as evidence of competing magnetic interactions at the interface. [14][15][16] Motivated by these res...