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Magnetic skyrmions are topologically protected chiral spin textures which present opportunities for next-generation magnetic data storage and logic information technologies. The topology of these structures originates in the geometric configuration of the magnetic spins - more generally described as the structure. While the skyrmion structure is most often depicted using a 2D projection of the three-dimensional structure, recent works have emphasized the role of all three dimensions in determining the topology and their response to external stimuli. In this work, grazing-incidence small-angle neutron scattering and polarized neutron reflectometry are used to determine the three-dimensional structure of hybrid skyrmions. The structure of the hybrid skyrmions, which includes a combination of N\'eel-like and Bloch-like components along their length, is expected to significantly contribute to their notable stability, which includes ambient conditions. To interpret the neutron scattering data, micromagnetic simulations of the hybrid skyrmions were performed, and the corresponding diffraction patterns were determined using a Born approximation transformation. The converged magnetic profile reveals the magnetic structure along with the skyrmion depth profile, including the thickness of the Bloch and N\'eel segments and the diameter of the core.

We present a temperature-dependent investigation of the local structure and magnetic dynamics of the FeAs binary. The magnetic susceptibility $\ensuremath{\chi}(T)$ result shows an anomalous broad feature up to 550 K, with \ensuremath{\chi} continuing to increase above the N\'eel antiferromagnetic ordering temperature (${T}_{\mathrm{N}}=70\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), peaking at \ensuremath{\sim}250 K, then decreasing gently above. It is remarkable that this peak susceptibility temperature corresponds to the onset of anisotropic negative thermal expansion in both the $a$ and $c$ axes, suggesting that magnetic interactions are affecting the structure even well above the N\'eel point. A systematic investigation into local bonding correlations, from time-of-flight neutron pair distribution function analyses, shows the octahedral volume around each Fe site growing monotonically while adjacent octahedra tilt toward one another before relaxing away past this peak in the anomalous magnetic susceptibility. We use inelastic-neutron scattering to map spin-wave excitations in FeAs at temperatures above and below the ${T}_{\mathrm{N}}$. We find magnetic excitations near ${T}_{\mathrm{N}}$ to be very different from the excitations in the ground state at 1.5 K. Spin waves measured at 1.5 K are three dimensional (3D), however, in the vicinity of the magnetic transition, the magnetic fluctuations clearly indicate two-dimensional (2D) character in this intrinsically 3D crystal structure. Unlike the undoped 2D parents of iron-arsenide superconductors, where the magnetic correlations are considerably weaker along the $c$ axis than in the $\mathit{ab}$ plane, inelastic neutron scattering here shows that the spin fluctuations in the 3D FeAs binary are nearly 2D in the $\mathit{bc}$ plane at 90 K. These results demonstrate the importance of short-range correlations in understanding the magnetic properties of transition-metal binaries, and suggest how 2D excitations, even in a 3D structure, can potentially become a breeding ground for unconventional superconductivity.

We have investigated the structural, magnetic, thermodynamic, and charge-transport properties of $\mathrm{M}{\mathrm{n}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$ single crystals through x-ray and neutron diffraction, magnetization, specific heat, magnetoresistance, and Hall-effect measurements. $\mathrm{M}{\mathrm{n}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$ displays a magnetic transition at ${T}_{C}\ensuremath{\sim}45\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with highly anisotropic behavior expected for a hexagonal-structured material. Below ${T}_{C}$, neutron diffraction reveals increased scattering near the structural Bragg peaks having a wider Q dependence along the $c$ axis than the nuclear Bragg peaks. This indicates either a short-range ferromagnetic (FM) order with a domain size of \ensuremath{\sim}250 nm along the $c$ axis or a possible magnetic modulation with a large pitch length. The expectation of a significant Dzyaloshinskii-Moriya interaction in this chiral-structured magnet, along with the helical state discovered in isostructural $\mathrm{C}{\mathrm{r}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$, suggest either a long period helical state with $q\ensuremath{\sim}0.0025\phantom{\rule{0.16em}{0ex}}{\AA{}}^{\ensuremath{-}1}$, or FM regions separated by magnetic solitons, may be responsible for the apparent small size of the FM domains. Here, the domain length along the $c$ axis is substantially larger than the pitch length of 48 nm found for the helimagnetic state in $\mathrm{C}{\mathrm{r}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$. Specific-heat-capacity measurements confirm a second-order magnetic phase transition with a substantial magnetic contribution that persists to low temperature. The low-temperature specific-heat capacity is consistent with a large density of low-lying magnetic excitations that are likely associated with topologically interesting magnetic modes. Changes to the magnetoresistance, the magnetization, and the magnetic neutron diffraction, which become more apparent below 20 K, imply a modification in the character of the magnetic ordering corresponding to the magnetic contribution to the specific-heat capacity. These observations signify a more complex magnetic structure both at zero and finite fields for $\mathrm{M}{\mathrm{n}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$ than for the well-investigated $\mathrm{C}{\mathrm{r}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$.

Artificial magnetic honeycomb lattices are expected to exhibit a broad and tunable range of novel magnetic phenomena that would be difficult to achieve in natural materials, such as long-range spin ice, entropy-driven magnetic charge-ordered state and spin-order due to the spin chirality. Eventually, the spin correlation is expected to develop into a unique spin solid state density ground state, manifested by the distribution of the pairs of vortex states of opposite chirality. Here we report the creation of a new artificial permalloy honeycomb lattice of ultra-small connecting bonds, with a typical size of $\simeq$ 12 nm. Detail magnetic and neutron scattering measurements on the newly fabricated honeycomb lattice demonstrate the evolution of magnetic correlation as a function of temperature. At low enough temperature, neutron scattering measurements and micromagnetic simulation suggest the development of loop state of vortex configuration in this system., 6 pages, 3 figures

By using magnetization and small-angle neutron scattering (SANS) measurements, we have investigated the magnetic behavior of Mn_{1-x}Ir_{x}Si system to explore the effect of increased carrier density and spin-orbit interaction on the magnetic properties of MnSi. We determine estimates of the spin wave stiffness and the Dzyalloshinski-Moria, DM, interaction strength and compare with Mn_{1-x}Co_{x}Si and Mn_{1-x}Fe_{x}Si. Despite the large differences in atomic mass and size of the substituted elements, Mn_{1-x}Co_{x}Si and Mn_{1-x}Ir_{x}Si show nearly identical variations in their magnetic properties with substitution. We find a systematic dependence of the transition temperature, the ordered moment, the helix period and the DM interaction strength with electron count for Mn{1-x}Ir{x}Si, Mn_{1-x}Co_{x}Si, and Mn_{1-x}Fe_{x}Si indicating that the magnetic behavior is primarily dependent upon the additional carrier density rather than on the mass or size of the substituting species. This indicates that the variation in magnetic properties, including the DM interaction strength, are primarily controlled by the electronic structure as Co and Ir are isovalent. Our work suggests that although the rigid band model of electronic structure along with Moira's model of weak itinerant magnetism describe this system surprisingly well, phenomenological models for the DM interaction strength are not adequate to describe this system., Comment: 17 pages, 7 Figures

Here we report small-angle neutron scattering (SANS) measurements and theoretical modeling of U$_3$Al$_2$Ge$_3$. Analysis of the SANS data reveals a phase transition to sinusoidally modulated magnetic order, at $T_{\mathrm{N}}=63$~K to be second order, and a first order phase transition to ferromagnetic order at $T_{\mathrm{c}}=48$~K. Within the sinusoidally modulated magnetic phase ($T_{\mathrm{c}} < T < T_{\mathrm{N}}$), we uncover a dramatic change, by a factor of three, in the ordering wave-vector as a function of temperature. These observations all indicate that U$_3$Al$_2$Ge$_3$ is a close realization of the three-dimensional Axial Next-Nearest-Neighbor Ising model, a prototypical framework for describing commensurate to incommensurate phase transitions in frustrated magnets.

We present inelastic neutron scattering measurements of the antiferromagnetic insulator ${\mathrm{CaMn}}_{2}{\mathrm{Sb}}_{2}$, which consists of corrugated honeycomb layers of Mn. The dispersion of magnetic excitations has been measured along the $\mathbf{H}$ and $\mathbf{L}$ directions in reciprocal space, with a maximum excitation energy of $\ensuremath{\approx}24$ meV. These excitations are well described by spin waves in a Heisenberg model, including first- and second-neighbor exchange interactions ${J}_{1}$ and ${J}_{2}$ in the Mn plane and also an exchange interaction between planes. The determined ratio ${J}_{2}/{J}_{1}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}1/6$ suggests that ${\mathrm{CaMn}}_{2}{\mathrm{Sb}}_{2}$ is an example of a compound that lies very close to the mean field tricritical point, known for the classical Heisenberg model on the honeycomb lattice, where the N\'eel phase and two different spiral phases coexist. The magnitude of the determined exchange interactions reveals a mean field ordering temperature $\ensuremath{\approx}4$ times larger than the reported N\'eel temperature ${T}_{N}=85$ K, suggesting significant frustration arising from proximity to the tricritical point.

We present high temperature inelastic neutron scattering and magnetic susceptibility measurements of the antiferromagnetic insulator LaMnPO that are consistent with the presence of two-dimensional magnetic correlations up to a temperature ${T}_{\text{max}}\ensuremath{\approx}700\phantom{\rule{0.28em}{0ex}}\text{K}\ensuremath{\gg}{T}_{N}=375$ K, the N\'eel temperature. Optical transmission measurements show the $T=300$ K direct charge gap $\ensuremath{\Delta}=1$ eV has decreased only marginally by 500 K and suggest it decreases by only 10% at ${T}_{\text{max}}$. Density functional theory and dynamical mean-field theory calculations reproduce a direct charge gap in paramagnetic LaMnPO only when a strong Hund's coupling ${J}_{H}=0.9$ eV is included, as well as on-site Hubbard $U=8$ eV. Our results show that LaMnPO is a Mott-Hund's insulator, in which the charge gap is rather insensitive to antiferromagnetic exchange coupling.

We have observed a magnetic vortex lattice (VL) in BaFe2(As0.67P0.33)(2) (BFAP) single crystals by small-angle neutron scattering. With the field along the c axis, a nearly isotropic hexagonal VL was formed in the field range from 1 to 16 T, and no symmetry changes in the VL were observed. The temperature dependence of the VL signal was measured and confirms the presence of (non-d-wave) nodes in the superconducting gap structure for measurements at 5 T and below. The nodal effects were suppressed at high fields. At low fields, a VL reorientation transition was observed between 1 and 3 T, with the VL orientation changing by 45 degrees. Below 1 T, the VL structure was strongly affected by pinning and the diffraction pattern had a fourfold symmetry. We suggest that this (and possibly also the VL reorientation) is due to pinning to defects aligned with the crystal structure, rather than being intrinsic. The temperature dependence of the scaled intensity suggests that BFAP possesses at least one full gap and one nodal gap with circular symmetry. Judging from the symmetry, the node structure should take the form of an "accidental" circular line node, which is consistent with recent angle-resolved photoemission spectroscopy results.

Using small angle neutron scattering, the anisotropy of the magnetic vortex lattice (VL), in the heavily hole-doped pnictide superconductor KFe2As2, was studied. Well-ordered VL scattering patterns were measured with fields applied in directions between B parallel to c and the basal plane, rotating either towards [100] or [110]. Slightly distorted hexagonal patterns were observed when B parallel to c. However, the scattering pattern distorted more strongly as the field was rotated away from the c axis. At low field, the arrangement of vortices is affected by the anisotropy of penetration depth in the plane perpendicular to the field. By fitting the distortion with the anisotropic London model, we obtain an estimate of similar to 3.4 for the anisotropy factor gamma between the in-plane and c-axis penetration depths at the lowest temperature studied. The results further reveal VL phase transitions as a function of field direction. We discuss these transitions using the "hairy ball" theorem.

Using small-angle neutron scattering, we have measured the misalignment between an applied field of $4\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ and the flux-line lattice in $\mathrm{Mg}{\mathrm{B}}_{2}$, as the field is rotated away from the $c$ axis by an angle $\ensuremath{\theta}$. The measurements, performed at $4.9\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, showed the vortices canting towards the $c$ axis for all field orientations. Using a two-band/two-gap model to calculate the magnetization, we are able to fit our results yielding a penetration depth anisotropy, ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}=1.1\ifmmode\pm\else\textpm\fi{}0.1$.