Magnetic toroidicity is an uncommon type of magnetic structure in solid-state materials. Here, we experimentally demonstrate that collinear spins in a material with R -3 lattice symmetry can host a significant magnetic toroidicity, even parallel to the ordered spins. Taking advantage of a single crystal sample of CoTe 6 O 13 with an R -3 space group and a Co 2+ triangular sublattice, temperature-dependent magnetic, thermodynamic, and neutron diffraction results reveal A-type antiferromagnetic order below 19.5 K, with magnetic point group -3′ and k = (0,0,0). Our symmetry analysis suggests that the missing mirror symmetry in the lattice could lead to the local spin canting for a toroidal moment along the c axis. Experimentally, we observe a large off-diagonal magnetoelectric coefficient of 41.2 ps/m that evidences the magnetic toroidicity. In addition, the paramagnetic state exhibits a large effective moment per Co 2+ , indicating that the magnetic moment in CoTe 6 O 13 has a significant orbital contribution. CoTe 6 O 13 embodies an excellent opportunity for the study of next-generation functional magnetoelectric materials.

Hexagonal rare-earth manganites (h-RMnO3, R = Sc, Y, Ho-Lu) exhibit intricate multiferroic properties and may be utilized for various technological applications ranging from non-volatile memories to solar cells and photo detectors. Here, we report the epitaxial growth of hexagonal YMnO3 (h-YMO) films with different thicknesses deposited on yttria-stabilized zirconia (YSZ) substrates and their photoresponses to ultraviolet (UV) light. The structural characterizations illustrate that all the films present high quality epitaxy with out-of-plane h-YMO (0001)//YSZ(111) and in-plane h-YMO[1230]//YSZ[110] relationship. Meanwhile, UV light driven photo responses of these as-prepared films based on the planar photodetector structure with transparent Al-doped ZnO (AZO) as top electrodes are explored. It is revealed that the photocurrent will first increase and then decrease with increasing film thickness. The highest photoconductivity gain is similar to 387.4 observed at +4 V bias for the device with 160 nm YMO film. These results provide a guideline for hexagonal rare-earth manganites to work for UV photodetector application.

We report on the intriguing evolution of the dynamical spin correlations of the frustrated spinel ZnMn2O4. Inelastic neutron scattering and magnetization studies reveal that the dynamical correlations at high temperatures are 1D. At lower temperature, these dynamical correlations become 2D. Surprisingly, the dynamical correlations condense into a quasi 2D Ising-like ordered state, making this a rare observation of two dimensional order on the spinel lattice. Remarkably, 3D ordering is not observed down to temperatures as low as 300 mK. This unprecedented dimensional crossover stems from frustrated exchange couplings due to the huge Jahn-Teller distortions around Mn3+ ions on the spinel lattice.

Nitrogen gas accidentally sealed in a sample container produces various spurious effects in elastic neutron scattering measurements. These effects are systematically investigated and the details of the spurious scattering are presented.

Single crystal neutron diffraction studies have been performed on the S = 3/2 kagome staircase compound Co3V2O8 with a magnetic field applied along the magnetization easy-axis (H || a). Previous zero field measurements [Y. Chen, et al., Phys. Rev. B 74, 014430 (2006)] reported a rich variety of magnetic phases, with a ferromagnetic ground state as well as incommensurate, transversely polarized spin density wave (SDW) phases (with a propagation vector of k = (0 delta 0)) interspersed with multiple commensurate lock-in transitions. The magnetic phase diagram with H || a adds further complexity. For small applied fields, H = 0.05 T, the commensurate lock-in phases are destabilized in favor of the incommensurate SDW, while slightly larger applied fields restore the commensurate lock-in phase with delta = 1/2 and yield a new commensurate phase with delta = 2/5. For all measurements in an applied field, higher-order scattering is observed that corresponds to the second-harmonic.

Magnetization, specific heat, and neutron scattering measurements were performed to study a magnetic transition in jarosite, a spin-5/2 kagome lattice antiferromagnet. When a magnetic field is applied perpendicular to the kagome plane, magnetizations in the ordered state show a sudden increase at a critical field H_c, indicative of the transition from antiferromagnetic to ferromagnetic states. This sudden increase arises as the spins on alternate kagome planes rotate 180 degrees to ferromagnetically align the canted moments along the field direction. The canted moment on a single kagome plane is a result of the Dzyaloshinskii-Moriya interaction. For H < H_c, the weak ferromagnetic interlayer coupling forces the spins to align in such an arrangement that the canted components on any two adjacent layers are equal and opposite, yielding a zero net magnetic moment. For H > H_c, the Zeeman energy overcomes the interlayer coupling causing the spins on the alternate layers to rotate, aligning the canted moments along the field direction. Neutron scattering measurements provide the first direct evidence of this 180-degree spin rotation at the transition.

We have used neutron-diffraction measurements to study the zero-field magnetic structure of the intermetallic compound Yb3Pt4, which was earlier found to order antiferromagnetically at the Neel temperature T-N = 2.4 K, and displays a field-driven quantum-critical point at 1.6 T. In Yb3Pt4, the Yb moments sit on a single low-symmetry site in the rhombohedral lattice with space group R (3) over bar. The Yb ions form octahedra with edges that are twisted with respect to the hexagonal unit cell, a twisting that results in every Yb ion having exactly one Yb nearest neighbor. Below T-N, we found new diffracted intensity due to a k=0 magnetic structure. This magnetic structure was compared to all symmetry-allowed magnetic structures and was subsequently refined. The best-fitting magnetic-structure model is antiferromagnetic and involves pairs of Yb nearest neighbors on which the moments point almost exactly toward each other. This structure has moment components within the ab plane as well as parallel to the c axis although the easy magnetization direction lies in the ab plane. Our magnetization results suggest that besides the crystal-electric-field anisotropy, anisotropic exchange favoring alignment along the c axis is responsible for the overall direction of the ordered moments. The magnitude of the ordered Yb moments in Yb3Pt4 is 0.81 mu(B)/Yb at 1.4 K. The analysis of the bulk properties, the size of the ordered moment, and the observation of well-defined crystal-field levels argue that the Yb moments are spatially localized in zero field.

The crystal and magnetic structures of DyCrO 4 were studied using neutron powder diffraction. Complete diffraction data at 3.6, 17, 27, and 40 K show that a crystal structural phase transition from tetragonal I4 1 / amd to orthorhombic Imma symmetry is found to take place between 27 and 40 K. This transition does not involve a significant change in the unit cell volume. Strong ferromagnetic reflections are observed at 3.6 and 17 K, and can be fit well using the magnetic model of space group Im'ma' , with the moments of both Dy 3+ and Cr 5+ ions aligning along the y -axis. Detailed temperature dependent magnetic intensities of 101/011 and 211/121 peaks reveal a Curie temperature of T c =22.35(15) K.

Inelastic neutron scattering and susceptibility measurements have been performed on the optimally doped Fe-based superconductor FeTe0.5Se0.5, which has a critical temperature, T-c of 14 K. The magnetic scattering at the stripe antiferromagnetic wave vector Q=(0.5,0.5) exhibits a "resonance" at similar to 6 meV, where the scattering intensity increases abruptly when cooled below T-c. In a 7-T magnetic field parallel to the a-b plane, T-c is slightly reduced to similar to 12 K, based on susceptibility measurements. The resonance in the neutron-scattering measurements is also affected by the field. The resonance intensity under field cooling starts to rise at a lower temperature similar to 12 K, and the low-temperature intensity is also reduced from the zero-field value. Our results provide clear evidence for the intimate relationship between superconductivity and the resonance measured in magnetic excitations of Fe-based superconductors.

Electric control of multiferroic domains is demonstrated through polarized magnetic neutron diffraction. Cooling to the cycloidal multiferroic phase of Ni3V2O8 in an electric field E causes the incommensurate Bragg reflections to become neutron spin polarizing, the sense of neutron polarization reversing with E. Quantitative analysis indicates the E-treated sample has a handedness that can be reversed by E. We further show a close association between cycloidal and ferroelectric domains through E-driven spin and electric polarization hysteresis. We suggest that a definite cycloidal handedness is achieved through magnetoelastically induced Dzyaloshinskii-Moriya interactions.

We use bulk magnetic susceptibility, electronic specific heat, and neutron scattering to study structural and magnetic phase transitions in Fe1+ySexTe1-x. Fe1.068Te exhibits a first-order phase transition near 67 K with a tetragonal-to-monoclinic structural transition and simultaneously develops a collinear antiferromagnetic (AF) order responsible for the entropy change across the transition. Systematic studies of the FeSe1-xTex system reveal that the AF structure and lattice distortion in these materials are different from those of FeAs-based pnictides. These results call into question the conclusions of present density-functional calculations, where FeSe1-xTex and FeAs-based pnictides are expected to have similar Fermi surfaces and therefore the same spin-density wave AF order.

Single-crystal neutron and high-energy x-ray diffraction measurements have identified the phase lines corresponding to transitions among the ambient-pressure paramagnetic tetragonal (T), the antiferromagnetic orthorhombic (O), and the nonmagnetic collapsed tetragonal (cT) phases of CaFe2As2. We find no evidence of additional structures for pressures of up to 2.5 GPa (at 300 K). Both the T-cT and O-cT transitions exhibit significant hysteresis effects, and we demonstrate that coexistence of the O and cT phases can occur if a nonhydrostatic component of pressure is present. Measurements of the magnetic diffraction peaks show no change in the magnetic structure or ordered moment as a function of pressure in the O phase, and we find no evidence of magnetic ordering in the cT phase. Band-structure calculations show that the transition into the cT phase results in a strong decrease in the iron 3d density of states at the Fermi energy, consistent with a loss of the magnetic moment.

Neutron-diffraction and magnetization measurements have been carried out on a series of samples of the magnetorefrigerant Mn1+yFe1-yP1-xGex. The data reveal that the ferromagnetic and paramagnetic phases correspond to two very distinct crystal structures, with the magnetic-entropy change as a function of magnetic field or temperature being directly controlled by the phase fraction of this first-order transition. By tuning the physical properties of this system we have achieved a magnetic-entropy change [magnetocaloric effect (MCE)] for the composition Mn1.1Fe0.9P0.80Ge0.20 that has a similar shape for both increasing and decreasing field, with the maximum MCE exceeding 74 J/kg K-substantially higher than the previous record. The diffraction results also reveal that there is a substantial variation in the Ge content in the samples which causes a distribution of transition temperatures that reduces the MCE. It therefore should be possible to improve the MCE to exceed 100 J/kg K under optimal conditions.

Elastic and inelastic neutron-scattering studies reveal details of the antiferromagnetic transition and intriguing spin dynamics in the magnetoelectric effect single crystal LiMnPO4. The elastic-scattering studies confirm that the system is antiferromagnetic below T-N=33.75 K with local magnetic moments (Mn2+; S=5/2) that are aligned along the crystallographic a axis. The spin-wave dispersion curves propagating along the three principal axes, determined by inelastic scattering, are adequately modeled in the linear spin-wave framework assuming a spin Hamiltonian that is parametrized by inter- and in-plane nearest- and next-nearest-neighbor interactions and by easy-plane anisotropy. The temperature dependence of the spin dynamics makes this an excellent model many-body spin system to address the question of the relationship between spin-wave excitations and the order parameter.

Neutron powder-diffraction studies of the crystal and magnetic structures of the magnetocaloric compound Mn1.1Fe0.9(P0.8Ge0.2) have been carried out as a function of temperature, applied magnetic field, and pressure. The data reveal that there is only one transition observed over the entire range of variables explored, which is a combined magnetic and structural transformation between the paramagnetic (PM) and ferromagnetic phases (T-c approximate to 255 K for this composition). The structural part of the transition is associated with an expansion of the hexagonal unit cell in the direction of the a and b axes and a contraction of the c axis as the FM phase is formed, which originates from an increase in the intralayer metal-metal bond distance. The application of pressure is found to have an adverse effect on the formation of the FM phase since pressure opposes the expansion of the lattice and hence decreases T-c. The application of a magnetic field, on the other hand, has the expected effect of enhancing the FM phase and increasing T-c. We find that the substantial range of temperature/field/pressure coexistence of the PM and FM phases observed is due to compositional variations in the sample. In situ high-temperature diffraction measurements were carried out to explore this issue, and reveal a coexisting liquid phase at high temperatures that is the origin of this variation. We show that this range of coexisting phases can be substantially reduced by appropriate heat treatment to improve the sample homogeneity.

Magnetization, susceptibility, specific heat, resistivity, neutron and x-ray diffraction have been used to characterize the properties of single-crystalline ${\text{CaFe}}_{2\ensuremath{-}x}{\text{Ni}}_{x}{\text{As}}_{2}$ as a function of Ni doping for $x$ varying from 0 to 0.1. The combined first-order structural and magnetic phase transitions occur together in the undoped system at 172 K with a small decrease in the area of the $a\text{\ensuremath{-}}b$ plane along with an abrupt increase in the length of the $c$ axis in the orthorhombic phase. With increasing $x$ the ordered moment and transition temperature decrease but the transition remains sharp at modest doping while the area of the $a\text{\ensuremath{-}}b$ plane quickly decreases and then saturates. Warming and cooling data in the resistivity and neutron diffraction indicate hysteresis of $\ensuremath{\approx}2\text{ }\text{K}$. At larger doping the transition is more rounded and decreases to zero for $x\ensuremath{\approx}0.06$. The susceptibility is anisotropic for all values of $x$. Electrical resistivity for $x=0.053$ and 0.06 shows a superconducting transition with an onset of nearly 15 K which is further corroborated by substantial diamagnetic susceptibility. For the fully superconducting sample there is no long-range magnetic order and the structure remains tetragonal at all temperature but there is an anomalous increase in the area of the $a\text{\ensuremath{-}}b$ plane in going to low $T$. Heat-capacity data show that the density of states at the Fermi level increases for $x\ensuremath{\ge}0.053$ as inferred from the value of Sommerfeld coefficient $\ensuremath{\gamma}$. The regime of superconductivity is quite restrictive with a maximum ${T}_{C}$ of 15 K and an upper critical field ${H}_{c2}=14\text{ }\text{T}$. Superconductivity disappears in the overdoped region.

We use neutron spectroscopy to determine the nature of the magnetic excitations in superconducting BaFe1.9Ni0.1As2(T-c=20 K). Above T-c the excitations are gapless and centered at the commensurate antiferromagnetic wave vector of the parent compound, while the intensity exhibits a sinusoidal modulation along the c axis. As the superconducting state is entered a spin gap gradually opens, whose magnitude tracks the T dependence of the superconducting gap as observed by angle-resolved photoemission. Both the spin-gap and magnetic-resonance energies are temperature and wave-vector dependent, but their ratio is the same within uncertainties. These results suggest that the spin resonance is a singlet-triplet excitation related to electron pairing and superconductivity.