Herein, we report the synthesis of multiscale nanostructured p-type (Bi,Sb)(2)Te(3) bulk materials by melt-spinning single elements of Bi, Sb, and Te followed by a spark plasma sintering process. The samples that were most optimized with the resulting composition (Bi(0.48)Sb(1.52)Te(3)) and specific nanostructures showed an increase of approximately 50% or more in the figure of merit, ZT, over that of the commercial bulk material between 280 and 475 K, making it suitable for commercial applications related to both power generation and refrigeration. The results of high-resolution electron microscopy and small angle and inelastic neutron scattering along with corresponding thermoelectric property measurements corroborate that the 10-20 nm nanocrystalline domains with coherent boundaries are the key constituent that accounts for the resulting exceptionally low lattice thermal conductivity and significant improvement of ZT.

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A new development in small-angle neutron scattering with polarization analysis allows us to directly extract the average spatial distributions of magnetic moments and their correlations with threedimensional directional sensitivity in any magnetic field. Applied to a collection of spherical magnetite nanoparticles 9.0 nm in diameter, this enhanced method reveals uniformly canted, magnetically active shells in a nominally saturating field of 1.2 T. The shell thickness depends on temperature, and it disappears altogether when the external field is removed, confirming that these canted nanoparticle shells are magnetic, rather than structural, in origin.

The pyrochlore material Yb2Ti2O7 displays unexpected quasi-two-dimensional (2D) magnetic correlations within a cubic lattice environment at low temperatures, before entering an exotic disordered ground state below T=265mK. We report neutron scattering measurements of the thermal evolution of the 2D spin correlations in space and time. Short range three dimensional (3D) spin correlations develop below 400 mK, accompanied by a suppression in the quasi-elastic (QE) scattering below ∼ 0.2 meV. These show a slowly fluctuating ground state with spins correlated over short distances within a kagome-triangular-kagome (KTK) stack along [111], which evolves to isolated kagome spin-stars at higher temperatures. Furthermore, low-temperature specific heat results indicate a sample dependence to the putative transition temperature that is bounded by 265mK, which we discuss in the context of recent mean field theoretical analysis.

Polarized small-angle neutron scattering studies of single-crystalline multiferroic BiFeO(3) reveal a long-wavelength spin density wave generated by ∼1° spin canting of the spins out of the rotation plane of the antiferromagnetic cycloidal order. This signifies weak ferromagnetism within mesoscopic regions of dimension 0.03 microns along [110], to several microns along [111], confirming a long-standing theoretical prediction. The average local magnetization is 0.06 μ(B)/Fe. Our results provide an indication of the intrinsic macroscopic magnetization to be expected in ferroelectric BiFeO(3) thin films under strain, where the magnetic cycloid is suppressed.

We study the structural and magnetic orders in electron-doped BaFe2−xNixAs2 by high-resolution synchrotron X-ray and neutron scatterings. Upon Ni-doping x, the nearly simultaneous tetragonalto-orthorhombic structural (Ts) and antiferromagnetic (TN ) phase transitions in BaFe2As2 are gradually suppressed and separated, resulting in Ts > TN with increasing x as was previously observed. However, the temperature separation between Ts and TN decreases with increasing x for x ≥ 0.065, tending towards a quantum bi-critical point near optimal superconductivity at x ≈ 0.1. The zerotemperature transition is preempted by the formation of a secondary incommensurate magnetic phase in the region 0.088 x 0.104, resulting in a finite value of TN ≈ Tc + 10 K above the superconducting dome around x ≈ 0.1. Our results imply an avoided quantum critical point, which is expected to strongly influence the properties of both the normal and superconducting states. A determination of the structural and magnetic phase diagram in correlated electron materials is important for understanding their underlying electronic excitations. In the iron pnictides, superconductivity arises at the border of both antiferromagnetic (AF) and structural orders [1][2][3][4][5]. This motivates the exploration of quantum critical points, where the transition temepratures for such orders are continuously suppressed to zero by a non-thermal control parameter. For the iron pnictide superconductors derived from electron or hole doping of their parent compounds, the most heavily studied materials are probably the electron-doped BaFe 2−x T x As 2 (where T = Co, Ni) because of the availability of high-quality single crystals [1, 6-15, 17, 18]. In the undoped state, BaFe 2 As 2 exhibits a tetragonal-to-orthorhombic structural transition at temperature T s and an AF phase transitions below nearly the same temperature T N ≈ T s ≈ 138 K [3, 4]. Upon electron-doping of BaFe 2 As 2 via partially replacing Fe by Co or Ni, various experiments, including transport [8,9], neutron [1,[11][12][13][14][15], and highresolution X-ray scattering [4,18] reveal that the structural (T s ) and magnetic (T N ) phase transition temperatures in BaFe 2−x T x As 2 gradually decrease and separate with increasing x, and have T s > T N for all doping levels. In the initial X-ray [10] and neutron [11] scattering work on BaFe 2−x Co x As 2 , it was suggested that the separated T s and T N smoothly extend into the superconducting dome, resulting in distinct structural and magnetic quantum critical points at different x. Subsequent X-ray [18] and neutron [12][13][14] scattering experiments on superconducting BaFe 2−x T x As 2 samples with coexisting AF order revealed that superconductivity actually competes with the static AF order and lattice orthorhombicity. As a consequence, the smoothly decreasing T s and T N are reported to bend back below T c , and the orthorhombic structure above T c for optimally doped sample evolves back to a tetragonal structure well below T c (termed the "re-entrant" tet...

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CaFe_{2}O_{4} is a S=5/2 anisotropic antiferromagnet based upon zig-zag chains having two competing magnetic structures, denoted as the A (↑↑↓↓) and B (↑↓↑↓) phases, which differ by the c-axis stacking of ferromagnetic stripes. We apply neutron scattering to demonstrate that the competing A and B phase order parameters result in magnetic antiphase boundaries along c which freeze on the time scale of ∼1  ns at the onset of magnetic order at 200 K. Using high resolution neutron spectroscopy, we find quantized spin wave levels and measure 9 such excitations localized in regions ∼1-2 c-axis lattice constants in size. We discuss these in the context of solitary magnons predicted to exist in anisotropic systems. The magnetic anisotropy affords both competing A+B orders as well as localization of spin excitations in a classical magnet.