No abstract

Modern materials and their properties are often characterized by varying degrees of disorder. Routine crystallographic structure solution only reveals the average structure. The study of Bragg and diffuse scattering yields the local atomic arrangements holding the key to understanding increasingly complex materials. In this paper we review the pair distribution function technique used to unravel the local structure. We aim to give a practical overview and make this method easily accessible to the wider scientific community.

Using bulk magnetization along with elastic and inelastic neutron scattering techniques, we have investigated the phase diagram of Fe 1þy Se x Te 1Àx and the nature of magnetic correlations in three nonsuperconducting samples of Fe 1:01 Se 0:1 Te 0:9 , Fe 1:01 Se 0:15 Te 0:85 , and Fe 1:02 Se 0:3 Te 0:7 . A cusp and hysteresis in the temperature dependence of the magnetization for the x ¼ 0:15 and 0.3 samples indicates spin-glass (SG) ordering below T sg ¼ 23 K. Neutron scattering measurements indicate that the spin-glass behavior is associated with short-range spin density wave (SDW) ordering characterized by a static component and a low-energy dynamic component with a characteristic incommensurate wave vector of Q m ¼ ð0:46; 0; 0:50Þ and an anisotropy gap of $2:5 meV. Our high Q-resolution data also show that the systems undergo a glassy structural distortion that coincides with the short-range SDW order.Following the discovery of superconductivity in Febased pnictides, 1) a resurgence of interest in the field of high temperature superconductivity ensued. 2-8) There has been particular interest in the possible connection between magnetism and superconductivity. In the iron pnictides, an antiferromagnetically ordered phase is in close proximity to optimal superconductivity. 9) In some cases, such as SmFeAsO 1Àx F x and Ba(Fe 1Àx Co x ) 2 As 2 , there is evidence for coexisting antiferromagnetic order and superconductivity. [10][11][12][13][14] The situation is somewhat different in the chalcogenide system, Fe 1þy Se x Te 1Àx . Here the details are sensitive to the Fe as well as the Se concentration, and we will focus on the situation for minimized excess Fe (i.e., y % 0). The Néel temperature drops rapidly for x 0:1, but our measurements indicate that bulk superconductivity only appears for x ! 0:4.One reason for a difference between the pnictides and chalcogenides concerns the nature of the antiferromagnetic order. To discuss that order, we first have to consider the crystal structure. In the -PbO structure of Fe 1þy Se x Te 1Àx (FST), the Fe layers have a square lattice structure; however, the positions of the Se/Te atoms above and below those planes break the translational symmetry. Thus, it is crystallographically appropriate to choose a unit cell with two Fe atoms per layer, such that the lattice parameter is a % 3:8 Å . We will specify reciprocal lattice vectors, Q ¼ ðh; k; lÞ, in reciprocal lattice units (rlu) of ð2=a; 2=b; 2=cÞ. In Fe 1þy Te, the long-range SDW state is accompanied by a tetragonal-to-monoclinic (or orthorhombic, depending on y) structural transition. 15,16) The spin arrangement is ferromagnetic along the b-direction and alternates in a þþÀÀ fashion along the a-direction, leading to a characteristic wave vector of ð0:5; 0; 0:5Þ. For larger y (e.g., y ¼ 0:14), the in-plane component of the magnetic wave vector becomes slightly incommensurate. 15) In Fe 1þy Se x Te 1Àx with 0:25x 0:33, static, but short-range, incommensurate magnetic order with Q m ¼ ð0:5 À ; 0; 0:5Þ is observed. 15,17,18) ...

Our powder inelastic neutron scattering data indicate that ZnV2O4 is a system of spin chains that are three-dimensionally tangled in the cubic phase above 50 K due to randomly occupied t(2g) orbitals of V3+ (3d(2)) ions. Below 50 K in the tetragonal phase, the chains become straight due to antiferro-orbital ordering. This is evidenced by the characteristic wave vector dependence of the magnetic structure factor that changes from symmetric to asymmetric at the cubic-to-tetragonal transition.

Inelastic cold neutron scattering on LaCoO 3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic (AFM) correlations that are dynamic follow the activation to the excited state, identified as the intermediate S = 1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La 1−x Sr x CoO 3 , the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism. [6], the system can be thermally excited to a magnetic state. Evidence for this is provided by a broad peak in the bulk susceptibility, χ, [1,7,8,9] ∼ 100 K that marks the onset to a paramagnetic state. The excited state can either be the so-called S = 1 intermediate spin (IS) state with a t and is Jahn-Teller active while the S = 2 state is not. The low temperature dependence of χ has been fit equally well with models that consider either spin state [8,10] making this a controversial issue for several decades [7,8,9,11]. Identifying the excited state that leads to the magnetic transition is important as it provides insights to the nature of the Co incident energy E i of 3.27 meV). The single crystals were measured using the cold-neutron triple-axis spectrometer SPINS at the NCNR, with a fixed final energy of E f = 3.7 meV.The instrumental energy resolution was ∆E = 0.22 meV. The single crystals were mounted in the (h,h,l) scattering plane using the pseudocubic notation and a room temperature lattice constant a = 3.8377Å. A cooled BeO filter was placed in the scattered beam to eliminate higher order contaminations.LaCoO 3 was measured using the DCS spectrometer at temperatures ranging from 10 to 300 K. As this system is not magnetic at low temperatures, the 10 K data were used as the 3 background and subtracted from data at the other temperatures to obtain the dynamical