[en] The fascinating magnetic properties of the rare earths are further enriched by alloying the chemically alike elements. The periodic magnetic structures of Ho and to some extent Er are well described by a mean field spin model, and therefore provide an ideal starting point for an examination of their alloys. Single crystals of random alloyed Ho₉₀Er₁₀ and Ho₅₀Er₅₀ have been studied using neutron scattering, which is the perfect probe in magnetic structure analysis. The experiments show quantitative as well as qualitative differences between the two alloys with respect to transition temperatures T_N and T_C, periodicities Q(T) and types of structures. For the interpretation of these data, the spin models for the elements have been merged to an essentially parameter free MF model using the virtual crystal approximation. The indirect exchange coupling is mediated by an averaged electron gas, whereas the one additional 4f electron in erbium causes opposite crystal field anisotropy. The competition between the periodicity of the former and the bunching of the spins along easy axes due to the latter results in a phase diagram where the detailed structure of the overall phases pose interesting phenomena. At low temperatures the pure Ho periodicity of 1/6 c^* is changed into an overall periodicity of 7/36 c^* but with a disordered sub-periodicity for the Ho₉₀Er₁₀ crystal. This is in contrast to the very ideally ordered structures found in most of the rare earth elements. A coincidence between periodicity and temperature quenches the structure in a random distribution of the two block types. At the transition from a helical to a conical ordered structure the Ho₅₀Er₅₀ alloy reveals an intermediate phase which has not yet been identified. The MF-VC models capability of describing the static structures has been extended to random phase approximation (RPA) calculations of the spin wave dynamics in the alloys. (au) 38 refs

[en] We have discovered that a recently reported weak excitation branch in the spin-Peierls material CuGeO₃ is in fact a ghost image of the primary magnetic excitation shifted in reciprocal space by a novel multiple scattering process. A model is developed that predicts the occurrence of such multiple scattering and accounts for the observations in CuGeO₃. New 'ghostons' can occur when the magnetic unit cell is smaller than the structural, while mixing of intensities from different reciprocal space zones jeopardize accurate polarisation analysis and the study of weak modes in general

[en] A novel multiplexing three-axis spectrometer for thermal neutrons has been tested for single-crystal spectroscopy. Measuring the entire 2D dispersion manifold in the bc plane of the spin-Peierls compound CuGeO₃, we demonstrate that the spectrometer fascilitates investigations of large volumes in (Q,ω)-space from small single crystals. The performance of this multi-analyser spectrometer at a steady-state source compares well to an existing time-of-flight spectrometer at a pulsed source

[en] The RITA spectrometer at Risoe National Laboratory was the first to incorporate a complete re-thinking of the neutron-path from source, through detector to analysis. Since then, other RITA-type spectrometers such as SPINS at NIST, RITA-II at PSI have been built, and several new spectrometers around the world are adapting the same philosophy. The main novelty of RITA was the introduction of a single back-end tank featuring both an analyser block with multiple individually turnable analyser blades and a 2D position sensitive detector. Several new triple-axis spectrometers are presently being built at existing and future sources, and almost all of them have learnt from the experience with RITA. (R.P.)

[en] The quasi-1D S=1/2 antiferromagnet CuGeO₃ undergoes a spin-Peierls transition to a dimerised singlet quantum ground state, with S=1 carrying pairs of domain walls as elementary excitations. Applying a large magnetic field, the domain walls - solitons - can be condensed into the ground state, forming a static incommensurate soliton lattice, which has been investigated by neutron scattering. The static soliton structure was found to be in good accordance to field theoretical and numerical calculations, while a low energy incommensurate mode is interpreted as a gapped phason. We anticipate that theoretical description of this and higher energy excitations will provide general insight to the effect of structural fluctuations on correlated electron systems

[en] We will review recent results obtained by longitudinal polarization analysis (LPA) and spherical neutron polarimetry (SNP) on typical 'low-dimensional' strongly correlated electronic systems (spin-Peierls CuGeO₃, High-T_c YBa₂Cu₃O_6+x...), which all demonstrate the importance of polarized neutrons for studies of the magnetic and structural inelastic contributions. In such systems, the hybridization between the structural and magnetic degrees of freedom is expected to give rise to non-negligible 'hybrid' correlation functions, which can be detected and investigated accurately by neutron polarimetry. The interest of the SNP in inelastic scattering will be discussed through measurements on CuGeO₃

[en] A new focusing supermirror guide has been designed for the IN14 three-axis spectrometer at ILL. The guide is made of four sections, tangent to an ideal parabolic shape. Commissioning on the instrument has shown a gain of 2 to 5, for k_i between 2.662 and 1.05 A^-1, on a 1 cm² section at sample position. The energy resolution is unchanged, compared to the instrument with relaxed collimation. Although the Q-resolution is degraded, it is shown that this setup is perfectly convenient for the study of non-dispersive excitations

[en] The inelastic neutron-scattering signal from magnetic nanoparticles contains information on magnetic dynamics like superparamagnetic relaxation and collective magnetic excitations. Often another, very broad quasi-elastic component is observed in addition. We have studied this quasi-elastic neutron signal from 4 nm ferrimagnetic maghemite (γ-Fe₂O₃) particles, and by means of time-of-flight polarised neutron scattering we have identified the source of (most of) this signal to be water adsorbed at the surface of the nanoparticles. A minor part of the signal has its origin in dynamics of disordered surface spins and vibrations of individual nanoparticles

[en] We will review recent results obtained by longitudinal polarization analysis and spherical neutron polarimetry (SNP) on typical low-dimensional strongly correlated electronic systems (spin-Peierls CuGeO₃, high-T_c La_0.85Sr_0.15CuO₄ and YBa₂Cu₃O_6+x, or spin-ladder Sr₁₄Cu₂₄O₄₁), which all demonstrate the importance of polarized neutrons for studies of magnetic and structural inelastic contributions. In such systems, the hybridization between the structural and magnetic degrees of freedom is expected to give rise to non-negligible hybrid correlation functions, which can be detected and accurately investigated by neutron polarimetry. The interest of the SNP in inelastic scattering will be discussed through measurements on CuGeO₃

[en] The crystallographic structure of the cuprate Sr₁₄Cu₂₄O₄₁ is a misfit stacking of layers of two distinct quantum-spin systems: linear edge-sharing CuO₂ chains and 2-leg Cu₂O₃ spin-ladders. Interestingly, the CuO₂-chains contain a large amount of holes which give rise to a quasi-2D charge order at low temperature, associated with a strong lattice distortion. In order to understand better the nature of excitation spectra in both sub-systems (in particular their possible hybrid or orbital nature), we have recently performed accurate polarimetric inelastic-neutron-scattering studies of the chain and ladder excitations in Sr₁₄Cu₂₄O₄₁ as a function of both the wave vector and the temperature. By measuring the spin-flip and non-spin-flip contributions for the incident polarization applied parallel and perpendicular to the scattering vector Q we have been able to separate the nuclear and the various magnetic components. Quite unexpectedly, our results for the CuO₂ chains and Cu₂O₃ ladders reveal the existence of a strong anisotropy of the magnetic inelastic structure factors, which could be the signature of orbital currents theoretically predicted