Thomas Brückel

Shape Induced Symmetry in Self-Assembled Mesocrystals of Iron Oxide Nanocubes

Grazing incidence small-angle scattering and electron microscopy have been used to show for the first time that nonspherical nanoparticles can assemble into highly ordered body-centered tetragonal mesocrystals. Energy models accounting for the directionality and magnitude of the van der Waals and dipolar interactions as a function of the degree of truncation of the nanocubes illustrated the importance of the directional dipolar forces for the formation of the initial nanocube clusters and the dominance of the van der Waals multibody interactions in the dense packed arrays.

KWS-1 high-resolution small-angle neutron scattering instrument at JCNS: current state

The KWS-1 small-angle neutron scattering (SANS) instrument operated by the Julich Centre for Neutron Science (JCNS) at the research reactor FRM II of the Heinz Maier-Leibnitz Zentrum in Garching near Munich has been recently upgraded. The KWS-1 instrument was updated, from its active collimation apertures to the detector cabling. Most of the parts of the instrument were installed for the first time, including a broadband polarizer, a large-cross-section radio-frequency spin flipper, a chopper and neutron lenses. A custom-designed hexapod in the sample position allows heavy loads and precise sample positioning in the beam for conventional SANS experiments as well as for grazing-incidence SANS under applied magnetic field. With the foreseen in situ polarization analysis the main scientific topic of the instrument tends towards magnetism. The performance of the polarizer and flipper was checked with a polarized 3He cell at the sample position. The results of these checks and a comparison of test measurements on a ferrofluid in a magnetic field with polarized and nonpolarized neutrons are presented.

Structural diversity in iron oxide nanoparticle assemblies as directed by particle morphology and orientation

The mesostructure of ordered arrays of anisotropic nanoparticles is controlled by a combination of packing constraints and interparticle interactions, two factors that are strongly dependent on the particle morphology. We have investigated how the degree of truncation of iron oxide nanocubes controls the mesostructure and particle orientation in drop cast mesocrystal arrays. The combination of grazing incidence small-angle X-ray scattering and scanning electron microscopy shows that mesocrystals of highly truncated cubic nanoparticles assemble in an fcc-type mesostructure, similar to arrays formed by iron oxide nanospheres, but with a significantly reduced packing density and displaying two different growth orientations. Strong satellite reflections in the GISAXS pattern indicate a commensurate mesoscopic superstructure that is related to stacking faults in mesocrystals of the anisotropic nanocubes. Our results show how subtle variation in shape anisotropy can induce oriented arrangements of nanoparticles of different structures and also create mesoscopic superstructures of larger periodicity.

2D to 3D crossover of the magnetic properties in ordered arrays of iron oxide nanocrystals

The magnetic 2D to 3D crossover behavior of well-ordered arrays of monodomain γ-Fe(2)O(3) spherical nanoparticles with different thicknesses has been investigated by magnetometry and Monte Carlo (MC) simulations. Using the structural information of the arrays obtained from grazing incidence small-angle X-ray scattering and scanning electron microscopy together with the experimentally determined values for the saturation magnetization and magnetic anisotropy of the nanoparticles, we show that MC simulations can reproduce the thickness-dependent magnetic behavior. The magnetic dipolar particle interactions induce a ferromagnetic coupling that increases in strength with decreasing thickness of the array. The 2D to 3D transition in the magnetic properties is mainly driven by a change in the orientation of the magnetic vortex states with increasing thickness, becoming more isotropic as the thickness of the array increases. Magnetic anisotropy prevents long-range ferromagnetic order from being established at low temperature and the nanoparticle magnetic moments instead freeze along directions defined by the distribution of easy magnetization directions.

Possible magnetic-polaron-switched positive and negative magnetoresistance in the GdSi single crystals

Magnetoresistance (MR) has attracted tremendous attention for possible technological applications. Understanding the role of magnetism in manipulating MR may in turn steer the searching for new applicable MR materials. Here we show that antiferromagnetic (AFM) GdSi metal displays an anisotropic positive MR value (PMRV), up to ~415%, accompanied by a large negative thermal volume expansion (NTVE). Around TN the PMRV translates to negative, down to ~−10.5%. Their theory-breaking magnetic-field dependencies [PMRV: dominantly linear; negative MR value (NMRV): quadratic] and the unusual NTVE indicate that PMRV is induced by the formation of magnetic polarons in 5d bands, whereas NMRV is possibly due to abated electron-spin scattering resulting from magnetic-field-aligned local 4f spins. Our results may open up a new avenue of searching for giant MR materials by suppressing the AFM transition temperature, opposite the case in manganites, and provide a promising approach to novel magnetic and electric devices.

Superlattice growth and rearrangement during evaporation-induced nanoparticle self-assembly

Understanding the assembly of nanoparticles into superlattices with well-defined morphology and structure is technologically important but challenging as it requires novel combinations of in-situ methods with suitable spatial and temporal resolution. In this study, we have followed evaporation-induced assembly during drop casting of superparamagnetic, oleate-capped γ-Fe2O3 nanospheres dispersed in toluene in real time with Grazing Incidence Small Angle X-ray Scattering (GISAXS) in combination with droplet height measurements and direct observation of the dispersion. The scattering data was evaluated with a novel method that yielded time-dependent information of the relative ratio of ordered (coherent) and disordered particles (incoherent scattering intensities), superlattice tilt angles, lattice constants, and lattice constant distributions. We find that the onset of superlattice growth in the drying drop is associated with the movement of a drying front across the surface of the droplet. We couple the rapid formation of large, highly ordered superlattices to the capillary-induced fluid flow. Further evaporation of interstitital solvent results in a slow contraction of the superlattice. The distribution of lattice parameters and tilt angles was significantly larger for superlattices prepared by fast evaporation compared to slow evaporation of the solvent.

POWTEX – the high-intensity time-of-flight diffractometer at FRM II for structure analysis of polycrystalline materials

In order to provide the large chemistry and materials science as well as the geosciences communities with a powerful tool for rapid data acquisition, a time-of-flight powder diffractometer to be installed at the new Munich reactor has been designed. The time-of-flight technique is expected to outperform a monochromator instrument by at least an order of magnitude in data acquisition time, particularly on small samples of less than a cubic centimetre. The construction of this innovative type of diffractometer utilizes modern components such as focusing super-mirror neutron guides, a four-unit high-speed disk chopper system and linear position-sensitive detectors covering a solid angle of about 2π steradian. The diffractometer design enables an easy enlargement of the focal spot size and is therefore equally well suited for the texture analysis of large geological and archaeological samples.

Approaching the true ground state of frustrated A-site spinels: A combined magnetization and polarized neutron scattering study

, macroscopic measurements evidence a ”cusp” in zero field-cooled susceptibility around13 K. Dynamic magnetic susceptibility and memory effect experiments provide results that do notconform with a canonical spin-glass scenario in this material. Through polarized neutron scatteringstudies, absence of long-range magnetic order down to 4 K is confirmed in FeAl

Incommensurate antiferromagnetic order in the manifoldly-frustrated SrTb2O4 with transition temperature up to 4.28 K

The Neel temperature of the new frustrated family of SrRE2O4 (RE = rare earth) compounds is yet limited to 0.9 K, which more or less hampers a complete understanding of the magnetic frustrations and spin interactions. Here we report on a new frustrated member to the family, SrTb2O4 with a record TN = 4.28(2) K, and an experimental study of the magnetic interacting and frustrating mechanisms by polarized and unpolarized neutron scattering. The compound of SrTb2O4 displays an incommensurate antiferromagnetic (AFM) order with a transverse wave vector Q = (0.5924(1), 0.0059(1), 0) albeit with partially-ordered moments, 1.92(6) uB at 0.5 K, stemming from only one of the two inequivalent Tb sites by virtue of their different octahedral distortions. The localized moments are confined to the bc plane, 11.9(66) degree away from the b axis by single-ion anisotropy. We reveal that this AFM order is dominated mainly by dipole-dipole interactions and disclose that the octahedral distortion, nearest-neighbour (NN) ferromagnetic (FM) arrangement, different next NN FM and AFM configurations, and in-plane anisotropic spin correlations are vital to the magnetic structure and associated multiple frustrations. The discovery of the thus far highest AFM transition temperature renders SrTb2O4 a new friendly frustrated platform in the family for exploring the nature of magnetic interactions and frustrations.

Spin-phonon coupling in K_{0.8}Fe_{1.6}Se_{2} and KFe_{2}Se_{2}: Inelastic neutron scattering and ab initio phonon calculations

We report measurements of the temperature dependence of phonon densities of states in K0.8Fe1.6Se2 using inelastic neutron scattering technique. While cooling down to 150 K, a phonon peak splitting around 25 meV is observed and a new peak appears at 31 meV. The measurements support the recent Raman and infra-red measurements indicating a lowering of symmetry of K0.8Fe1.6Se2 upon cooling below 250 K. Ab-initio phonon calculations have been carried out for K0.8Fe1.6Se2 and KFe2Se2. The comparison of the phonon spectra as obtained from the magnetic as well as non magnetic calculations show pronounced differences. We show that in the two calculations the energy range of the vibrational contribution from both Fe and Se are quite different. We conclude that Fe magnetism is correlated to the phonon dynamics and it plays an important role in stabilizing the structure of K0.8Fe1.6Se2 as well as that of KFe2Se2. The calculations highlight the presence of low energy librational modes in K0.8Fe1.6Se2 as compared to KFe2Se2.