W. Schweika

Approaching the ground state of the Kagomé antiferromagnet

Y0.5Ca0.5BaCo4O7 contains kagomé layers of Co ions, whose spins are strongly coupled, with a Curie-Weiss temperature of -2200 K. At low temperature, T=1.2 K, our diffuse neutron scattering study with polarization analysis reveals characteristic spin correlations close to a predicted two-dimensional coplanar ground state with staggered chirality. The absence of three-dimensional long-range antiferromagnetic order indicates negligible coupling between the kagomé layers. The scattering intensities are consistent with high spin S=3/2 states of Co2+ in the kagomé layers and low spin S=0 states for Co3+ ions on interlayer sites. Our observations agree with previous Monte Carlo simulations indicating a ground state of effectively short range, staggered chiral spin order.

Competing ferri- and antiferromagnetic phases in geometrically frustrated LuFe2O4.

We present a detailed study of magnetism in LuFe(2)O(4), combining magnetization measurements with neutron and soft x-ray diffraction. The magnetic phase diagram in the vicinity of T(N) involves a metamagnetic transition separating an antiferro- and a ferrimagnetic phase. For both phases the spin structure is refined by neutron diffraction. Observed diffuse magnetic scattering far above T(N) is explained in terms of near degeneracy of the magnetic phases.

Incommensurate Charge Order Phase in Fe2OBO3 due to Geometrical Frustration

The temperature dependence of charge order in Fe2OBO3 was investigated by resistivity and differential scanning calorimetry measurements, Mössbauer spectroscopy, and synchrotron x-ray scattering, revealing an intermediate phase between room temperature and 340 K, characterized by coexisting mobile and immobile carriers, and by incommensurate superstructure modulations with temperature-dependent propagation vector (1/2, 0, tau). The incommensurate modulations arise from specific antiphase boundaries with low energy cost due to geometrical charge frustration.

The structure of liquid water by polarized neutron diffraction and reverse Monte Carlo modelling

The coherent static structure factor of water has been investigated by polarized neutron diffraction. Polarization analysis allows us to separate the huge incoherent scattering background from hydrogen and to obtain high quality data of the coherent scattering from four different mixtures of liquid H(2)O and D(2)O. The information obtained by the variation of the scattering contrast confines the configurational space of water and is used by the reverse Monte Carlo technique to model the total structure factors. Structural characteristics have been calculated directly from the resulting sets of particle coordinates. Consistency with existing partial pair correlation functions, derived without the application of polarized neutrons, was checked by incorporating them into our reverse Monte Carlo calculations. We also performed Monte Carlo simulations of a hard sphere system, which provides an accurate estimate of the information content of the measured data. It is shown that the present combination of polarized neutron scattering and reverse Monte Carlo structural modelling is a promising approach towards a detailed understanding of the microscopic structure of water.

Time-of-flight and vector polarization analysis for diffuse neutron scattering

The potential of pulsed neutron sources for diffuse scattering including time-of-flight (TOF) and polarization analysis is discussed in comparison to the capabilities of the present instrument diffuse neutron scattering at the research center Julich. We present first results of a new method for full polarization analysis using precessing neutron polarization. A proposal is made for a new type of instrument at pulsed sources, which allows for vector polarization analysis in TOF instruments with multi-detectors.

Structural invariance upon antiferromagnetic ordering in geometrically frustrated swedenborgite, CaBaCo2Fe2O7

Centimetre-sized single crystals of high-quality CaBaCo2Fe2O7 were synthesized by the optical floating zone technique. The metal-to-metal stoichiometry and oxygen content were confirmed by spectroscopy and thermal reduction experiments. The hexagonal symmetry P63mc (No. 186) well describes the powder X-ray and neutron diffraction as well as single-crystal neutron diffraction at all measured temperatures. This symmetry is also consistent with optical second harmonic generation data obtained between 10 and 295 K. However, a satisfactory structure description from single-crystal neutron diffraction data needs an oxygen split position. Specific heat, magnetic susceptibility and powder neutron diffraction data indicate a magnetic phase transition at TN = 159 K to an antiferromagnetic ground state, but with a persisting hexagonal symmetry and intrinsic geometric frustration.

High spin frustration in Co based Swedenborgites

The spin correlations in a series of cobaltate polycrystalline samples with Swedenborgite structure, ABaCo3BO7 (A = Y, Ca, and B = Co, Fe, Al, Zn), were studied by means of diffuse polarized neutron scattering. The alternate stacking of kagome and triangular Co layers forms a tetrahedral Co-network with geometrical frustration of antiferromagnetically (AF) coupled spins, which typically suppresses long-range order even at low temperatures (~1K) despite Curie-Weiss temperatures in the order of 1000 K. The generic diffuse magnetic scattering as found in Y0.5Ca0.5BaCo4O7 reveals two dimensional (2D) spin correlations on the kagome sublattices towards the entropically favoured × structure and suggests a decoupling of layers by low spin states S = 0 on triangular sites. Co-substitution by Al and Zn yields similar diffuse magnetic scattering, however, spin dilution results in even more disordered spin liquid or spin glass states. With B = Fe or Co, differences in the magnetic scattering evolve, indicating the onset of spin correlations perpendicular to the kagome layers and non-vanishing spin states on the triangular sites. Geometrical frustration prohibits any 3D long-range order when the characteristic 2D spin correlations with the × motif is present. We observe long-range order only in YBaCo4O7 and CaBaCo3FeO7 showing rather distinct ordering wave-vectors.

XYZ-polarisation analysis of diffuse magnetic neutron scattering from single crystals

Studies of diffuse magnetic scattering largely benefit from the use of a multi-detector covering wide scattering angles. Therefore, the different contributions to the diffuse scattering that originate from magnetic, nuclear coherent, and nuclear spin-incoherent scattering can be separated by the so-called XYZ-polarization analysis. In the past this method has been successfully applied to the analysis of diffuse scattering by polycrystalline samples of magnetic disordered materials. Single crystal studies that exploit the vector properties of spin correlations are of particular interest for furthering our understanding of frustration effects in magnetism. Based on the symmetry properties of polarised scattering a suitable extension of the conventional XYZ method has been derived, which allows for the complete separation and the analysis of features of diffuse magnetic scattering from single crystals.

Monte Carlo Simulation of Order-Disorder Phenomena in Binary Alloys

An introduction is given into the Monte Carlo technique of phase diagram calculations. Beside the standard approaches used to determine first and second order phase transitions, a most promising development is discussed, namely the analysis of the probability distributions by histograms. Applications to real alloys are considered, such as the decomposition in Cu-Co and Cu-Ni, the short-range order and critical scattering in Cu-Au type alloys, the phenomenon of surface induced order in Cu-Au models having a free (100) surface, and the interplay of magnetism and chemical ordering in Fe-Al.

A Rietveld refinement method for angular- and wavelength-dispersive neutron time-of-flight powder diffraction data

This paper introduces a novel approach to Rietveld refinements of two-dimensional angular- and wavelength-dispersive powder diffraction data as measured at time-of-flight neutron diffraction instruments. To do so, the authors’ ansatz for diffraction data obtained from the POWGEN diffractometer has been verified, and furthermore its feasibility and benefit in simulations for the novel POWTEX instrument presently under construction are demonstrated.