# Publications

## The commensurate phase of multiferroic HoMn2O5 studied by X-ray magnetic scattering

ArXiv (0)

The commensurate phase of multiferroic HoMn2O5 was studied by X-ray magnetic scattering, both off resonance and in resonant conditions at the Ho-L3 edge. Below 40 K, magnetic ordering at the Ho sites is induced by the main Mn magnetic order parameter, and its temperature dependence is well accounted for by a simple Curie-Weiss susceptibility model. A lattice distortion of periodicity twice that of the magnetic order is also evidenced. Azimuthal scans confirm the model of the magnetic structure recently refined from neutron diffraction data for both Mn and Ho sites, indicating that the two sublattices interact via magnetic superexchange.

## A Neutron diffraction study of multiferroics RMn2O5

ArXiv (0)

The magnetic properties of RMn2O5 multiferrroics as obtained by unpolarized and polarized neutron diffraction experiments are reviewed. We discuss the qualitative features of the magnetic phase diagram both in zero magnetic field and in field and analyze the commensurate magnetic structure and its coupling to an applied electric field. The origin of ferrolectricity is discussed based on calculations of the ferroelectric polarization predicted by different microscopic coupling mechanisms (exchange striction and cycloidal spin-orbit models). A minimal model containing a small set of parameters is also presented in order to understand the propagation of the magnetic structure along the c-direction.

## Competing magnetic interactions in the extended Kagome system YBaCo4O7

ArXiv (0)

YBaCo4O7 belongs to a new class of geometrically frustrated magnets like the pyrochlores, in which Co-spins occupy corners of tetrahedra. The structure can be viewed as an alternating stacking of Kagome and triangular layers. Exactly half of the triangular units of the Kagome plane are capped by Co ions to form columns running perpendicular to the Kagome sheets. Neutron powder diffraction reveals a broad temperature range of diffuse magnetic scattering, followed by long range magnetic ordering below 110K. A unique low-temperature magnetic structure simultaneously satisfies an S=0 arrangement in the uncapped triangular units and antiferromagnetic coupling along the columns. A spin reorientation above 30K tracks the relative strengths of the in-plane and out-of-plane interactions.

## Ferroelectricity induced by acentric spin-density waves in YMn$_2$O$_5$

ArXiv (0)

The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn$_{2}$O$_{5}$, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magneto-elastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature-dependence of the spontaneous polarization, in particular its sign reversal at the commensurate-incommensurate transition.

## Order to disorder transition in the XY-like quantum magnet Cs2CoCl4 induced by noncommuting applied fields

ArXiv (0)

We explore the effects of noncommuting applied fields on the ground-state ordering of the quasi-one-dimensional spin-1/2 XY-like antiferromagnet Cs2CoCl4 using single-crystal neutron diffraction. In zero field interchain couplings cause long-range order below T_N=217(5) mK with chains ordered antiferromagnetically along their length and moments confined to the (b,c) plane. Magnetic fields applied at an angle to the XY planes are found to initially stabilize the order by promoting a spin-flop phase with an increased perpendicular antiferromagnetic moment. In higher fields the antiferromagnetic order becomes unstable and a transition occurs to a phase with no long-range order in the (b,c) plane, proposed to be a spin liquid phase that arises when the quantum fluctuations induced by the noncommuting field become strong enough to overcome ordering tendencies. Magnetization measurements confirm that saturation occurs at much higher fields and that the proposed spin-liquid state exists in the region 2.10 < H_SL < 2.52 T || a. The observed phase diagram is discussed in terms of known results on XY-like chains in coexisting longitudinal and transverse fields.

## The properties of Haldane excitations and multi-particle states in the antiferromagnetic spin-1 chain compound CsNiCl

ArXiv (0)

We report inelastic time-of-flight and triple-axis neutron scattering measurements of the excitation spectrum of the coupled antiferromagnetic spin-1 Heisenberg chain system CsNiCl3. Measurements over a wide range of wave-vector transfers along the chain confirm that above T_N CsNiCl3 is in a quantum-disordered phase with an energy gap in the excitation spectrum. The spin correlations fall off exponentially with increasing distance with a correlation length xi=4.0(2) sites at T=6.2K. This is shorter than the correlation length for an antiferromagnetic spin-1 Heisenberg chain at this temperature, suggesting that the correlations perpendicular to the chain direction and associated with the interchain coupling lower the single-chain correlation length. A multi-particle continuum is observed in the quantum-disordered phase in the region in reciprocal space where antiferromagnetic fluctuations are strongest, extending in energy up to twice the maximum of the dispersion of the well-defined triplet excitations. We show that the continuum satisfies the Hohenberg-Brinkman sum rule. The dependence of the multi-particle continuum on the chain wave-vector resembles that of the two-spinon continuum in antiferromagnetic spin-1/2 Heisenberg chains. This suggests the presence of spin-1/2 degrees of freedom in CsNiCl3 for T < 12K, possibly caused by multiply-frustrated interchain interactions.

## Spin dynamics of the model 2D quantum antiferromagnet CFTD

ArXiv (0)

The magnetic excitation spectrum in the two-dimensional (2D) S=1/2 Heisenberg antiferromagnet copper deuteroformate tetradeuterate (CFTD) has been measured for temperatures up to T\sim J/2, where J=6.31+-0.02 meV is the 2D exchange coupling. For T\ll J, a dispersion of the zone boundary energy is observed, which is attributed to a wavevector dependent quantum renormalization. At higher temperatures, spin-wave-like excitations persist, but are found to broaden and soften. By combining our data with numerical calculations, and with existing theoretical work, a consistent description of the behaviour of the model system is found over the whole temperature interval investigated.

## SMALL-ANGLE NEUTRON-SCATTERING FROM STAR-BRANCHED POLYMERS IN DILUTE-SOLUTION

MACROMOLECULES **22** (1989) 3130-3137

## Evidence of the B and C superconducting phases in the Bi-Ca-Sr-Cu-O system

Il Nuovo Cimento D **11** (1989) 903-906

A sintering process of Bi-Ca-Sr-Cu-O superconductor is reported. The electrical and microstructural analysis of the pellets shows the coexistence of B and C phases with transition temperatures of about 85 K and 105 K, respectively. © 1989 Società Italiana di Fisica.

## SMALL-ANGLE NEUTRON-SCATTERING FROM STAR-BRANCHED POLYMERS IN THE MOLTEN STATE

MACROMOLECULES **22** (1989) 681-686

## Small-Angle Neutron Scattering from Star-Branched Polymers

Springer Proceedings in Physics Springer-Verlag **42** (1989) 200-205

## OBSERVATION OF RESIDUAL 3-BODY INTERACTIONS IN STAR-BRANCHED POLYMERS AT THE THETA-POINT

EUROPHYSICS LETTERS **9** (1989) 327-332

## THE EFFECT OF GRAVITY ON THE RESOLUTION OF SMALL-ANGLE NEUTRON-SCATTERING

JOURNAL OF APPLIED CRYSTALLOGRAPHY **22** (1989) 252-255

## Small-Angle Neutron Scattering (SANS) Study of Star-Branched Polymers in the Molten State

Materials Science Forum Trans Tech Publications **27-28** (1988) 445-450

## Small-Angle Neutron Scattering (SANS) Study of Star-Branched Polymers in Dilute Solution

Materials Science Forum Trans Tech Publications **27&28** (1988) 451-456

## Superconducting compounds in the Bi-Ca-Sr-Cu-O system from citrate polymer precursor: production and preliminary characterization

Vuoto Bologna **18** (1988) 208-211

In this work a procedure followed for the preparation of superconducting compounds in the Bi-Ca-Sr-Cu-O system using the cytrate polymer precursor route is reported. A preliminary characterization of the structure and superconducting properties of the samples so obtained is given through X-ray diffraction analysis, resistivity and a.c. susceptibility measurements. All the samples so obtained showed the presence of superconducting phases at 80 K and 110 K beyond other non superconducting ones.

## MULTIPLE-SCATTERING CORRECTION TO THE 2ND VIRIAL-COEFFICIENT MEASURED BY SMALL-ANGLE SCATTERING

MACROMOLECULES **21** (1988) 3328-3329

## POLYDISPERSITY CORRECTIONS TO SMALL-ANGLE SCATTERING DATA FROM POLYMERS IN THE MELT AND IN DILUTE-SOLUTION

POLYMER **29** (1988) 1555-1561

## Interplay between localized and itinerant d electrons in a frustrated metallic antiferromagnet, 2H-AgNiO2

ArXiv (0)

We report the electronic and magnetic behaviour of the frustrated triangular metallic antiferromagnet 2H-AgNiO2 in high magnetic fields (54 T) using thermodynamic and transport measurements. Here localized d electrons are arranged on an antiferromagnetic triangular lattice nested inside a honeycomb lattice with itinerant d electrons. When the magnetic field is along the easy axis we observe a cascade of field-induced transitions, attributed to the competition between easy-axis anisotropy, geometrical frustration and coupling of the localized and itinerant system. The quantum oscillations data suggest that the Fermi surface is reconstructed by the magnetic order but in high fields magnetic breakdown orbits are possible. The itinerant electrons are extremely sensitive to scattering by spin fluctuations and a significant mass enhancement (~ 3) is found.