Structure and Melting of Two-Species Charged Clusters in a Parabolic Trap

We consider a system of charged particles interacting with an unscreened Coulomb repulsion in a two-dimensional parabolic confining trap. The static charge on a portion of the particles is twice as large as the charge on the remaining particles. The particles separate into a shell structure with those of greater charge situated farther from the center of the trap. As we vary the ratio of the number of particles of the two species, we find that for certain configurations, the symmetry of the arrangement of the inner cluster of singly-charged particles matches the symmetry of the outer ring of doubly-charged particles. These matching configurations have a higher melting temperature and a higher thermal threshold for intershell rotation between the species than the nonmatching configurations.Comment: 4 pages, 6 postscript figure

Elucidating Individual Magnetic Contributions in Bi-Magnetic Fe3O4/Mn3O4 Core/Shell Nanoparticles by Polarized Powder Neutron Diffraction

Heterogeneous bi-magnetic nanostructured systems have had a sustained interest during the last decades owing to their unique magnetic properties and the wide range of derived potential applications. However, elucidating the details of their magnetic properties can be rather complex. Here, a comprehensive study of Fe3O4/Mn3O4 core/shell nanoparticles using polarized neutron powder diffraction, which allows disentangling the magnetic contributions of each of the components, is presented. The results show that while at low fields the Fe3O4 and Mn3O4 magnetic moments averaged over the unit cell are antiferromagnetically coupled, at high fields, they orient parallel to each other. This magnetic reorientation of the Mn3O4 shell moments is associated with a gradual evolution with the applied field of the local magnetic susceptibility from anisotropic to isotropic. Additionally, the magnetic coherence length of the Fe3O4 cores shows some unusual field dependence due to the competition between the antiferromagnetic interface interaction and the Zeeman energies. The results demonstrate the great potential of the quantitative analysis of polarized neutron powder diffraction for the study of complex multiphase magnetic materials

Topological Defects and Non-homogeneous Melting of Large 2D Coulomb Clusters

The configurational and melting properties of large two-dimensional clusters of charged classical particles interacting with each other via the Coulomb potential are investigated through the Monte Carlo simulation technique. The particles are confined by a harmonic potential. For a large number of particles in the cluster (N>150) the configuration is determined by two competing effects, namely in the center a hexagonal lattice is formed, which is the groundstate for an infinite 2D system, and the confinement which imposes its circular symmetry on the outer edge. As a result a hexagonal Wigner lattice is formed in the central area while at the border of the cluster the particles are arranged in rings. In the transition region defects appear as dislocations and disclinations at the six corners of the hexagonal-shaped inner domain. Many different arrangements and type of defects are possible as metastable configurations with a slightly higher energy. The particles motion is found to be strongly related to the topological structure. Our results clearly show that the melting of the clusters starts near the geometry induced defects, and that three different melting temperatures can be defined corresponding to the melting of different regions in the cluster.Comment: 7 pages, 11 figures, submitted to Phys. Rev.

High-resolution optical spectroscopy, magnetic properties, and single-crystal neutron diffraction of multiferroic HoFe3(BO3)4: Magnetic structure

The magnetic structure is usually determined by the neutron diffraction measurements. However, in the case of complex multisublattice magnetics, this method fails to give an unambiguous result. Here, on the example of multiferroic HoFe3(BO3)4, we show that in the case of rare-earth (RE) compounds the right magnetic structure can be determined by additionally using optical spectroscopy and a theoretical analysis based on spectroscopic data. HoFe3(BO3)4 demonstrates a series of phase transitions and interesting magnetic and magnetoelectric properties. The available information on the magnetic structure of the compound, necessary for understanding and utilizing these properties, is contradictory. To resolve the existing ambiguities, we apply a combined approach. The high-resolution spectroscopy data deliver a set of the Ho3+ crystal-field (CF) levels in the paramagnetic and both easy-plane and easy-axis magnetic phases. These data are used to determine CF and Ho3+-Fe3+ exchange parameters and, then, to calculate the temperature dependencies of the magnetic susceptibility tensor of HoFe3(BO3)4. Based on these calculations, we suggest an easy-plane antiferromagnetic structure with a collinear arrangement of the Fe spins along the a axis and induced noncolinear moments of magnetically nonequivalent Ho ions. The suggested structure is further confirmed by single-crystal elastic neutron scattering experiments. We argue that specific features of the magnetic properties of RE iron borates isostructural to HoFe3(BO3)4 are governed by the energy patterns and the symmetry properties of the wave functions of the lower CF levels of the RE ground multiplet in the crystal field of the C2 symmetry

Spin-wave dynamics and exchange interactions in multiferroic NdFe3(BO3)4 explored by inelastic neutron scattering

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Crystal structure and phase transition in the doped super-ionic conductor bismuth vanadate Bi4(V,Fe)2O11 revealed by neutron diffraction

Bismuth vanadate Bi4V2O11-y doped with Fe was studied by neutron diffraction. It was shown that Fe substitutes V only partially and the redundant iron forms an impurity hematite phase. In the sample with nominal content Bi4(V0.96Fe0.04)2O11-y a new phase transition within the frame of the monoclinic structure with the appearing of inversion center was discovered in the temperature region 200-300°C. This transition was also confirmed by DTA experiments. Slow stabilization of the crystal structure with stabilization times of the order of hours, accompanied by a decrease of the unit cell volume, was observed. At high level of Fe doping, in addition to the dominant tetragonal γ-phase some amount of the nanosized monoclinic α-phase was detected. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Neutron powder diffraction and single crystal X ray magnetic resonant and non resonant scattering studies of the doped multiferroic Tb Bi MnO3

The results of powder neutron diffraction, X ray resonant and non resonant scattering in Tb0.95Bi0.05MnO3 demonstrate that substitution of Bi for Tb leads to partial suppression of intrinsic long range magnetic order in the Tb sub lattice, which transforms to correlated short range order. The magnetic order in the Mn sub lattice remains similar to that of TbMnO3. In the Bi substituted sample an anomaly in the temperature dependencies of the unit cell parameters was detected at about 100