Spin-reorientation transitions in the Cairo pentagonal magnet Bi4 Fe5 O13 F

© 2017 American Physical Society. We show that interlayer spins play a dual role in the Cairo pentagonal magnet Bi4Fe5O13F, on one hand mediating the three-dimensional magnetic order, and on the other driving spin-reorientation transitions both within and between the planes. The corresponding sequence of magnetic orders unraveled by neutron diffraction and Mössbauer spectroscopy features two orthogonal magnetic structures described by opposite local vector chiralities, and an intermediate, partly disordered phase with nearly collinear spins. A similar collinear phase has been predicted theoretically to be stabilized by quantum fluctuations, but Bi4Fe5O13F is very far from the relevant parameter regime. While the observed in-plane reorientation cannot be explained by any standard frustration mechanism, our ab initio band-structure calculations reveal strong single-ion anisotropy of the interlayer Fe3+ spins that turns out to be instrumental in controlling the local vector chirality and the associated interlayer order

Cubic lead perovskite PbMoO3 with anomalous metallic behavior

A previously unreported Pb-based perovskite PbMoO$_3$ is obtained by high-pressure and high-temperature synthesis. This material crystallizes in the $Pm\bar{3}m$ cubic structure at room temperature, making it distinct from typical Pb-based perovskite oxides with a structural distortion. PbMoO$_3$ exhibits a metallic behavior down to 0.1 K with an unusual $T$-sub linear dependence of the electrical resistivity. Moreover, a large specific heat is observed at low temperatures accompanied by a peak in $C_P/T^3$ around 10 K, in marked contrast to the isostructural metallic system SrMoO$_3$. These transport and thermal properties for PbMoO$_3$, taking into account anomalously large Pb atomic displacements detected through diffraction experiments, are attributed to a low-energy vibrational mode, associated with incoherent off-centering of lone pair Pb$^{2+}$ cations. We discuss the unusual behavior of the electrical resistivity in terms of a polaron-like conduction, mediated by the strong coupling between conduction electrons and optical phonons of the local low-energy vibrational mode.Comment: 5 pages, 5 figure

The Director’s Method in Contemporary Visual Effects Film: The Influence of Digital Effects on Film Directing

The director’ s method – meant as the organisation of the filmmaking process – is usually characterised by common procedures such as work on the script, shot design and the actors’ performance. For films involving a large-scale use of digital effects, directors consistently approach such procedures with a particular attitude dictated by the digital pipeline, the step-by-step technical procedure through which computer-generated images are created. In light of this, the use of digital effects might influence the director’s method. This thesis aims to define what is considered to be a consensual methodological approach to direct films with no or few digital effects and then compares this approach to when such effects are conspicuously involved. This analysis is conducted through interviews with working directors, visual effects companies and practitioners, and integrated with the current literature. The frame of the research is represented by a large spectrum of contemporary films produced in western countries and which involve digital effects at different scales and complexity but always in interaction with live-action. The research focuses on commercial films and excludes computer-animated and experimental films. The research is intended to address an area in production studies which is overlooked. In fact, although the existent literature examines both digital effects and film directing as distinct elements, there is to date no detailed analysis on the influence that the former has on the latter. In light of this, this dissertation seeks to fill a gap in production studies. The research looks to argue that the director’s method has been changed by the advent of digital effects; it describes a common workflow for digital effects film and notes the differences between this method and the method applied when digital effects are not involved. This is of significant importance for a film industry which is heavily dependent on such effects, as the analysis on contemporary filmmaking reveal

Phosphate Ion Functionalization of Perovskite Surfaces for Enhanced Oxygen Evolution Reaction

Recent findings revealed that surface oxygen can participate in the oxygen evolution reaction (OER) for the most active catalysts, which eventually triggers a new mechanism for which the deprotonation of surface intermediates limits the OER activity. We propose in this work a “dual strategy” in which tuning the electronic properties of the oxide, such as La1–xSrxCoO3??, can be dissociated from the use of surface functionalization with phosphate ion groups (Pi) that enhances the interfacial proton transfer. Results show that the Pi functionalized La0.5Sr0.5CoO3?? gives rise to a significant enhancement of the OER activity when compared to La0.5Sr0.5CoO3?? and LaCoO3. We further demonstrate that the Pi surface functionalization selectivity enhances the activity when the OER kinetics is limited by the proton transfer. Finally, this work suggests that tuning the catalytic activity by such a “dual approach” may be a new and largely unexplored avenue for the design of novel high-performance catalysts

Experimental determination of the magnetic interactions of frustrated Cairo pentagon lattice materials

We present inelastic neutron scattering measurements of the Cairo pentagon lattice magnets Bi$_2$Fe$_4$O$_9$ and Bi$_4$Fe$_5$O$_{13}$F, supported by high field magnetisation measurements of Bi$_2$Fe$_4$O$_9$. Using linear spin wave theory and mean field analyses we determine the spin exchange interactions and single-ion anisotropy in these materials. The Cairo lattice is geometrically frustrated and consists of two inequivalent magnetic sites, both occupied by Fe$^{3+}$ ions and connected by two competing nearest neighbour interactions. We found that one of these interactions, coupling nearest neighbour spins on the three-fold symmetric sites, is extremely strong and antiferromagnetic. These strongly coupled dimers are then weakly coupled to a framework formed from spins occupying the other inequivalent site. In addition we found that the Fe$^{3+}$ $S=5/2$ spins have a non-negligible single-ion anisotropy, which manifests as a spin anisotropy gap in the neutron spectrum and a spin-flop transition in high field magnetisation measurements.Comment: 10 pages, 9 figure

Room Temperature Magnetically Ordered Polar Corundum GaFeO3 Displaying Magnetoelectric Coupling

The polar corundum structure type offers a route to new room temperature multiferroic materials, as the partial LiNbO3-type cation ordering that breaks inversion symmetry may be combined with long-range magnetic ordering of high spin d5 cations above room temperature in the AFeO3 system. We report the synthesis of a polar corundum GaFeO3 by a high-pressure, high-temperature route and demonstrate that its polarity arises from partial LiNbO3-type cation ordering by complementary use of neutron, X-ray, and electron diffraction methods. In situ neutron diffraction shows that the polar corundum forms directly from AlFeO3-type GaFeO3 under the synthesis conditions. The A3+/Fe3+ cations are shown to be more ordered in polar corundum GaFeO3 than in isostructural ScFeO3. This is explained by DFT calculations which indicate that the extent of ordering is dependent on the configurational entropy available to each system at the very different synthesis temperatures required to form their corundum structures. Polar corundum GaFeO3 exhibits weak ferromagnetism at room temperature that arises from its Fe2O3-like magnetic ordering, which persists to a temperature of 408 K. We demonstrate that the polarity and magnetization are coupled in this system with a measured linear magnetoelectric coupling coefficient of 0.057 ps/m. Such coupling is a prerequisite for potential applications of polar corundum materials in multiferroic/magnetoelectric devices