Understanding complex magnetic order in disordered cobalt hydroxides through analysis of the local structure

In many ostensibly crystalline materials, unit-cell-based descriptions do not always capture the complete physics of the system due to disruption in long-range order. In the series of cobalt hydroxides studied here, Co(OH)$_{2-x}$(Cl)$_x$(H$_2$O)$_{n}$, magnetic Bragg diffraction reveals a fully compensated N\'eel state, yet the materials show significant and open magnetization loops. A detailed analysis of the local structure defines the aperiodic arrangement of cobalt coordination polyhedra. Representation of the structure as a combination of distinct polyhedral motifs explains the existence of locally uncompensated moments and provides a quantitative agreement with bulk magnetic measurements and magnetic Bragg diffraction

Magnetodielectric coupling in Mn3O4

We have investigated the dielectric anomalies associated with spin ordering transitions in the tetragonal spinel Mn$_3$O$_4$, using thermodynamic, magnetic, and dielectric measurements. We find that two of the three magnetic ordering transitions in Mn$_3$O$_4$ lead to decreases in the temperature dependent dielectric constant at zero applied field. Applying a magnetic field to the polycrystalline sample leaves these two dielectric anomalies practically unchanged, but leads to an increase in the dielectric constant at the intermediate spin-ordering transition. We discuss possible origins for this magnetodielectric behavior in terms of spin-phonon coupling. Band structure calculations suggest that in its ferrimagnetic state, Mn$_3$O$_4$ corresponds to a semiconductor with no orbital degeneracy due to strong Jahn-Teller distortion.Comment: 6 pages, 7 figure

Tuning magnetic frustration on the diamond lattice of the A-site magnetic spinels CoAl2−x_{2-x}Gax_xO4_4: Lattice expansion and site disorder

The spinels CoB$_2$O$_4$ with magnetic Co$^{2+}$ ions on the diamond lattice A site can be frustrated because of competing near-neighbor ($J_1$) and next-near neighbor ($J_2$) interactions. Here we describe attempts to tune the relative strengths of these interactions by substitution on the non-magnetic B-site. The system we employ is CoAl$_{2-x}$Ga$_x$O$_4$, where Al is systematically replaced by the larger Ga, ostensibly on the B site. As expected, Ga substitution expands the lattice, resulting in Co atoms on the A-site being pushed further from one other and thereby weakening magnetic interactions. In addition, Ga distributes between the B and the A site in a concentration dependent manner displacing an increasing amount of Co from the A site with increasing $x$. This increased inversion, which is confirmed by neutron diffraction studies carried out at room temperature, affects magnetic ordering very significantly, and changes the nature of the ground state. Modeling of the magnetic coupling illustrates the complexity that arises from the cation site disorder.Comment: 9 pages, 10 figure

Nova edició del workshop sobre "Mètodes Ràpids i Automatització en Microbiologia Alimentària" - memorial DYCFung

El XIX workshop sobre Mètodes Ràpids i Automatització en Microbiologia Alimentària (MRAMA) - memorial DYCFung, va ser organitzat en línia des de la Facultat de Veterinària de la UAB el mes de novembre passat. Celebrat anualment, amb un contingut aplicat i de futur, el MRAMA amplia i difon els coneixements teòrics i pràctics sobre mètodes innovadors per a detectar, comptar, aïllar i caracteritzar ràpidament els microorganismes, i els seus metabòlits, habituals en els aliments i l'aigua.El XIX workshop sobre Métodos Rápidos y Automatización en Microbiología Alimentaria (MRAMA) - memorial DYCFung, fue organizado en línea desde la Facultad de Veterinaria de la UAB el pasado mes de noviembre. Celebrado anualmente, con un contenido aplicado y de futuro, el MRAMA amplía y difunde los conocimientos teóricos y prácticos sobre métodos innovadores para detectar, contar, aislar y caracterizar rápidamente los microorganismos, y sus metabolitos, habituales en los alimentos y el agua

Risks management and cobots. Identifying critical variables

Trabajo presentado en: 29th European Safety and Reliability Conference (ESREL), 22–26 September 2019, HannoverA collaborative robot or a "Cobot" is the name of a robot that can share a workspace with operators in the absence of a protective fence or with only partial protection. They represent a new and expanding sector of industrial robotics. This investigation draws from the latest international rules and safety parameters related to work with collaborative robots. Its detailed research is motivated by the design of a collaborative industrial robot system, hazard elimination, risk reduction, and different collaborative operations, such as power and force limiting, collaborative operation design, and end-effector safety requirements, among others. The purpose of our study is to analyze the most important variables that must be controlled in accordance with the desired use of the Cobot, according to ISO / TS 15066, ISO / TR 20218-1and some other generic safety regulations on machines and industrial robots. A series of observations and appreciations on the use of the Cobot will also be presented

Magnetic phase evolution in the spinel compounds Zn1−x_{1-x}Cox_xCr2_2O4_4

We present the magnetic properties of complete solid solutions of ZnCr$_2$O$_4$ and CoCr$_2$O$_4$: two well-studied oxide spinels with very different magnetic ground states. ZnCr$_2$O$_4$, with non-magnetic $d^{10}$ cations occupying the A site and magnetic $d^3$ cations on the B site, is a highly frustrated antiferromagnet. CoCr$_2$O$_4$, with magnetic $d^7$ cations (three unpaired electrons) on the A site as well, exhibits both N\'eel ferrimagnetism as well as commensurate and incommensurate non-collinear magnetic order. More recently, CoCr$_2$O$_4$ has been studied extensively for its polar behavior which arises from conical magnetic ordering. Gradually introducing magnetism on the A site of ZnCr$_2$O$_4$ results in a transition from frustrated antiferromagnetism to glassy magnetism at low concentrations of Co, and eventually to ferrimagnetic and conical ground states at higher concentrations. Real-space Monte-Carlo simulations of the magnetic susceptibility suggest that the first magnetic ordering transition and features of the susceptibility across $x$ are captured by near-neighbor self- and cross-couplings between the magnetic A and B atoms. We present as a part of this study, a method for displaying the temperature dependence of magnetic susceptibility in a manner which helps distinguish between compounds possessing purely antiferromagnetic interactions from compounds where other kinds of ordering are present.Comment: 10 pages, 5 figures, 1 tabl

The role of static disorder in negative thermal expansion in ReO3

Time-of-flight neutron powder diffraction and specific heat measurements were used to study the nature of thermal expansion in rhenium trioxide, an electrically conducting oxide with cubic symmetry. The temperature evolution of the lattice parameters show that ReO3 can exhibit negative thermal expansion at low temperatures and that the transition from negative to positive thermal expansion depends on sample preparation; the single crystal sample demonstrated the highest transition temperature, 300 K, and largest negative value for the coefficient of thermal expansion, alpha = -1.1(1)x 10^-6 K^-1. For the oxygen atoms, the atomic displacement parameters are strongly anisotropic even at 15 K, indicative of a large contribution of static disorder to the displacement parameters. Further inspection of the temperature evolution of the oxygen displacement parameters for different samples reveals that the static disorder contribution is greater for the samples with diminished NTE behavior. In addition, specific heat measurements show that ReO3 lacks the low energy Einstein-type modes seen in other negative thermal expansion oxides such as ZrW2O8.Comment: 6 pages, 5 figure

Polarizable Anionic Sublattices Can Screen Molecular Dipoles in Noncentrosymmetric Inorganic-Organic Hybrids

We report the growth and photophysical characterization of two polar hybrid lead halide phases, methylenedianiline lead iodide and bromide, (MDA)Pb2I6 and (MDA)Pb2Br6, respectively. The phases crystallize in noncentrosymmetric space group Fdd2, which produces a highly oriented molecular dipole moment that gives rise to second harmonic generation (SHG) upon excitation at 1064 nm. While both compositions are isostructural, the size dependence of the SHG signal suggests that the bromide exhibits a stronger phase-matching response whereas the iodide exhibits a significantly weaker non-phase-matching signal. Similarly, fluorescence from (MDA)Pb2Br6 is observed around 630 nm below 75 K whereas only very weak luminescence from (MDA)Pb2I6 can be seen. We attribute the contrasting optical properties to differences in the character of the halide sublattice and postulate that the increased polarizability of the iodide ions acts to screen the local dipole moment, effectively reducing the local electric field in the crystals