Dielectric anomalies and spiral magnetic order in CoCr2O4

We have investigated the structural, magnetic, thermodynamic, and dielectric properties of polycrystalline CoCr$_2$O$_4$, an insulating spinel exhibiting both ferrimagnetic and spiral magnetic structures. Below $T_c$ = 94 K the sample develops long-range ferrimagnetic order, and we attribute a sharp phase transition at $T_N$ $\approx$ 25 K with the onset of long-range spiral magnetic order. Neutron measurements confirm that while the structure remains cubic at 80 K and at 11 K; there is complex magnetic ordering by 11 K. Density functional theory supports the view of a ferrimagnetic semiconductor with magnetic interactions consistent with non-collinear ordering. Capacitance measurements on CoCr$_2$O$_4$, show a sharp decrease in the dielectric constant at $T_N$, but also an anomaly showing thermal hysteresis falling between approximately $T$ = 50 K and $T$ = 57 K. We tentatively attribute the appearance of this higher temperature dielectric anomaly to the development of \textit{short-range} spiral magnetic order, and discuss these results in the context of utilizing dielectric spectroscopy to investigate non-collinear short-range magnetic structures.Comment: & Figure

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

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

Structural and magnetic characterization of the complete delafossite solid solution (CuAlO2){1-x}(CuCrO2){x}

We have prepared the complete delafossite solid solution series between diamagnetic CuAlO2 and the t2g^3 frustrated antiferromagnet CuCrO2. The evolution with composition x in CuAl(1-x)Cr(x)O2 of the crystal structure and magnetic properties has been studied and is reported here. The room-temperature unit cell parameters follow the Vegard law and increase with x as expected. The effective moment is equal to the Cr^3+ spin-only S = 3/2 value throughout the entire solid solution. Theta is negative, indicating that the dominant interactions are antiferromagnetic, and its magnitude increases with Cr substitution. For dilute Cr compositions, J_BB was estimated by mean-field theory to be 2.0 meV. Despite the sizable Theta, long-range antiferromagnetic order does not develop until very large x, and is preceeded by glassy behavior. Data presented here, and that on dilute Al-substitution from Okuda et al., suggest that the reduction in magnetic frustration due to the presence of non-magnetic Al does not have as dominant an effect on magnetism as chemical disorder and dilution of the magnetic exchange. For all samples, the 5 K isothermal magnetization does not saturate in fields up to 5 T and minimal hysteresis is observed. The presence of antiferromagnetic interactions is clearly evident in the sub-Brillouin behavior with a reduced magnetization per Cr atom. An inspection of the scaled Curie plot reveals that significant short-range antiferromagnetic interactions occur in CuCrO2 above its Neel temperature, consistent with its magnetic frustration. Uncompensated short-range interactions are present in the Al-substituted samples and are likely a result of chemical disorder

Understanding the role of crystallographic shear on the electrochemical behavior of niobium oxyfluorides

The effects of shear planes in perovskite materials have been studied in order to identify their role in the electrochemical behavior of Li⁺ intercalation hosts. These planes modulate the structural stability and ionic transport pathways and therefore play an intimate role in the characteristics and performance of shear compounds. Herein, two Nb-based compounds, NbO₂F and Nb₃O₇F, were chosen as representative perovskite and shear derivatives respectively to investigate the role of crystallographic shear. A series of operando measurements, including X-ray diffraction and X-ray absorption spectroscopy, in conjunction with structural analysis, Raman spectroscopy, and detailed electrochemical studies identified the effect of shear planes. It was found that shear planes led to increased structural stability during Li⁺ (de)intercalation with shear layers being maintained, while perovskite layers were seen to degrade rapidly. However, disordering in the shear plane stacking introduced during delithiation ultimately led to poor capacity retention despite structural maintenance as Li⁺ diffusion channels are disrupted

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

Evolution of magnetic properties in the normal spinel solid solution Mg(1-x)Cu(x)Cr2O4

We examine the evolution of magnetic properties in the normal spinel oxides Mg(1-x)Cu(x)Cr2O4 using magnetization and heat capacity measurements. The end-member compounds of the solid solution series have been studied in some detail because of their very interesting magnetic behavior. MgCr2O4 is a highly frustrated system that undergoes a first order structural transition at its antiferromagnetic ordering temperature. CuCr2O4 is tetragonal at room temperature as a result of Jahn-Teller active tetrahedral Cu^2+ and undergoes a magnetic transition at 135 K. Substitution of magnetic cations for diamagnetic Mg^2+ on the tetrahedral A site in the compositional series Mg(1-x)Cu(x)Cr2O4 dramatically affects magnetic behavior. In the composition range 0 < x < 0.3, the compounds are antiferromagnetic. A sharp peak observed at 12.5K in the heat capacity of MgCr2O4 corresponding to a magnetically driven first order structural transition is suppressed even for small x suggesting glassy disorder. Uncompensated magnetism - with open magnetization loops - develops for samples in the x range 0.43 < x < 1. Multiple magnetic ordering temperatures and large coercive fields emerge in the intermediate composition range 0.43 < x < 0.47. The Neel temperature increases with increasing x across the series while the value of the Curie-Weiss Theta decreases. A magnetic temperature-composition phase diagram of the solid solution series is presented

What research agenda could be generated from the European General Practice Research Network concept of Multimorbidity in Family Practice?

This is the final version of the article. Available from the publisher via the DOI in this record.BACKGROUND: Multimorbidity is an intuitively appealing, yet challenging, concept for Family Medicine (FM). An EGPRN working group has published a comprehensive definition of the concept based on a systematic review of the literature which is closely linked to patient complexity and to the biopsychosocial model. This concept was identified by European Family Physicians (FPs) throughout Europe using 13 qualitative surveys. To further our understanding of the issues around multimorbidity, we needed to do innovative research to clarify this concept. The research question for this survey was: what research agenda could be generated for Family Medicine from the EGPRN concept of Multimorbidity? METHODS: Nominal group design with a purposive panel of experts in the field of multimorbidity. The nominal group worked through four phases: ideas generation phase, ideas recording phase, evaluation and analysis phase and a prioritization phase. RESULTS: Fifteen international experts participated. A research agenda was established, featuring 6 topics and 11 themes with their corresponding study designs. The highest priorities were given to the following topics: measuring multimorbidity and the impact of multimorbidity. In addition the experts stressed that the concept should be simplified. This would be best achieved by working in reverse: starting with the outcomes and working back to find the useful variables within the concept. CONCLUSION: The highest priority for future research on multimorbidity should be given to measuring multimorbidity and to simplifying the EGPRN model, using a pragmatic approach to determine the useful variables within the concept from its outcomes.The study had a Grant of 8000 Euros from the EGPRN