Avant-propos

International audienceForeword to the special issue of ARIMA Journal dedicated to CARI'14Avant propos au numéro spécial de la revue ARIMA dédié au CARI'14

Un modèle intégré pour explorer les trajectoires d’utilisation de l’espace

International audienceDynamic spatial models are important tools for the study of complex systems like environmental systems. This paper presents an integrated model that has been designed to explore land use trajectories in a small region around Maroua, located in the far north of Cameroon. The model simulates competition between land use types taking into account a set of biophysical, socio-demographic and geo-economics driving factors. The model includes three modules. The dynamic simulation module combines results of the spatial analysis and prediction modules. Simulation results for each scenario can help to identify where changes occur. The model developed constitutes an efficient knowledge support system for exploratory research and land use planning.Les modèles spatiaux dynamiques sont des outils de très grande importance pour l'étude des systèmes complexes comme les systèmes environnementaux. De plus, une approche intégrée est indispensable lorsqu'on veut avoir une compréhension plus complète du comportement de ces systèmes. Cet article décrit les bases d'un modèle intégré développé pour explorer les trajectoires d'utilisation de l'espace dans la région autour de Maroua, à l'Extrême Nord du Cameroun. Le modèle simule la compétition entre différentes catégories d'utilisation de l'espace en prenant en compte l'influence d'un ensemble de facteurs biophysiques, sociodémographiques et géoéconomiques. On distingue trois principaux modules. Le module de simulation dynamique combine les résultats des modules d'analyse spatiale et de prédiction. La calibration et la validation du modèle ont été effectuées pour la période entre 1987 et 1999, et la simulation des changements entre 1999 et 2010. Trois scénarios ont été formulés en s'appuyant sur l'analyse des tendances observées et les hypothèses de transition du système d'utilisation de l'espace. Les principales dynamiques observées concernent le développement de la culture maraîchère et l'extension de la culture du sorgho de contre saison qui induisent une compétition plus importante et des conflits. Les résultats de simulation pour chaque scénario permettent d'identifier des zones prioritaires pour toute intervention allant dans le sens de l'intensification ou d'une gestion intégrée et plus durable de l'espace. Le modèle développé constitue ainsi un outil de recherche exploratoire et un support de connaissances utilisable pour la planification de l'utilisation de l'espace. Une utilisation est envisageable pour initier toute concertation ou négociation entre les acteurs concernés par la gestion de l'espace

High-temperature terahertz optical diode effect without magnetic order in polar FeZnMo3_3O8_8

We present a terahertz spectroscopic study of polar ferrimagnet FeZnMo$_3$O$_8$. Our main finding is a giant high-temperature optical diode effect, or nonreciprocal directional dichroism, where the transmitted light intensity in one direction is over 100 times lower than intensity transmitted in the opposite direction. The effect takes place in the paramagnetic phase with no long-range magnetic order in the crystal, which contrasts sharply with all existing reports of the terahertz optical diode effect in other magnetoelectric materials, where the long-range magnetic ordering is a necessary prerequisite. In \fzmo, the effect occurs resonantly with a strong magnetic dipole active transition centered at 1.27 THz and assigned as electron spin resonance between the eigenstates of the single-ion anisotropy Hamiltonian. We propose that the optical diode effect in paramagnetic FeZnMo$_3$O$_8$ is driven by signle-ion terms in magnetoelectric free energy

Non-collinear magnetic structure and multipolar order in Eu2_2Ir2_2O7_7

The magnetic properties of the pyrochlore iridate material Eu$_2$Ir$_2$O$_7$ (5$d^5$) have been studied based on the first principle calculations, where the crystal field splitting $\Delta$, spin-orbit coupling (SOC) $\lambda$ and Coulomb interaction $U$ within Ir 5$d$ orbitals are all playing significant roles. The ground state phase diagram has been obtained with respect to the strength of SOC and Coulomb interaction $U$, where a stable anti-ferromagnetic ground state with all-in/all-out (AIAO) spin structure has been found. Besides, another anti-ferromagnetic states with close energy to AIAO have also been found to be stable. The calculated nonlinear magnetization of the two stable states both have the d-wave pattern but with a $\pi/4$ phase difference, which can perfectly explain the experimentally observed nonlinear magnetization pattern. Compared with the results of the non-distorted structure, it turns out that the trigonal lattice distortion is crucial for stabilizing the AIAO state in Eu$_2$Ir$_2$O$_7$. Furthermore, besides large dipolar moments, we also find considerable octupolar moments in the magnetic states.Comment: 6 pages, 4 figures, supplemental material is included in the source file, accepted for publication in PR

Magneto-elastic coupling and unconventional magnetic ordering in triangular multiferroic AgCrS2

The temperature evolution of the crystal and magnetic structures of ferroelectric sulfide AgCrS2 have been investigated by means of neutron scattering. AgCrS2 undergoes at TN = 41.6 K a first-order phase transition, from a paramagnetic rhombohedral R3m to an antiferromagnetic monoclinic structure with a polar Cm space group. In addition to being ferroelectric below TN, the low temperature phase of AgCrS2 exhibits an unconventional collinear magnetic structure that can be described as double ferromagnetic stripes coupled antiferromagnetically, with the magnetic moment of Cr+3 oriented along b within the anisotropic triangular plane. The magnetic couplings stabilizing this structure are discussed using inelastic neutron scattering results. Ferroelectricity below TN in AgCrS2 can possibly be explained in terms of atomic displacements at the magneto-elastic induced structural distortion. These results contrast with the behavior of the parent frustrated antiferromagnet and spin-driven ferroelectric AgCrO2