DETERMINATION OF THE DEGREE OF CRYSTALLITES ORIENTATION IN PERMANENT MAGNETS BY X-RAY SCATTERING AND MAGNETIC MEASUREMENTS

Les aimants permanents sont essentiellement des composés ferromagnétiques et uniaxiaux. Certains aimants sont élaborés en frittant des poudres préalablement orientées et comprimées sous champ. La valeur de 1'induction rémanente Br dépend directement du degré d'alignement des cristallites et nous présentons dans cet article deux méthodes quantitatives différentes pour le déterminer. L'analyse aux rayons X permet de comparer les intensités des réflexions de Bragg pour les différentes valeurs de l'angle entre une direction [hkl] et l'axe cristallographique principal. L'analyse magnétique permet de comparer les courbes d'aimantation expérimentales et celles calculées, pour différentes distributions statistiques des orientations des grains. Les deux méthodes ont été appliquées à des aimants Nd-Fe-B et SmCo5. Elles révèlent que la désorientation des particules dans ces systèmes conduit à une réduction d'environ 10 % de l'induction rémanente.Permanent magnet compounds are basically ferromagnets with uniaxial anisotropy. In different cases, the magnets are elaborated by sintering of powders previously oriented and compressed under field. The value of the remanent induction Br is directly dependent on the degree of crystallites alignment, and we present in this paper two different quantitative methods to determine it. X-ray analysis involves the comparison of the intensities of Bragg reflections, for different values of the angle of a direction [hkl] with respect to the unique crystallographic axis of the compound. Magnetic analysis involves the comparison between experimental magnetization curves and calculated ones for different statistical distributions of crystallites orientations. Both methods were applied to NdFeB and SmCo5 magnets. They reveal that the misorientation of particles in these systems leads to a reduction of the remanent induction by about 10 %

Ascl1/Mash1 Promotes Brain Oligodendrogenesis during Myelination and Remyelination

International audienceOligodendrocytes are the myelin-forming cells of the CNS. They differentiate from oligodendrocyte precursor cells (OPCs) that are produced from progenitors throughout life but more actively during the neonatal period and in response to demyelinating insults. An accurate regulation of oligodendrogenesis is required to generate oligodendrocytes during these developmental or repair processes. We hypothesized that this regulation implicates transcription factors, which are expressed by OPCs and/or their progenitors. Ascl1/Mash1 is a proneural transcription factor previously implicated in embryonic oligodendrogenesis and operating in genetic interaction with Olig2, an essential transcriptional regulator in oligodendrocyte development. Herein, we have investigated the contribution of Ascl1 to oligodendrocyte development and remyelination in the postnatal cortex. During the neonatal period, Ascl1 expression was detected in progenitors of the cortical subventricular zone and in cortical OPCs. Different genetic approaches to delete Ascl1 in cortical progenitors or OPCs reduced neonatal oligodendrogenesis, showing that Ascl1 positively regulated both OPC specification from subventricular zone progenitors as well as the balance between OPC differentiation and proliferation. Examination of remyelination processes, both in the mouse model for focal demyelination of the corpus callosum and in multiple sclerosis lesions in humans, indicated that Ascl1 activity was upregulated along with increased oligodendrogenesis observed in remyelinating lesions. Additional genetic evidence indicated that remyelinating oligodendrocytes derived from Ascl1(+) progenitors/OPCs and that Ascl1 was required for proper remyelination. Together, our results show that Ascl1 function modulates multiple steps of OPC development in the postnatal brain and in response to demyelinating insults