X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism investigation of size effects on field-induced N\'eel-cap reversal

X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism is used to investigate the influence of an applied magnetic field on N\'eel caps (i.e., surface terminations of asymmetric Bloch walls). Self-assembled micron-sized Fe(110) dots displaying a moderate distribution of size and aspect ratios serve as model objects. Investigations of remanent states after application of an applied field along the direction of N\'eel-cap magnetization give clear evidence for the magnetization reversal of the N\'eel caps around 120 mT, with a $\pm$20 mT dispersion. No clear correlation could be found between the value of the reversal field and geometrical features of the dots

Isolement des bactéries lactiques à partir des produits laitiers traditionnels Marocains et formulation d’un lait fermenté proche du Kéfir

Les bactéries lactiques par leur biodiversité sont largement utilisées dans l’industrie agroalimentaire. L’objectif de cette étude est donc l’isolement des bactéries lactiques à partir de lben et jben marocains, l’étude de la vitesse d’acidification des isolats en culture pure et en culture mixte afin de trouver la meilleure combinaison à caractère acidifiant très rapide et très élevé et son utilisation pour formuler une boisson lactée fermentée proche du Kéfir. Les résultats montrent que les isolats obtenus appartiennent aux genres Lactobacillus, Lactococcus et Leuconostoc, que les Lactobacillus et les Lactococcus isolés en culture pure provoquent tous une diminution du pH du lait de 6,5 à 4,2 au bout de 24 heures de fermentation et que la souche Lb4 du genre Lactobacillus possède une forte propriété acidifiante. Cette souche a été utilisée en combinaison avec les Leuconostoc et les Lactococcus (CM1, CM2, CM3) et a donné une vitesse d’acidification plus rapide et un pH plus bas. Une boisson lactée fermentée proche du Kéfir qui est une boisson très reconnue par ses effets sanitaires et thérapeutiques est alors produite grâce à cette dernière combinaison et à l’ajout des levures (saccharomycess cerevisiae) et des bactéries acétiques (Acetobacter). Le résultat de ce travail a noté une bonne appréciation du nouveau produit par le jury de dégustation et une forte ressemblance au Kéfir qui est un produit d’origine Russe.Mots-clés: bactéries lactiques, Lactobacillus, Leuconostoc, Lactococcus, Lben, Jben, Kéfir. Isolation of lactic acid bacteria from traditional Moroccan dairy products and formulation of a loved fermented milk KefirLactic acid bacteria for their biodiversity and their therapeutic properties are widely used in the food industry. the interest of this study is the isolation of lactic acid bacteria from Lben and Jben Moroccan studying the acidification rate of isolates in pure culture and in mixed culture in order to find the best combination for fast acidifying character and very expensive and its use to make a fermented milk close Kefir . The results show that the isolates belong to the genera Lactobacillus, Lactococcus and Leuconostoc, Lactobacillus and the isolated pure culture Lactococcus all cause a decrease in pH of the milk to 6.5 to 4.2 after 24 hours fermentation and the LB4 strain of Lactobacillus has a strong acidifying property. This strain was used in combination with Lactococcus and Leuconostoc (CM1, CM2, CM3) gave a faster speed of acidification and a low pH. A fermented milk drink close Kefir is a highly recognized by its health and therapeutic effects beverage is then produced by the latter combination and the addition of yeast (saccharomycess cerevisiae) and acetic acid bacteria (Acetobacter). The result of this work has been a good appreciation of the new product by the tasting panel and a strong resemblance to Kefir is a Russian original product.Keywords: lactic acid bacteria, Lactobacillus, Leuconostoc, Lactococcus, Lben, Jben, Kefir

360 degree domain wall generation in the soft layer of magnetic tunnel junctions

High spatial resolution X-ray photo-emission electron microscopy technique has been used to study the influence of the dipolar coupling taking place between the NiFe and the Co ferromagnetic electrodes of micron sized, elliptical shaped magnetic tunnel junctions. The chemical selectivity of this technique allows to observe independently the magnetic domain structure in each ferromagnetic electrode. The combination of this powerful imaging technique with micromagnetic simulations allows to evidence that a 360 degree domain wall can be stabilized in the NiFe soft layer. In this letter, we discuss the origin and the formation conditions of those 360 degree domain walls evidenced experimentally and numerically

Indirect localization of a magnetic domain wall mediated by quasi walls

International audienceThe manipulation of magnetic domain walls in thin films and nanostructures opens new opportunities for fundamental and applied research. But controlling reliably the position of a moving domain wall still remains challenging. So far, most of the studies aimed at understanding the physics of pinning and depinning processes in the magnetic layer in which the wall moves (active layer). In these studies, the role of other magnetic layers in the stack has been often ignored. Here, we report an indirect localization process of 180° domain walls that occurs in magnetic tunnel junctions, commonly used in spintronics. Combining Scanning Transmission X-Ray Microscopy and micromagnetic simulations, magnetic configurations in both layers are resolved. When nucleating a 180° domain wall in the active layer, a quasi wall is created in the reference layer, atop the wall. The wall and its quasi wall must then be moved or positioned together, as a unique object. As a mutual effect, a localized change of the magnetic properties in the reference layer induces a localized quasi wall in the active layer. The two types of quasi walls are shown to be responsible for an indirect localization process of the 180° domain wall in the active layer

Artificial Kagome Arrays of Nanomagnets: A Frozen Dipolar Spin Ice

Magnetic frustration effects in artificial kagome arrays of nanomagnets are investigated using x-ray photoemission electron microscopy and Monte Carlo simulations. Spin configurations of demagnetized networks reveal unambiguous signatures of long range, dipolar interaction between the nanomagnets. As soon as the system enters the spin ice manifold, the kagome dipolar spin ice model captures the observed physics, while the short range kagome spin ice model fails.Comment: 4 pages, 4 figures, 1 tabl

Controlled switching of N\'eel caps in flux-closure magnetic dots

While magnetic hysteresis usually considers magnetic domains, the switching of the core of magnetic vortices has recently become an active topic. We considered Bloch domain walls, which are known to display at the surface of thin films flux-closure features called N\'eel caps. We demonstrated the controlled switching of these caps under a magnetic field, occurring via the propagation of a surface vortex. For this we considered flux-closure states in elongated micron-sized dots, so that only the central domain wall can be addressed, while domains remain unaffected.Comment: 4 pages, 3 figure

Magnetostructure of MnAs on GaAs revisited

The ferromagnetic to nonferromagnetic (α-β) phase transition in epitaxial MnAs layers on GaAs(100) is studied by x-ray magnetic circular dichroism and x-ray magnetic linear dichroism photoemission electron microscopy in order to elucidate the nature of the controversial nonferromagnetic state of β-MnAs. In the coexistence region of the two phases the β phase shows a clear XMLD signal characteristic of antiferromagnetism. The nature and the possible causes of the elusiveness of this magnetic state are discussed

Dynamical evolution of Ge quantum dots on Si(111): from island formation to high temperature decay

Heteroepitaxial growth is a process of profound fundamental importance as well as an avenue to realize nanostructures such as Ge/Si quantum dots (QDs), with appealing properties for applications in opto- and nanoelectronics. However, controlling the Ge/Si QD size, shape, and composition remains a major obstacle to their practical implementation. Here, Ge nanostructures on Si(111) were investigated in situ and in real-time by low energy electron microscopy (LEEM), enabling the observation of the transition from wetting layer formation to 3D island growth and decay. The island size, shape, and distribution depend strongly on the growth temperature. As the deposition temperature increases, the islands become larger and sparser, consistent with Brownian nucleation and capture dynamics. At 550 degrees C, two distinct Ge/Si nanostructures are formed with bright and dark appearances that correspond to flat, atoll-like and tall, faceted islands, respectively. During annealing, the faceted islands increase in size at the expense of the flat ones, indicating that the faceted islands are thermodynamically more stable. In contrast, triangular islands with uniform morphology are obtained from deposition at 600 degrees C, suggesting that the growth more closely follows the ideal shape. During annealing, the islands formed at 600 degrees C initially show no change in morphology and size and then rupture simultaneously, signaling a homogeneous chemical potential of the islands. These observations reveal the role of dynamics and energetics in the evolution of Ge/Si QDs, which can serve as a step towards the precise control over the Ge nanostructure size, shape, composition, and distribution on Si(111)