The use of Lorentz microscopy for the determination of magnetic reversal mechanism of exchange-biased Co30Fe70/NiMn bilayer

Lorentz transmission electron microscopy (LTEM) combined with in-situ magnetizing experiments is a powerful tool for the investigation of the magnetization of the reversal process at the micron scale. We have implemented this tool on a conventional transmission electron microscope (TEM) to study the exchange anisotropy of a polycrystalline Co35Fe65/NiMn bilayer. Semi-quantitative maps of the magnetic induction were obtained at different field values by the differential phase contrast (DPC) technique adapted for a TEM (SIDPC). The hysteresis loop of the bilayer has been calculated from the relative intensity of magnetic maps. The curve shows the appearance of an exchange-bias field reveals with two distinct reversal modes of the magnetization: the first path corresponds to a reversal by wall propagation when the applied field is parallel to the anisotropy direction whereas the second is a reversal by coherent rotation of magnetic moments when the field is applied antiparallel to unidirectional anisotropy direction

Probing punctual magnetic singularities during magnetization process in FePd films

We report the use of Lorentz microscopy to observe the domain wall structure during the magnetization process in FePd thin foils. We have focused on the magnetic structure of domain walls of bubble-shaped magnetic domains near saturation. Regions are found along the domain walls where the magnetization abruptly reverses. Multiscale magnetic simulations shown that these regions are vertical Bloch lines (VBL) and the different bubble shapes observed are then related to the inner structure of the VBLs. We were thus able to probe the presence of magnetic singularities as small as Bloch points in the inner magnetization of the domain walls

Dimensionality cross-over in magnetism: from domain walls (2D) to vortices (1D)

Dimensionality cross-over is a classical topic in physics. Surprisingly it has not been searched in micromagnetism, which deals with objects such as domain walls (2D) and vortices (1D). We predict by simulation a second-order transition between these two objects, with the wall length as the Landau parameter. This was conrmed experimentally based on micron-sized ux-closure dots

Micromagnetic study of flux-closure states in Fe dots using quantitative Lorentz Microscopy

A micromagnetic study of epitaxial micron-sized iron dots is reported through the analysis of Fresnel contrast in Lorentz Microscopy. Their use is reviewed and developed through analysis of various magnetic structures in such dots. Simple Landau configuration is used to investigate various aspects of asymmetric Bloch domain walls. The experimental width of such a complex wall is first derived and its value is discussed with the help of micromagnetic simulations. Combination of these two approaches enables us to define what is really extracted when estimating asymmetric wall width in Lorentz Microscopy. Moreover, quantitative data on the magnetization inside the dot is retrieved using phase retrieval as well as new informations on the degrees of freedom of such walls. Finally, it is shown how the existence and the propagation of a surface vortex can be characterized and monitored. This demonstrates the ability to reach a magnetic sensitivity a priori hidden in Fresnel contrast, based on an original image treatment and backed-up by the evaluation of contrasts obtained from micromagnetic simulations

Asymmetric hysteresis of N\'eel caps in flux-closure magnetic dots

We investigated with XMCD-PEEM magnetic imaging the magnetization reversal processes of N\'eel caps inside Bloch walls in self-assembled, micron-sized Fe(110) dots with flux-closure magnetic state. In most cases the magnetic-dependent processes are symmetric in field, as expected. However, some dots show pronounced asymmetric behaviors. Micromagnetic simulations suggest that the geometrical features (and their asymmetry) of the dots strongly affect the switching mechanism of the N\'eel caps.Comment: Proceeding for MMM-Intermag 2010 (Washington

Electron Microscopy Investigation of Magnetization Process in Thin Foils and Nanostructures

International audienceThis paper presents an investigation of magnetization configuration evolution during insitu magnetic processes, in materials exhibiting planar and perpendicular magnetic anisotropy. Transmission electron microscopy (TEM) has been used to perform magnetic imaging. Fresnel contrast in Lorentz Transmission Electron Microscopy (LTEM), phase retrieval methods such as Transport of Intensity Equation (TIE) solving and electron holography have all been implemented. These techniques are sensitive to magnetic induction perpendicular to the electron beam, allowing the mapping of magnetic induction distribution with a spatial resolution better than 10nm and can be extended to allow dynamical studies during in-situ observation. Thin foils of FePd alloys with a strong perpendicular magnetic anisotropy (PMA) and self-assembled Fe dots have been examined. Both are studied during magnetization processes, exhibiting the capacities of in-situ magnetic imaging in a TEM

Probing domain walls in cylindrical magnetic nanowires with electron holography

3 pages, 2 figuresInternational audienceWe probe magnetic domain walls in cylindrical soft magnetic nanowires using electron holography. We detail the modelling of expected contrast for both transverse and Bloch point domain walls and provide comparison with experimental observations performed on NiCo nanowires, involving also both magnetic and electrostatic contribution to the electron holography map. This allows the fast determination of the domain wall type without the need for uneasy and time-consuming experimental removal of the electrostatic contribution. Finally, we describe and implement a new efficient algorithm for calculating the magnetic contrast

How many bits may fit in a single magnetic dot? XMCD-PEEM evidences the switching of Néel caps inside Bloch domain walls

Elettra Highlights 2008-2009Data storage relies on the handling of two states, called bits. The market of mass storage is currently still dominated by magnetic technology, hard disk drives for the broad public and tapes for massive archiving. In these devices each bit is stored in the form of the direction of magnetization of nanosized magnetic domains, i.e. areas of ferromagnetic materials displaying a uniform magnetization. While miniaturization is the conventional way to fuel the continuous increase of device density, disruptive solutions are also sought. To these pertain in recent years many fundamental studies no longer considering the magnetic domains themselves, but the manipulation of the domain walls (DWs) that separate such domains. Concepts of storage and logic based on the propagation of DWs along lithographically-patterned stripes have been patented, while many fundamental aspects of DW propagation deeply related to condensed matter physics are still hotly debated. If one now considers magnetic dots of submicrometer dimensions, the magnetization has a tendency to curl along the outer edges of the nanostructure to close its magnetic flux and thereby reduce its magnetostatic energy. Then both domains and DWs of well-defined geometries arise, whose combined manipulation has been proposed as a multilevel magnetic storage scheme..

De la microscopie de Lorentz in situ à la l’interférométrie électronique in operando

Ce travail présente mes travaux autour de la microscopie électronique en transmission et plus particulièrement de l’holographie électronique. L’apport des techniques de nanofabrication ont permis de faire progresser la microscopie in situ vers une quantification à l’échelle nanométrique avec de nouvelles sensibilités. L’ouverture des mesures à la fréquence excitatrice ou à la 3 e dimension durant les expériences offrent par exemple un gain exhaustif et quantitatif dans la caractérisation physique offerte par la microscopie in situ. Ces travaux ont permis le développement d’une nouvelle source électronique de forte brillance basée sur les nanostructures de carbone. Dans ce cadre, des avancées sur la caractérisation de nanotubes dopés, remplis ou décorés ont été obtenues via l’holographie à basse tension. L’apport dans le domaine du micromagnétisme des travaux présentés ici concerne l’analyse dynamique de systèmes 1D et l’analyse du déplacement, du piégeage et de la modification des parois de domaines magnétiques. Les contributions de ces travaux aux techniques de nanofabrication couvrent un spectre large allant du dépôt localisé à la nano-manipulation en passant par la lithographie ionique et l’ingénierie de systèmes transparents aux électrons. Enfin, de nombreux développements instrumentaux ont permis de développer une approche de la mesure physique en étroite corrélation avec la nanocaractérisation, tout en proposant une formation à large spectre d’étudiants au métier de la Recherche