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 conf
iguration 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.
The structure of domain walls delimiting magnetic bubbles in L10 FePd thin layers is described on the basis of Lorentz transmission electron microscopy (LTEM) and multiscale magnetic simulations. Images obtained by high resolution LTEM show the exist
ence of magnetization reversal areas inside domain walls, called vertical Bloch lines (VBLs). Combining these observations and multiscale simulations on various geometries, we can identify the structure of these VBLs, notably the presence or not of magnetic singularities.
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.
Data 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...
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.
Thin film alloys with perpendicular anisotropy were studied using Lorentz Transmission Electron Microscopy (LTEM). This work focuses on the configuration of domain walls and demonstrates the suitability and accuracy of LTEM for the magnetic character
isation of perpendicular magnetic anisotropy materials. Thin films of chemically ordered ($unit{L1_0}$) FePd alloys were investigated by micro-magnetic modelling and LTEM phase retrieval approach. The different components of magnetization described by the modeling were studied on experimental images and confirmed by LTEM contrast simulation. Furthermore, quantitative measurements of magnetic induction inside the domain walls were made by using an original method to separate the electrical and magnetical contributions to the phase information. Irregularities were also observed along the domain walls which could play a major role during the magnetization processes.
Off-axis electron holography was used to observe and quantify the magnetic microstructure of a perpendicular magnetic anisotropic (PMA) recording media. Thin foils of PMA materials exhibit an interesting up and down domain configuration. These domain
s are found to be very stable and were observed at the same time with their stray field, closing magnetic flux in the vacuum. The magnetic moment can thus be determined locally in a volume as small as few tens of cubic nanometers.
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 t
ransmission 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.
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 f
eatures called Neel 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.
