Aurélien Masseboeuf

Development of TEM and SEM high brightness electron guns using cold-field emission from a carbon nanotip

A newly developed carbon cone nanotip (CCnT) has been used as field emission cathode both in low voltage SEM (30 kV) electron source and high voltage TEM (200 kV) electron source. The results clearly show, for both technologies, an unprecedented stability of the emission and the probe current with almost no decay during 1h, as well as a very small noise (rms less than 0.5%) compared to standard sources which use tungsten tips as emitting cathode. In addition, quantitative electric field mapping around the FE tip have been performed using in situ electron holography experiments during the emission of the new tip. These results show the advantage of the very high aspect ratio of the new CCnT which induces a strong enhancement of the electric field at the apex of the tip, leading to very small extraction voltage (some hundred of volts) for which the field emission will start. The combination of these experiments with emission current measurements has also allowed to extract an exit work function value of 4.8 eV.

Quantitative Observation of Magnetic Flux Distribution in New Magnetic Films for Future High Density Recording Media

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 domains 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().

Controlled Switching of Néel 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 Néel 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.

Dimensionality crossover in magnetism: from domain walls (2D) to vortices (1D).

Dimensionality crossover 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 confirmed experimentally based on micron-sized flux-closure dots.

Off-Axial Aberration Correction using a B-COR for Lorentz and HREM Modes

A dedicated Hitachi HF3300C microscope, “I2TEM” , has recently been installed in CEMES. This microscope has been specially designed to carry on electron interferometry and in - situ TEM experiments. I2TEM is a 300 kV cold FEG microscope fitted with a multibrism set - up, two stages capability, a GIF quantum ER, a 4k X 4k camera and a Cs - corrector “B - COR” from CEOS. The first sta ge location within the objective pole piece allows performing classical HREM experiments while the second location is in a field free region above the objective lens and below the third condenser lens and allows carrying TEM imaging or electron holography in Lorentz mode. Contrary to non - dedicated microscope, I2TEM allows, in Lorentz mode, using apertures in the focal plane of the objective lens (i.e. used as “ Lorentz lens ” ) to select diffracted beams. In addition, the B - COR can be adjusted to correct for t he objective lens aberrations when excited in HREM or in Lorentz modes at voltages of 60kV, 80kV, 200kV and 300kV. This unique multipolar optical system (also called “ Aplanator ” ) has been specially designed to correct not only for the Cs, the axial coma (B 2), the three - fold astigmatism (A2), but also to compensate for the radial and azimuthal off - axial coma [1]. These off - axis corrections are achieved thanks to two additional pair of short hexapoles located in between two image planes inside the corrector. These planes are located between three long hexapoles used to compensate the axial aberrations (Cs, B2, A2, . . . ) like in the standard C - COR (Fig. 1). As conventional Cs - correctors allow for correcting most of the important first and second order aberration s confined close to the optic axis in HREM images of few ten of nanometers wide, the Aplanator compensates for aberrations in much larger field of view images (the number of equally resolved point regarding the standard π/4 limit is indeed considerably hig her). It is therefore of huge interest for large field of view HREM images recorded with a 4k X 4k camera and for low magnification images or holograms obtained in Lorentz mode. We will present recent results showing the capacity of the B - COR to correct f or the axial and off - axial aberrations of the 11 mm pole piece gap of the I2TEM objective lens and achieve 80pm spatial resolution in HREM mode (Fig. 2a). Results will also be presented showing the capacity of the Aplanator to correct for the objective len s aberration when used in Lorentz mode where 0.5 nm spatial resolution has been achieved (Fig. 2b).

Laser-induced electron emission from a tungsten nanotip: identifying above threshold photoemission using energy-resolved laser power dependencies

We present an experiment studying the interaction of a strongly focused 25 fs laser pulse with a tungsten nanotip, investigating the different regimes of laser-induced electron emission. We study the dependence of the electron yield with respect to the static electric field applied to the tip. Photoelectron spectra are recorded using a retarding field spectrometer and peaks separated by the photon energy are observed with a 45% contrast. They are a clear signature of above threshold photoemission (ATP), and are confirmed by extensive spectrally resolved studies of the laser power dependence. Understanding these mechanisms opens the route to control experiment in the strong-field regime on nanoscale objects.

Lorentz microscopy mapping for domain wall structure study in L10 FePd thin films

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 characterization of perpendicular magnetic anisotropy materials. Thin films of chemically ordered (L1(0)) FePd alloys were investigated by micro-magnetic modeling 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.

Electron holography study of the local magnetic switching process in magnetic tunnel junctions

We present an electron holography experiment enabling the local and quantitative study of magnetic properties in magnetic tunnel junction. The junction was fully characterized during the switching process and each possible magnetic configuration was highlighted with magnetic induction maps. No magnetic coupling was found between the two layers. We plot a local hysteresis loop that was compared with magnetometry measurement at the macroscopic scale confirming the validity of the local method.

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.

In situ electron holography of the dynamic magnetic field emanating from a hard-disk drive writer

The proliferation of mobile devices in society accessing data via the “cloud” is imposing a dramatic increase in the amount of information to be stored on hard disk drives (HDD) used in servers. Forecasts are that areal densities will need to increase by as much as 35% compound per annum and by 2,020 cloud storage capacity will be around 7 zettabytes corresponding to areal densities of 2 Tb/in2. This requires increased performance from the magnetic pole of the electromagnetic writer in the read/write head in the HDD. Current state-of-art writing is undertaken by morphologically complex magnetic pole of sub 100 nm dimensions, in an environment of engineered magnetic shields and it needs to deliver strong directional magnetic field to areas on the recording media around 50 nm × 13 nm. This points to the need for a method to perform direct quantitative measurements of the magnetic field generated by the write pole at the nanometer scale. Here we report on the complete in situ quantitative mapping of the magnetic field generated by a functioning write pole in operation using electron holography. The results point the way towards a new nanoscale magnetic field source to further develop in situ transmission electron microscopy.