Search

We report on a wide variety of magnetic states in Co/Cu multilayered nanocylinders grown by electrodeposition with different thicknesses of both elements. The remnant magnetic states in individual Co layers have quantitatively been determined at the nanoscale by micromagnetic reconstruction of the magnetic phase shift image obtained by electron holography. We demonstrate that the magnetization in the Co layers can present either uniform or vortex states. Also, different magnetic configurations can be observed within the same nanocylinder. In the case of vortices, the direction of the core can rotate almost at 90° from the nanocylinder axis for layers with an aspect ratio close to 1. We show that the occurrence of each magnetic configuration depends on the aspect ratio of the layers, the direction of magnetocrystalline anisotropy, and, in some cases, the interlayer dipolar coupling. Such a wide variety of magnetic states is observed due to lower values of the Co magnetic constants (magnetization, exchange, anisotropy) with respect to bulk, typical of the electrodeposition process in a single bath, and to the local geometrical variation of the layers. We also calculated the phase diagram of the remnant magnetic states in a single layer for various amplitudes and orientations of the magnetocrystalline anisotropy and different directions of the saturation field. In particular cases, these phase diagrams, in addition to statistics of occurrence of each kind of magnetic configurations in the multilayer and the application of a saturation field in different directions, allow for recovering information on the preferential orientation of the crystalline anisotropy. [ABSTRACT FROM AUTHOR]

The signal-to-noise ratio of measurements by electron holography could be considerably improved if longer exposure times were possible: increasing the number of electrons contributing to the hologram improves the counting statistics. However, instrumental instabilities causing drift in the hologram fringes and specimen position make acquisition times of above a few seconds counterproductive. The current approach is to acquire image stacks of holograms, with short exposure times, followed by numerical realignment through sophisticated post-processing. The associated data storage and manipulation make in-situ and tomography experiments extremely cumbersome. Here, we implement dynamic automation of electron holography experiments to overcome these problems. The real-time drift measurement and feedback control of the instrument allow single holograms to be acquired with exposure times of 30 min or more. Indeed, there are no longer any limitations from instrumental instabilities and only those imposed by the specimen itself. Furthermore, automation allows the implementation of sophisticated phase reconstruction techniques based on precise control of the experimental conditions. Smart acquisition of electron holograms preludes future computer-controlled electron microscopy capabilities. [ABSTRACT FROM AUTHOR]

Most studies on MnAs material in its bulk form have been focused on its temperature-dependent structural phase transition accompanied by a magnetic one. Magnetostructural phase transition parameters in thin MnAs films grown on substrates present however some differences from the bulk behavior, and local studies become mandatory for a deeper understanding of the mechanisms involved within the transition. Up to now, only surface techniques have been carried out, while the transition is a three-dimensional phenomenon. We therefore developed an original nanometer scale methodology using electron holography to investigate the phase transition in an epitaxial MnAs thin film on GaAs(001) from the cross-section view. Using quantitative magnetic maps recorded at the nanometer scale as a function of the temperature, our work provides a direct in situ observation of the inhomogeneous spatial distribution of the transition in the layer depth and brings new insights on the fundamental transition mechanisms. [ABSTRACT FROM AUTHOR]

The distribution and movement of charge is fundamental to many physical phenomena, particularly for applications involving nanoparticles, nanostructures, and electronic devices. However, there are very few ways of quantifying charge at the necessary length scale. Here, we show that aberration-corrected electron holography is capable of counting the charge on individual nanoparticles to a precision of one elementary unit of charge. We present a method that measures charges within predefined contours by directly applying Gauss's law at the nanoscale. We perform a statistical analysis to reveal the relationship between the size of the contours and the precision of the charge measurement and present strategies to optimize the spatial and signal resolution for the presented method. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 1 h, 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. [ABSTRACT FROM AUTHOR]

• We report electron holography experiments performed with femtosecond electron pulses, using a coherent UTEM based on a laser triggered cold field emission source. • We have determined the optimum experimental conditions related to the long acquisition times imposed by the low emission/probe current available. • Holograms are acquired in vacuum and on a nano-object showing that useful physical properties can nevertheless be extracted from the hologram phase in pulsed condition. • We have implemented acquisition of short exposure time holograms assembled in a stack, combined with a computer-assisted fringes shift compensation which paves the way to ultrafast time-resolved electron holography using pump-probe setup. We report on electron holography experiments performed with femtosecond electron pulses in an ultrafast coherent Transmission Electron Microscope based on a laser-driven cold field emission gun. We first discuss the experimental requirements related to the long acquisition times imposed by the low emission/probe current available in these instruments. The experimental parameters are first optimized and electron holograms are then acquired in vacuum and on a nano-object showing that useful physical properties can nevertheless be extracted from the hologram phase in pulsed condition. Finally, we show that the acquisition of short exposure time holograms assembled in a stack, combined with a computer-assisted shift compensation of usual instabilities encountered in holography, such as beam and biprism wire instabilities, can yield electron holograms acquired with a much better contrast paving the way to ultrafast time-resolved electron holography. [ABSTRACT FROM AUTHOR]

Off-axis electron holography experiments have been combined with micromagnetic simulations to study the remnant magnetic states of electrodeposited Co/Cu multilayered nanocylinders. Structural and chemical data obtained by transmission electron microscopy have been introduced in the simulations. Three different magnetic configurations such as an antiparallel coupling of the Co layers, coupled vortices, and a monodomain-like state have been quantitatively mapped and simulated. While most of the wires present the same remnant state whatever the direction of the saturation field, we show that some layers can present a change from an antiparallel coupling to vortices. Such a configuration can be of particular interest to design nano-oscillators with two different working frequencies. [ABSTRACT FROM AUTHOR]

Here, we study the effect of dynamic scattering on the projected geometric phase and strain maps reconstructed using dark-field electron holography (DFEH) for non-uniformly strained crystals. The investigated structure consists of a {SiGe/Si} superlattice grown on a (001)-Si substrate. The three-dimensional strain field within the thin TEM lamella is modelled by the finite element method. The observed projected strain is simulated in two ways by multiplying the strain at each depth in the crystal by a weighting function determined from a recently developed analytical two-beam dynamical theory, and by simply taking the average value. We demonstrate that the experimental results need to be understood in terms of the dynamical theory and good agreement is found between the experimental and simulated results. Discrepancies do remain for certain cases and are likely to be from an imprecision in the actual two-beam diffraction conditions, notably the deviation parameter, and points to limitations in the 2-beam approximation. Finally, a route towards a 3D reconstruction of strain fields is proposed. [ABSTRACT FROM AUTHOR]

Abstract: A generalized procedure for the in situ application of magnetic fields by means of the excitation of the objective lens for magnetic imaging experiments in Lorentz microscopy and electron holography is quantitatively described. A protocol for applying magnetic fields with arbitrary in-plane magnitude and orientation is presented, and a freeware script for Digital Micrograph™ is provided to assist the operation of the microscope. Moreover, a method to accurately reconstruct hysteresis loops is detailed. We show that the out-of-plane component of the magnetic field cannot be always neglected when performing quantitative measurements of the local magnetization. Several examples are shown to demonstrate the accuracy and functionality of the methods. [Copyright &y& Elsevier]