L'EMCD, Energy Loss Magnetic Chiral Dichroism, est une technique récente, mise en œuvre dans le microscope électronique à transmission (TEM), qui utilise la spectroscopie de pertes d'énergie d'électrons (EELS). Elle a pour objectif la mesure du moment magnétique local d'un élément chimique donné. Son utilisation contribue à progresser dans la compréhension des phénomènes magnétiques à l'échelle nanométrique. Cette thèse propose d'élargir les domaines d'applications de l'EMCD. Nous avons exploité l'EMCD pour l'étude de films minces de MnAs épitaxiés sur un substrat de GaAs(001). Ce travail montre l'utilité de cette technique dans le cas de structures hexagonales présentant une anisotropie magnéto-cristalline élevée. Le rapport des moments orbital et de spin du Mn dans les films de MnAs ferromagnétique de structure hexagonale a été mesuré par EMCD et comparé à des calculs DFT, ceci le long des axes magnétiques facile, difficile et intermédiaire. Une rupture de l'ordre ferromagnétique a par ailleurs été observée et mesurée in situ dans le microscope grâce à un porte-objet chauffant, lors de la transition cristallographique de a-MnAs hexagonal à ß-MnAs quasi-hexagonal. La technique EMCD a également été mise en œuvre pour sonder le moment 4f de composés de terres rares à base de dysprosium. Il s'agissait d'étudier des super-réseaux DyFe2/YFe2. Les règles de somme ont été établies pour le seuil M4,5 du Dy. En outre, le couplage antiparallèle des moments Dy et Fe a été confirmé en comparant leurs signaux dichroïques et en prenant en compte la théorie dynamique de la diffraction. Ce travail de thèse illustre pour la première fois d'une part la faisabilité de la technique EMCD pour l'étude quantitative de l'anisotropie et des transitions magnétiques, et d'autre part son potentiel pour étudier les terres rares et leur moment 4f, ainsi que le couplage avec des éléments de transition. / EMCD (Energy-Loss Magnetic Chiral Dichoism) is an emerging technique based on energy-loss spectroscopy (EELS) in a transmission electron microscopy (TEM). It aims at measuring the element-specific local magnetic moment of solids at a nanometer scale, and hence improving our understanding of magnetic local magnetic phenomena. This thesis presents the exploring work on developing the EMCD technique and its applications. We have applied EMCD to epitaxial MnAs thin films grown on a GaAs(001) substrate, extending the application of this technique to hexagonal structure with high magnetocrystalline anisotropy. The 3d orbital-to-spin moment ratio of Mn in hexagonal ferromagnetic MnAs along easy, hard and intermediate magnetic axes has been respectively estimated and then compared to DFT calculations. Moreover, a breaking of the ferromagnetic order in MnAs thin film, together with the crystallographic transition from hexagonal a-MnAs to quasi-hexagonal ß-MnAs, has been locally studied in-situ by modifying the temperature of the crystal inside the electron microscope. EMCD has also been settled to probe 4f moment in rare earth compounds, by investigating Dy-M4,5 edges in DyFe2/YFe2 superlattices. We have derived sum rules which are specified for 4f moment and applied them to the obtained dichroic signal over Dy-M4,5 edges. In addition, antiparallel coupling of Dy and Fe moments has been confirmed by comparing their dichroic signals, taking into account the dynamic diffraction effect. The work in this thesis illustrates for the first time the feasibility of EMCD technique for quantitative study of magnetocrystalline anisotropy and magnetic transition, and also proves its potential as a tool to investigate 4f moment as well as moment coupling in magnetic materials.

<p>Two new methods for calculating the double differential scattering cross-section (DDSCS) in electron energy loss spectroscopy (EELS) have been developed, allowing for simulations of sample geometries which have been unavailable to earlier methods of calculation. The new methods concerns the calculations of the <em>thickness function</em> of the DDSCS. Earlier programs have used an analytic approximation of a sum over the lattice vectors of the sample that is valid for samples with parallel entrance and exit surfaces.The first of the new methods carries out the sum explicitly, first identifying the unit cells illuminated by the electron beam, which are the ones needed to be summed over. The second uses an approach with Fourier transforms, yielding a final expression containing the <em>shape amplitude</em>, the Fourier transform of the <em>shape function</em> defining the shape of the electron beam inside the sample. Approximating the shape with a polyhedron, one can quickly calculate the shape amplitude as sums over it’s faces and edges. The first method gives fast calculations for small samples or beams, when the number of illuminated unit cells is small. The second is more efficient in the case of large beams or samples, as the number of faces and edges of the polyhedron used in the calculation of the shape amplitude does not need to be increased much for large beams. A simulation of the DDSCS for magnetite has been performed, yielding diffraction patterns for the L₃ edge of the three Fe atoms in its basis.</p>

Two new methods for calculating the double differential scattering cross-section (DDSCS) in electron energy loss spectroscopy (EELS) have been developed, allowing for simulations of sample geometries which have been unavailable to earlier methods of calculation. The new methods concerns the calculations of the thickness function of the DDSCS. Earlier programs have used an analytic approximation of a sum over the lattice vectors of the sample that is valid for samples with parallel entrance and exit surfaces.The first of the new methods carries out the sum explicitly, first identifying the unit cells illuminated by the electron beam, which are the ones needed to be summed over. The second uses an approach with Fourier transforms, yielding a final expression containing the shape amplitude, the Fourier transform of the shape function defining the shape of the electron beam inside the sample. Approximating the shape with a polyhedron, one can quickly calculate the shape amplitude as sums over it’s faces and edges. The first method gives fast calculations for small samples or beams, when the number of illuminated unit cells is small. The second is more efficient in the case of large beams or samples, as the number of faces and edges of the polyhedron used in the calculation of the shape amplitude does not need to be increased much for large beams. A simulation of the DDSCS for magnetite has been performed, yielding diffraction patterns for the L3 edge of the three Fe atoms in its basis.

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Multislice HAADF - STEM image simulations of SrTiO 3 are performed at 300 K.The procedure of these simulations and the used techniques are briefly ex-plained and reasoned. The results are presented and discussed in a conciseway and in an attached paper a comparison to experimental images is made.The paper proofs that the electron optical setup developed in Dresden is indeed capable of producing atomic-sized EVBs, a precondition for measuring EMCD with atomic resolution.

Code written in the DigitalMicrograph® scripting language enables a new experimental design for acquiring the magnetic dichroism in EELS. Called the q-E STEM setup, it provides simultaneous acquisition of the dichroic pairs of spectra (eliminating major error sources) while preserving the real-space resolution of STEM. This gives the setup great potential for real-space maps of magnetic momenta which can be instrumental in furthering the understanding of e.g. interfacial magnetic effects. The report includes a thorough presentation of the created acquisition routine, a detailed outline of future work and a fast introduction to the DMscript language.

Structural characterization is essential for the understanding of the magnetic properties of thin films and multilayers. In this thesis, both crystalline and amorphous thin films and multilayers were analyzed utilizing transmission electron microscopy (TEM). High resolution TEM and electron diffraction studies emphasize on the growth of amorphous Fe91Zr9 and Co68Fe24Zr8 on both Al2O3 and Al70Zr30 in multilayer structures by magnetron sputtering. The properties of the growth surfaces were found to strongly influence the formation of nano-crystallites of the magnetic material at interfaces. Field induced uniaxial magnetic anisotropy was found to be possible to imprint into both fully amorphous and partially crystallized Co68Fe24Zr8 layers, yielding similar magnetic characteristics regardless of the structure. These findings are important for the understanding of both growth and magnetic properties of these amorphous thin films. As magnetic systems become smaller, new analysis techniques need to be developed. One such important step was the realization of electron energy-loss magnetic circular dichroism (EMCD) in the TEM, where information about the ratio of the orbital to spin magnetic moment (mL/mS) of a sample can be obtained. EMCD makes use of angular dependent inelastic scattering, which is characterized using electron energy-loss spectroscopy. The work of this thesis contributes to the development of EMCD by performing quantitative measurements of the mL/mS ratio. Especially, methods for obtaining energy filtered diffraction patterns in the TEM together with analysis tools of the data were developed. It was found that plural inelastic scattering events modify the determination of the mL/mS ratio, wherefore a procedure to compensate for it was derived. Additionally, utilizing special settings of the electron gun it was shown that EMCD measurements becomes feasible on the nanometer level through real space maps of the EMCD signal.

Dynamical effects are known to reduce the signal to noise ratio in EMCD measurements making them highly dependent on sample thickness. Precession of the electron beam has been shown to reduce these effects in ordinary crystallography. This work investigates precession of the electron beam as a method of reducing the dynamical effects in EMCD using simulations. Simulations are run on BCC Fe in two and three beam conditions. The results show significant effects on the EMCD signal. However, whether these improve the signal quality seems dependent on sample orientation and thickness range. The initial findings reported here are promising and motivate further research.