Towards atomically resolved magnetic measurements in the transmission electron microscope : A study of structure and magnetic moments in thin films
Sammanfattning: The magnetic properties of thin metallic films are significantly different from the bulk properties due to the presence of interfaces. The properties shown by such thin films are influenced by the atomic level structure of the films and the interfaces. Transmission electron microscope (TEM) has the potential to analyse the structure and the magnetic properties of such systems with atomic resolution. In this work, the TEM is employed to characterize the structure of the Fe/V and Fe/Ni multilayers and the technique of electron magnetic circular dichroism (EMCD) is developed to obtain the quantitative magnetic measurements with high spatial resolution.From TEM analysis of short period Fe/V multilayers, a coherent superlattice structure is found. In short period Fe/Ni multilayer samples with different repeat frequency, only the TEM technique could verify the existence of the multilayer structure in the thinnest layers. The methods of scanning TEM imaging and electron energy loss spectroscopy (EELS) results were used and refined to determine interdiffusion at the interfaces. The confirmation of the multilayer structure helped to explain the saturation magnetization of these samples.Electron magnetic circular dichroism (EMCD) has the potential to quantitatively measure the magnetic moments of the materials with atomic resolution, but the technique presents several challenges. First, the EMCD measurements need to acquire two EELS spectra at two different scattering angles. These spectra are mostly acquired one after the other which makes it difficult to guaranty the identical experimental conditions and the spatial registration between the two acquisitions. We have developed a technique to simultaneously acquire the two angle-resolved EELS spectra in a single acquisition. This not only ensures the accuracy of the measurements but also improves the signal to noise ratio as compared to the previously used methods. The second important question is the effect of crystal orientations on the measured EMCD signals, considering the fact that the crystal orientation of a real crystal does not remain the same in the measured area. We developed the methodology to simultaneously acquire the EMCD signals and the local crystal orientations with high precision and experimentally showed that the crystal tilt significantly changes the magnetic signal. The third challenge is to obtain EMCD measurements with atomic resolution which is hampered by the need of high beam convergence angles. We further developed the simultaneous acquisition technique to obtain the quantitative EMCD measurements with beam convergence angles corresponding to atomic size electron probes.
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