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Ján Rusz: First principles calculations of electron beam deflection by microscopic magnetic fields in crystals: magnetic differential phase contrast imaging at atomic scale
October 10, 2019 @ 14:00 - 15:00
Thursday, 10. 10. 2019, 14.00,
Lecture room F2 (1st floor), MFF UK, Ke Karlovu 5
Dept. of Physics and Astronomy, Uppsala University, Sweden
First principles calculations of electron beam deflection by microscopic magnetic fields in crystals: magnetic differential phase contrast imaging at atomic scale
Emergence of exotic magnetic behavior at nano-scale calls for magnetic characterization techniques capable to measure magnetism with sufficient spatial resolution. Scanning tunneling microscopy allows to measure magnetism with atomic spatial resolution, enabling fascinating insights into the atomic scale magnetism , however, this method is restricted to detection of properties of surface layers of atoms. Transmission electron microscope (TEM) appears to be a natural choice of an instrument to study magnetization inside materials, at atomic scale: it allows to focus illuminating electron probe to sub-atomic areas, while transmitting it through the sample. It is thus having the potential to obtain atomic-scale information from the bulk of the sample.
We will describe – from a theoretical perspective – a new technique with a potential to provide atomic-scale information about magnetism in samples. The technique is based on differential phase contrast imaging (DPC) at atomic resolution . Recent simulations utilizing the Pauli multislice equation , which includes the interaction of the electron beam with microscopic magnetic field inside the sample, show that the diffraction patterns (ronchigrams) carry information about the projected microscopic magnetic fields at atomic scale . Figure 1 shows an example result of simulations for ferromagnetic FePt crystal. The beam deflections due to microscopic magnetic fields are in the range between approximately 0.1% and 1% of the deflections due to microscopic electric fields, which is a comparatively weak signal. However, DPC measurements by their nature are very efficient per unit of electron dose, since a majority of elastically scattered signals are used to measure the beam deflection. Considering that EMCD signals  of strength about 1% have been detected  in the electron energy loss spectra, magnetic DPC at atomic resolution, which utilizes most of the elastically scattered electrons, could be a viable alternative for magnetic studies at atomic resolution.
Figure 1 Left: Projected magnetization within a unit cell of FePt crystal with magnetization along the x-axis, which is parallel to crystallographic c-axis – the easy axis of magnetization of FePt. Right: Magnetic DPC image of 2.7nm thick FePt crystal calculated by Pauli multislice method for convergence semi-angle of 30mrad and acceleration voltage of 1000kV.
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