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| 2. |
- Ali, Hasan, 1985-, et al.
(författare)
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Atomic resolution energy-loss magnetic chiral dichroism measurements enabled by patterned apertures
- 2020
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Ingår i: Physical Review Research. - College Park USA. - 2643-1564. ; 2:2
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Tidskriftsartikel (refereegranskat)abstract
- Electron energy-loss magnetic chiral dichroism (EMCD) has the potential to measure magnetic properties of the materials at atomic resolution but the complex distribution of magnetic signals in the zone axis and the overlapping diffraction discs at higher beam convergence angles make the EMCD signal acquisition challenging. Recently, the use of ventilator apertures to acquire the EMCD signals with atomic resolution was proposed. Here we give the experimental demonstration of several types of ventilator apertures and obtain a clear EMCD signal at beam semiconvergence angles of 5 mrad. To simplify the experimental procedures, we propose a modified ventilator aperture which not only simplifies the complex scattering conditions but reduces the influence of lens aberrations on the EMCD signal as compared to the originally proposed ventilator apertures. In addition, this modified aperture can be used to analyze magnetic crystals with various symmetries and we demonstrate this feature by acquiring EMCD signals on different zone axis orientations of an Fe crystal. With the same aperture we obtain EMCD signals with convergence angles corresponding to atomic resolution electron probes. After the theoretical demonstration of the EMCD signal on a zone axis orientation at high beam convergence angles, this work thus overcomes the experimental and methodological hurdles and enables atomic resolution EMCD on the zone axis by using apertures.
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| 3. |
- Ali, Hasan, 1985-, et al.
(författare)
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Quantitative EMCD by use of a double aperture for simultaneous acquisition of EELS
- 2019
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Ingår i: Ultramicroscopy. - : Elsevier BV. - 0304-3991 .- 1879-2723. ; 196, s. 192-196
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Tidskriftsartikel (refereegranskat)abstract
- The weak signal strength in electron magnetic circular dichroism (EMCD) measurements remains one of the main challenges in the quantification of EMCD related EELS spectra. As a consequence, small variations in peak intensity caused by changes of background intervals, choice of method for extraction of signal intensity and equally differences in sample quality can cause strong changes in the EMCD signal. When aiming for high resolution quantitative EMCD, an additional difficulty consists in the fact that the two angular resolved EELS spectra needed to obtain the EMCD signal are taken at two different instances and it cannot be guaranteed that the acquisition conditions for these two spectra are identical. Here, we present an experimental setup where we use a double hole aperture in the transmission electron microscope to obtain the EMCD signal in a single acquisition. This geometry allows for the parallel acquisition of the two electron energy loss spectra (EELS) under exactly the same conditions. We also compare the double aperture acquisition mode with the qE acquisition mode which has been previously used for parallel acquisition of EMCD. We show that the double aperture mode not only offers better signal to noise ratio as compared to qE mode but also allows for much higher acquisition times to significantly improve the signal quality which is crucial for quantitative analysis of the magnetic moments.
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| 4. |
- Ali, Hasan, 1985-, et al.
(författare)
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Simultaneous mapping of EMCD signals and crystal orientations in a transmission electron microscope
- 2021
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Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 11
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Tidskriftsartikel (refereegranskat)abstract
- When magnetic properties are analysed in a transmission electron microscope using the technique of electron magnetic circular dichroism (EMCD), one of the critical parameters is the sample orientation. Since small orientation changes can have a strong impact on the measurement of the EMCD signal and such measurements need two separate measurements of conjugate EELS spectra, it is experimentally non-trivial to measure the EMCD signal as a function of sample orientation. Here, we have developed a methodology to simultaneously map the quantitative EMCD signals and the local orientation of the crystal. We analyse, both experimentally and by simulations, how the measured magnetic signals evolve with a change in the crystal tilt. Based on this analysis, we establish an accurate relationship between the crystal orientations and the EMCD signals. Our results demonstrate that a small variation in crystal tilt can significantly alter the strength of the EMCD signal. From an optimisation of the crystal orientation, we obtain quantitative EMCD measurements.
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| 6. |
- Magnus, Fridrik, et al.
(författare)
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Sequential magnetic switching in Fe/MgO(001) superlattices
- 2018
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 97:17
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Tidskriftsartikel (refereegranskat)abstract
- Polarized neutron reflectometry is used to determine the sequence of magnetic switching in interlayer exchangecoupled Fe/MgO(001) superlattices in an applied magnetic field. For 19.6 Å thick MgO layers we obtain a 90◦periodic magnetic alignment between adjacent Fe layers at remanence. In an increasing applied field the toplayer switches first followed by its second-nearest neighbor. For 16.4 Å MgO layers, a 180◦periodic alignment isobtained at remanence and with increasing applied field the layer switching starts from the two outermost layersand proceeds inwards. This sequential tuneable switching opens up the possibility of designing three-dimensionalmagnetic structures with a predefined discrete switching sequence
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| 7. |
- Moubah, Reda, et al.
(författare)
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Discrete Layer-by-Layer Magnetic Switching in Fe/MgO(001) Superlattices
- 2016
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Ingår i: Physical Review Applied. - : American physical society. - 2331-7019. ; 5:4
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Tidskriftsartikel (refereegranskat)abstract
- We report on a discrete layer-by-layer magnetic switching in Fe/MgO superlattices driven by an antiferromagnetic interlayer exchange coupling. The strong interlayer coupling is mediated by tunneling through MgO layers with thicknesses up to at least 1.8 nm, and the coupling strength varies with MgO thickness. Furthermore, the competition between the interlayer coupling and magnetocrystalline anisotropy stabilizes both 90 degrees and 180 degrees periodic alignment of adjacent layers throughout the entire superlattice. The tunable layer-by-layer switching, coupled with the giant tunneling magnetoresistance of Fe/MgO/Fe junctions, is an appealing combination for three-dimensional spintronic memories and logic devices.
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| 8. |
- Skovdal, Björn Erik, et al.
(författare)
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Temperature-induced collapse of spin dimensionality in magnetic metamaterials
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:1
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Tidskriftsartikel (refereegranskat)abstract
- Spin and spatial dimensionalities are universal concepts, essential for describing both phase transitions and dynamics in magnetic materials. Lately, these ideas have been adopted to describe magnetic properties of metamaterials, replicating the properties of their atomic counterparts as well as exploring properties of ensembles of mesospins belonging to different universality classes. Here, we take the next step when investigating magnetic metamaterials not conforming to the conventional framework of continuous phase transitions. Instead of a continuous decrease in the moment with temperature, discrete steps are possible, resulting in a binary transition in the interactions of the elements. The transition is enabled by nucleation and annihilation of vortex cores, shifting topological charges between the interior and the edges of the elements. Consequently, the mesospins can be viewed as shifting their spin dimensionality, from 2 (XY-like) to 0 (vortices), at the transition. The results provide insight into how dynamics at different length scales couple, which can lead to thermally driven topological transitions in magnetic metamaterials.
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| 9. |
- Warnatz, Tobias
(författare)
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Magnetic Properties of Epitaxial Metal/Oxide Heterostructures
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work in this dissertation is devoted to tailoring and studying magnetic properties of epitaxial metal/oxide heterostructures. The aim is to understand the fundamental principles governing these properties and how they affect each other. The acquired knowledge can prove useful for the development of future spintronic devices. A variety of experimental techniques is used to fabricate and characterize the epitaxial structures. For fabrication, a combination of direct-current and radio-frequency sputtering is used, whereas x-ray reflectivity and diffraction measurements are the main tools for the structural characterization of the heterostructures. The magnetic characterization of these structures is done by a combination of longitudinal magneto-optical Kerr-effect measurements, Kerr-microscopy and polarized neutron reflectometry. First, it is shown how strain affects the magnetic properties of metal/oxide heterostructures by comparing Fe/MgO and Fe/MgAl2O4 superlattices. Subsequently, an antiferromagnetic interlayer exchange coupling in Fe/MgO superlattices is revealed and attributed to a spin-polarized-tunneling mechanism. The coupling strength can be tuned by changing the MgO thickness leading to the stabilization of different remanent states as well as to different reversal mechanisms. It is shown that the interlayer exchange coupling in Fe/MgO superlattices is a consequence of two distinct components. These components can be interpreted as beyond-nearest-neighbor interactions and a contribution arising from the total thickness of the heterostructures.The interlayer exchange coupling is further investigated via temperature dependent magnetization measurements. It is shown that different remanent states and reversal mechanisms occur at different temperatures. Furthermore, a large increase in interlayer exchange coupling strength with reduced temperature is revealed. Finally, it is shown that Fe84Cu16/MgO superlattices exhibit a reduced magnetocrystalline anisotropy and interlayer exchange coupling strength, as compared to pure Fe/MgO superlattices. Patterning such Fe84Cu16/MgO superlattices in circular islands leads to an increased saturation field with decreasing island diameter.
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| 10. |
- Warnatz, Tobias, et al.
(författare)
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The impact of number of repeats N on the interlayer exchange in [Fe/MgO]N(001) superlattices
- 2021
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Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 11
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Tidskriftsartikel (refereegranskat)abstract
- The strength of the interlayer exchange coupling in [Fe/MgO]N(001) superlattices with 2 <= N <= 10 depends on the number of bilayer repeats (N). The exchange coupling is antiferromagnetic for all the investigated thicknesses while being nine times larger in a sample with N = 4 as compared to N = 2. The sequence of the magnetic switching in two of the samples (N = 4, N = 8) is determined using polarized neutron reflectometry. The outermost layers are shown to respond at the lowest fields, consistent with having the weakest interlayer exchange coupling. The results are consistent with the existence of quantum well states defined by the thickness of the Fe and the MgO layers as well as the number of repeats (N) in [Fe/MgO]N(001)superlattices.
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