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Towards atomically resolved magnetic measurements in the transmission electron microscope: A study of structure and magnetic moments in thin films
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (ELMiN)ORCID iD: 0000-0002-8262-5893
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2020. , p. 83
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1886
Keywords [en]
Transmission electron microscope, thin films, magnetic moments, electron energy loss spectroscopy, simultaneous acquisition, electron magnetic circular dichroism
National Category
Nano Technology Condensed Matter Physics
Research subject
Engineering Science with specialization in Materials Analysis
Identifiers
URN: urn:nbn:se:uu:diva-398561ISBN: 978-91-513-0830-2 (print)OAI: oai:DiVA.org:uu-398561DiVA, id: diva2:1376057
Public defence
2020-02-07, Room 80101, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2020-01-15 Created: 2019-12-06 Last updated: 2020-03-05
List of papers
1. Shrinking of silicon nanocrystals embedded in an amorphous silicon oxide matrix during rapid thermal annealing in a forming gas atmosphere
Open this publication in new window or tab >>Shrinking of silicon nanocrystals embedded in an amorphous silicon oxide matrix during rapid thermal annealing in a forming gas atmosphere
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2016 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 27, no 36, article id 365601Article in journal (Refereed) Published
Abstract [en]

We report the effect of hydrogen on the crystallization process of silicon nanocrystals embedded in a silicon oxide matrix. We show that hydrogen gas during annealing leads to a lower sub-band gap absorption, indicating passivation of defects created during annealing. Samples annealed in pure nitrogen show expected trends according to crystallization theory. Samples annealed in forming gas, however, deviate from this trend. Their crystallinity decreases for increased annealing time. Furthermore, we observe a decrease in the mean nanocrystal size and the size distribution broadens, indicating that hydrogen causes a size reduction of the silicon nanocrystals.

Keywords
silicon nanocrystals, rapid thermal annealing, oxidation, forming gas, hydrogen, defects, passivation
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-306271 (URN)10.1088/0957-4484/27/36/365601 (DOI)000384064400019 ()27478921 (PubMedID)
Available from: 2016-10-26 Created: 2016-10-26 Last updated: 2019-12-06Bibliographically approved
2. Crystal perfection by strain engineering: The case of Fe/V (001)
Open this publication in new window or tab >>Crystal perfection by strain engineering: The case of Fe/V (001)
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2017 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 636, p. 608-614Article in journal (Refereed) Published
Abstract [en]

We study the effect of bilayer thickness at fixed volume fraction on the structural quality of Fe/V (001)superlattices. We find that such artificial metallic superlattices can be manufactured with excellent crystalquality and layering up to at least 50 Å in repeat distance (K = LFe +LV). For an intended fixed ratio of theconstituents: LFe/LV= 1/7, out-of-plane coherence lengths comparable to the thicknesses of the sampleswere obtained. We evaluate the strain in- and out-of-plane of both layers as a function of the bilayer thicknessand comment on the growth using the framework of linear elasticity theory. We interpret the stabilityof the superlattice against crystal degradation due to the alternating compressive and tensile strain, yieldingclose to ideal lattice matching to the substrate.

Keywords
Superlattice; Iron/Vanadium; Sputtering; Epitaxy; Reciprocal space mapping; X-ray diffraction; X-ray reflectivity; Linear elasticity
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:uu:diva-332050 (URN)10.1016/j.tsf.2017.07.005 (DOI)000408037800086 ()
Available from: 2017-10-23 Created: 2017-10-23 Last updated: 2019-12-06Bibliographically approved
3. Composition, structure and magnetic properties of ultra-thin Fe/Ni multilayers sputter deposited on epitaxial Cu/Si(001)
Open this publication in new window or tab >>Composition, structure and magnetic properties of ultra-thin Fe/Ni multilayers sputter deposited on epitaxial Cu/Si(001)
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2018 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 646, p. 117-125Article in journal (Refereed) Published
Abstract [en]

Sputter deposited symmetric multilayers of (n Fe)/(n Ni), with individual thicknesses from n = 4 to n = 48 monolayers (ML), were deposited on epitaxial Cu/Si(001), and their microstructural evolution and magnetic properties versus n have been studied. Elemental layering can be seen with transmission electron microscopy down to n = 4 ML layer thickness, although an intermixed region characterized by a finite interface width is found to be present. This width is composed of the interface roughness as well as the interdiffusion between layers, but the relative contributions from these two sources could not be concluded by the techniques used. The measured elemental layering and X-ray reflectivity (XRR) give an upper limit to the interface width which must be smaller than the thinnest layers, 4 ML. Electron energy loss spectroscopy (EELS), depth profiling X-ray photoelectron spectroscopy (XPS) and also XRR reveal that Fe has a higher tendency to mix with Ni than vice versa. XPS does not have the resolution to measure this thin elemental layering: composition variations for n = 8 ML which are clearly seen by EELS are barely resolved by XPS. The structure was determined by X-ray diffraction, and an epitaxial fcc (001) structure is found to be maintained throughout the multilayers up to n less than or similar to 8 ML. For larger n values, relaxation starts by Fe-fcc(001) layers changing into Fe-bcc(110), which is then followed by Ni-fcc(001) changing from (001) to (111) orientation along the growth direction. A decreased total measured magnetic moment for the fully epitaxial multilayers can be explained by the fcc Fe layers being partly anti-ferromagnetic, whereas the relaxed multilayers exhibit the expected magnetic properties of (bcc Fe) +(fcc Ni).

National Category
Condensed Matter Physics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-305519 (URN)10.1016/j.tsf.2017.11.023 (DOI)000418575900017 ()
Funder
Swedish Research Council
Available from: 2016-10-22 Created: 2016-10-18 Last updated: 2019-12-06Bibliographically approved
4. Quantitative EMCD by use of a double aperture for simultaneous acquisition of EELS
Open this publication in new window or tab >>Quantitative EMCD by use of a double aperture for simultaneous acquisition of EELS
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2019 (English)In: Ultramicroscopy, ISSN 0304-3991, E-ISSN 1879-2723, Vol. 196, p. 192-196Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Other Materials Engineering
Research subject
Materials Science
Identifiers
urn:nbn:se:uu:diva-364715 (URN)10.1016/j.ultramic.2018.10.012 (DOI)000451180800026 ()30439606 (PubMedID)
Funder
Swedish Research Council, C0367901Swedish Research Council, 2016-05259Knut and Alice Wallenberg Foundation
Available from: 2018-10-31 Created: 2018-10-31 Last updated: 2019-12-06Bibliographically approved
5. Simultaneous mapping of EMCD signals and crystal orientations in a transmission electron microscope
Open this publication in new window or tab >>Simultaneous mapping of EMCD signals and crystal orientations in a transmission electron microscope
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(English)Manuscript (preprint) (Other academic)
National Category
Nano Technology
Identifiers
urn:nbn:se:uu:diva-398559 (URN)
Available from: 2019-12-06 Created: 2019-12-06 Last updated: 2019-12-06
6. Atomic resolution electron probe magnetic circular dichroism measurements enabled by patterned apertures
Open this publication in new window or tab >>Atomic resolution electron probe magnetic circular dichroism measurements enabled by patterned apertures
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(English)Manuscript (preprint) (Other academic)
National Category
Nano Technology
Identifiers
urn:nbn:se:uu:diva-398560 (URN)
Available from: 2019-12-06 Created: 2019-12-06 Last updated: 2019-12-06
7. An electron energy loss spectrometer based streak camera for time resolved TEM measurements
Open this publication in new window or tab >>An electron energy loss spectrometer based streak camera for time resolved TEM measurements
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2017 (English)In: Ultramicroscopy, ISSN 0304-3991, E-ISSN 1879-2723, Vol. 176, p. 5-10Article in journal (Refereed) Published
Abstract [en]

We propose an experimental setup based on a streak camera approach inside an energy filter to measure time resolved properties of materials in the transmission electron microscope (TEM). In order to put in place the streak camera, a beam sweeper was built inside an energy filter. After exciting the TEM sample, the beam is swept across the CCD camera of the filter. We describe different parts of the setup at the example of a magnetic measurement. This setup is capable to acquire time resolved diffraction patterns, electron energy loss spectra (EELS) and images with total streaking times in the range between 100 ns and 10 μs.

Keywords
Time resolved; TEM; Energy filter; Streak camera; Sweep
National Category
Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-329982 (URN)10.1016/j.ultramic.2016.11.026 (DOI)000403992200003 ()
Available from: 2017-09-25 Created: 2017-09-25 Last updated: 2019-12-06Bibliographically approved

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