| 1. |
- 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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| 2. |
- 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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| 3. |
- Čiučiulkaitė, Agnė, MSc, 1991-
(författare)
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Dynamics in magnetic metamaterials
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Metamaterials are artificially created structures with properties that are not found in nature. They can be tailored to achieve desired response to external excitations such as external electric and magnetic fields, as well as to enhance materials’ optical or magnetic activity. Magnetic metamaterials comprised of arrays of sub-micrometer sized magnetic elements, can be used for a range of different applications, such as magnonic crystals and building blocks for magnetic memory elements.In this work, pathways for tuning magnetization dynamics are explored. Different magnetic metamaterials containing arrays of sub-micrometer sized elements, refered to as nanomagnets, were used as model systems for these explorations. The nano-magnets comrising these arrays are of two kinds: XY-rotors, with a magnetization direction rotating freely in-plane of a disk, and Ising-like spins, pointing along either of the two allowed magnetization directions. The Ising-like spins can be realized in either in-plane or out-of-plane magnetized materials.Collective magnetization dynamics were investigated in square arrays of coupled nanomagnets. Studies revealed that nanomagnet’s magnetization state cannot always be approximated by a ridig mesospin approximation. Instead, it was demonstrated that upon an external perturbation, such as an external magnetic or thermal field, internal magnetization experience texture excitations. The observed texture excitations have implications on the nanomagnet coupling in an array.Arrays, where collective phenomena emerge via excited plasmon resonances, were used for investigations of light-induced dynamics processes. Studies revealed importance of an array design for the observation of magneto-optical activity enhancement and more eÿcient ultrafast magnetization dynamics. It was reported that in arrays containing truncated Au nanocones with TbCo tip, enhancement of TbCo demagnetization can be achieved via resonant light illumination.Utilizing the array concept and magnetic anisotropy of a material, it is possible to create metamaterials, where a range of magnetization dynamics regimes can be investigated. Inter-element spacing defines on which lengthscale the nanomagnets will be coupled and which effects can be utilized for tuning system’s magneto-optical response and excited magnetization dynamics.
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| 4. |
- Gemma, Ryota, et al.
(författare)
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The impact of nanoscale compositional variation on the properties of amorphous alloys
- 2020
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Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 10:1
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Tidskriftsartikel (refereegranskat)abstract
- The atomic distribution in amorphous FeZr alloys is found to be close to random, nevertheless, the composition can not be viewed as being homogenous at the nm-scale. The spatial variation of the local composition is identified as the root of the unusual magnetic properties in amorphous Fe1-xZrx alloys. The findings are discussed and generalised with respect to the physical properties of amorphous and crystalline materials.
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| 5. |
- George, Sebastian, 1990-
(författare)
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Amorphous Magnetic Materials : A Versatile Foundation for Tomorrow’s Applications
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Amorphous magnetic materials exhibit a number of key differentiating properties with respect to crystalline magnets. In some cases, the differences may simply be in the values of macroscopic properties such as saturation magnetization, coercivity, Curie temperature, and electrical conductivity. Other cases are more fundamental, such as the possibility for many amorphous alloys to be produced with nearly arbitrary composition, something that is not always possible in crystal structures that may only be stable for certain specific compositions.Fundamentally, these properties arise due to the disordered arrangement of atoms in amorphous materials. However, this structure is challenging to probe and characterize, either experimentally or theoretically. A significant contribution of this thesis is the development of a new approach for studying the local atomic structure of amorphous materials, specifically amorphous SmCo and FeZr alloys. The strategy combines extended x-ray absorption spectroscopy (EXAFS) measurements with stochastic quenching (SQ) simulations in a way that provides more information than either method can offer alone. Additionally, this approach offers the potential for identifying any shortcomings in the theoretical models obtained via SQ.Having an accurate model of the atomic arrangement is not, however, a prerequisite for developing technical applications of amorphous magnetic materials. For that, it is sufficient to quantify those macroscopic properties that are relevant for a given application. Such is the value of the magnetic characterization of amorphous TbCo and CoFeZr alloy thin films presented here. Both investigations used methods such as vibrating sample magnetometry (VSM) and magneto-optic Kerr effect (MOKE) measurements to highlight the high tunability of the magnetic properties in these materials, which can be achieved simply by changing the chemical composition.The final portion of this thesis examines what can be achieved by combining amorphous SmCo and TbCo alloys together in bilayer structures. This is a step away from the alloy characterization studies, as it focuses on how new properties can be realized when multiple materials are brought together. MOKE measurements were used to identify the conditions under which the bilayers spontaneously become magnetized parallel to the film plane versus when the TbCo magnetization begins to tilt out of the plane. Further investigation combining x-ray circular magnetic dichroism (XMCD) measurements and micromagnetic simulations provided a depth-resolved model of the magnetization throughout the bilayers in the presence of a broad range of external field strengths and directions. These models also showed that the local magnetization just above and just below the SmCo/TbCo interface can be aligned either parallel or antiparallel to one another simply by varying the TbCo composition. This discovery offers a novel method for controlling the magnetic behavior in these materials, and may well be useful for all-optical switching or spintronics applications where amorphous TbCo alloys have already drawn attention.
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| 6. |
- Huang, Wen, et al.
(författare)
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Combined Light and Electron Scattering for Exploring Proximity Effects on Hydrogen Absorption in Vanadium
- 2021
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Ingår i: Energies. - : MDPI. - 1996-1073. ; 14:24
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Tidskriftsartikel (refereegranskat)abstract
- We investigate proximity effects on hydrogen absorption in ultra-thin vanadium layers through combing light transmission and electron scattering. We compare the thermodynamic properties of the vanadium layers, which are based on the superlattice structure of Cr/V (001) and Fe/V (001). We find an influence of the proximity effects on the finite-size scaling of the critical temperatures, which can be explained by a variation of dead layers in the vanadium. In addition to this, the proximity effects on hydrogen absorption are also verified from the changes of excess resistivity.
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| 7. |
- Huang, Wen, et al.
(författare)
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Size effect on excess resistivity induced by hydrogen in ultra-thin vanadium systems
- 2020
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 22:20, s. 11609-11613
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Tidskriftsartikel (refereegranskat)abstract
- Through combining a four-point probe and an optical technique, a profound vanadium(v) size effect on the change in excess resistivity during hydrogenation is observed in Fe-n/V-7n (n = 2, 4) superlattices at c >= 0.05 H/V. This phenomenon highly emphasizes the effect of interface and H-H interaction on the electrical properties of hydrogen in these systems.
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| 8. |
- Kaplan, Maciej, et al.
(författare)
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Phase formation and thermal stability of amorphous ZrNbCrMo thin films
- 2021
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Ingår i: Journal of Non-Crystalline Solids. - : Elsevier BV. - 0022-3093 .- 1873-4812 .- 2590-1591. ; 9-10, s. 100061-100061
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate that the atomic size mismatch, δ, can be used to predict the boundary between amorphous and crystalline phase formation ranges in ZrNbCrMo thin films. The concentrations of all alloying elements are varied systematically, which induces a change in δ. The formation of an amorphous phase is observed by x-ray diffraction for δ ≥ 6.9%, whereas crystallization is detected for δ ≤ 6.9%. The crystallization temperature increases from 575 to 625 ∘C by increasing Nb + Mo content from 10 to 34 at.%, and decreases to 475 ∘C upon further additions of Nb + Mo. Calphad calculated liquidus temperatures, Tl, increases monotonously and δ decreases monotonously with Nb and Mo additions. Consequently, δ and Tl do not serve as predictors for thermal stability of ZrNbCrMo thin films.
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| 9. |
- Magnus, Fridrik, et al.
(författare)
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Tuneable exchange-spring stiffness in amorphous magnetic trilayer structures
- 2021
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 33:44
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the magnetic properties of amorphous Sm10Co90/Co-60(Al70Zr30)(40)/Co-85(Al70Zr30)(15) exchange-spring magnet trilayers. The magnetically soft Co-85(Al70Zr30)(15) layer is coupled to the magnetically hard Sm10Co90 layer through the weakly magnetic low-T (c) Co-60(Al70Zr30)(40) spacer layer. The strength of the coupling can be controlled with temperature and the coupling persists above the intrinsic T (c) of the spacer layer due to a long-range magnetic proximity effect. Polarized neutron reflectivity is used to examine the magnetic profile of the trilayers during magnetization reversal. A two-step switching occurs, with the switching angle of the soft layer strongly dependent on the strength of the coupling. In the strong coupling regime a magnetic state can be achieved where the soft layer magnetization is perpendicular to the hard layer whereas in the weak coupling regime the soft layer reverses fully.
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| 10. |
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