| 11. |
- Cedervall, J., et al.
(författare)
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Magnetic and mechanical effects of Mn substitutions in AlFe 2 B 2
- 2019
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853. ; 482, s. 54-60
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Tidskriftsartikel (refereegranskat)abstract
- The mechanical and magnetic properties of the newly discovered MAB-phase class of materials based upon AlFe 2 B 2 were investigated. The samples were synthesised from stoichiometric amounts of all constituent elements. X-ray diffraction shows that the main phase is orthorhombic with an elongated b-axis, similar to AlFe 2 B 2 . The low hardness and visual inspection of the samples after deformation indicate that these compounds are deformed via a delamination process. When substituting iron in AlFe 2 B 2 with manganese, the magnetism in the system goes from being ferro- to antiferromagnetic via a disordered ferrimagnetic phase exhibited by AlFeMnB 2 . Density functional theory calculations indicate a weakening of the magnetic interactions among the transitions metal ions as iron is substituted by manganese in AlFe 2 B 2 . The Mn-Mn exchange interactions in AlMn 2 B 2 are found to be very small.
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| 12. |
- Cedervall, Johan, et al.
(författare)
-
Magnetic and mechanical effects of Mn substitutions in AlFe2B2
- 2019
-
Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 482, s. 54-60
-
Tidskriftsartikel (refereegranskat)abstract
- The mechanical and magnetic properties of the newly discovered MAB-phase class of materials based upon AlFe2B2 were investigated. The samples were synthesised from stoichiometric amounts of all constituent elements. X-ray diffraction shows that the main phase is orthorhombic with an elongated b-axis, similar to AlFe2B2. The low hardness and visual inspection of the samples after deformation indicate that these compounds are deformed via a delamination process. When substituting iron in AlFe2B2 with manganese, the magnetism in the system goes from being ferro- to antiferromagnetic via a disordered ferrimagnetic phase exhibited by AlFeMnB2. Density functional theory calculations indicate a weakening of the magnetic interactions among the transitions metal ions as iron is substituted by manganese in AlFe2B2. The Mn-Mn exchange interactions in AlMn2B2 are found to be very small.
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| 13. |
- Cedervall, Johan, et al.
(författare)
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Magnetic structure of the magnetocaloric compound AlFe2B2
- 2016
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Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 664, s. 784-791
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Tidskriftsartikel (refereegranskat)abstract
- The crystal and magnetic structures of AlFe2B2 have been studied with a combination of X-ray and neutron diffraction and electronic structure calculations. The magnetic and magnetocaloric properties have been investigated by magnetisation measurements. The samples have been produced using high temperature synthesis and subsequent heat treatments. The compound crystallises in the orthorhombic crystal system Cmmm and it orders ferromagnetically at 285 K through a second order phase transition. At temperatures below the magnetic transition the magnetic moments align along the crystallographic a-axis. The magnetic entropy change from 0 to 800 kA/m was found to be - 1.3 J/K kg at the magnetic transition temperature.
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| 14. |
- Cedervall, Johan, et al.
(författare)
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Magnetocaloric effect in Fe2 P: Magnetic and phonon degrees of freedom
- 2019
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 99:17
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Tidskriftsartikel (refereegranskat)abstract
- Devices based on magnetocaloric materials provide great hope for environmentally friendly and energy efficient cooling that does not rely on the use of harmful gasses. Fe2P based compounds are alloys that have shown great potential for magnetocaloric devices. The magnetic behavior in Fe2P is characterized by a strong magnetocaloric effect that coexists with a first-order magnetic transition (FOMT). Neutron diffraction and inelastic scattering, Mossbauer spectroscopy, and first-principles calculations have been used to determine the structural and magnetic state of Fe2P around the FOMT. The results reveal that ferromagnetic moments in the ordered phase are perturbed at the FOMT such that the moments cant away from the principle direction within a small temperature region. The acoustic-phonon modes reveal a temperature-dependent nonzero energy gap in the magnetically ordered phase that falls to zero at the FOMT. The interplay between the FOMT and the phonon energy gap indicates hybridization between magnetic modes strongly affected by spin-orbit coupling and phonon modes leading to magnon-phonon quasiparticles that drive the magnetocaloric effect.
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| 15. |
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| 16. |
- Chimata, Raghuveer, et al.
(författare)
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Magnetism and ultrafast magnetization dynamics of Co and CoMn alloys at finite temperature
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 95:21
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Tidskriftsartikel (refereegranskat)abstract
- Temperature-dependent magnetic experiments such as pump-probe measurements generated by a pulsed laser have become a crucial technique for switching the magnetization in the picosecond time scale. Apart from having practical implications on the magnetic storage technology, the research field of ultrafast magnetization poses also fundamental physical questions. To correctly describe the time evolution of the atomic magnetic moments under the influence of a temperature-dependent laser pulse, it remains crucial to know if the magnetic material under investigation has magnetic excitation spectrum that is more or less dependent on the magnetic configuration, e.g., as reflected by the temperature dependence of the exchange interactions. In this paper, we demonstrate from first-principles theory that the magnetic excitation spectra in Co in fcc, bcc, and hcp structures are nearly identical in a wide range of noncollinear magnetic configurations. This is a curious result of a balance between the size of the magnetic moments and the strength of the Heisenberg exchange interactions, that in themselves vary with configuration, but put together in an effective spin Hamiltonian results in a configuration-independent effective model. We have used such a Hamiltonian, together with ab initio calculated damping parameters, to investigate the magnon dispersion relationship as well as ultrafast magnetization dynamics of Co and Co-rich CoMn alloys.
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| 17. |
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| 18. |
- Delczeg-Czirjak, Erna, et al.
(författare)
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Effect of long-range order on elastic properties of Pd(0.5)Ag(0.5) alloy from first principles
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:9, s. 094205-
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Tidskriftsartikel (refereegranskat)abstract
- The effect of long-range order on single-crystal elastic constants of Pd(0.5)Ag(0.5) alloy has been investigated using first-principles electronic structure calculations. The lowest energy among the considered ordered, partially ordered, and disordered structures is found to be the L1(1) layered structure, which is formed by alternate (111) Pd and Ag layers. The ordering effect is found to follow a clear trend: in contrast to the disordered phase, for which the K(a) and K(c) compressibilities are equal, the L1(1) structure becomes less compressible along the c axis than along the a axis.
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| 19. |
- Delczeg-Czirjak, Erna-Krisztina, 1978-
(författare)
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Energy relavant materials: Investigations based on first principles
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Energy production, storage and efficient usage are all crucial factors for environmentally sound and sustainable future technologies. One important question concerns the refrigeration industry, where the energy efficiency of the presently used technologies is at best 40% of the theoretical Carnot limit. Magnetic refrigerators offer a modern low-energy demand and environmentally friendly alternative. The diiron phosphide-based materials have been proposed to be amongst the most promising candidates for working body of magnetic refrigerators. Hydrogen is one of the most promising sources of renewable energy. Considerable international research focuses on finding good solid state materials for hydrogen storage. On the other hand, hydrogen gas is obtained from hydrogen containing chemical compounds, which after breaking the chemical bonds usually yield to a mixture of different gases. Palladiumsilver alloys are frequently used for hydrogen separation membranes for producing purified hydrogen gas. All these applications need a fundamental understanding of the structural, magnetic, chemical and thermophysical properties of the involved solid state materials. In this thesis ab initio electronic structure methods are used to study the magnetic and crystallographic properties of Fe2P based magneto-caloric compounds and the thermophysical properties of Pd-Ag binary alloys. The nature of magnetism and the strong sensitivity of the Curie temperature of the Fe2P1−xTx (T = boron, silicon and arsenic) are investigated. Using first principles theory, the change in the average magnetic exchange interactions upon doping is decomposed into chemical and structural contributions, the latter including the c/a and vol-ume effects. It is demonstrated that for the investigated alloys the structural effect can´be ascribed mainly to the c/a ratio that strengthens the magnetic exchange interactions between the two Fe sublattices. On the other hand, it is shown that the two types of Fe atoms have a very complicated co-dependency, which manifests in a metamagnetic behavior of the FeI sublattice. This behavior is strongly influenced by doping the P sites. Lattice stability of pure Fe2P and the effect of Si doping on the phase stability are pre-sented. In contrast to the observation, for the ferromagnetic state the hexagonal structure (hex, space group P62m) has no the lowest energy. For the paramagnetic state, the hex structure is shown to be the stable phase and the computed total energy versuscomposition indicates a hex to bco (body centered orthorhombic, space group Imm2)crystallographic phase transition with increasing Si content. The mechanisms responsi-ble for the structural phase transition are discussed in details. The magnetic properties of Fe2P can be subtly tailored by Mn doping. It was shown experimentally that Mn atoms preferentially occupy one of the two different Fe sites of Fe2P. Theoretical results for the Mn site occupancy in MnFeP1−xSix are presented. The single crystal elastic constants, the polycrystalline elastic moduli and the Debye temperature of disordered Pd1−xAgx binary alloys are calculated for the whole range of concentration, 0 ≤ x ≤ 1. It is shown that the variation of the elastic parameters of Pd-Ag alloys with chemical composition strongly deviates from the simple linear trend. The complex electronic origin of these anomalies is demonstrated. The effect of long range order on the single crystal elastic constants of Pd0.5Ag0.5 alloy is also investigated. Within this thesis most of the calculations were performed using the Exact Muffin-Tin Orbitals method. The chemical and magnetic disorder are treated via the Coherent Potential Approximation. The paramagnetic phase is modeled by the Disordered Local Magnetic Moments approach.
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| 20. |
- Delczeg-Czirjak, Erna-Krisztina, 1978-
(författare)
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Energy Relevant Materials: Investigations Based on First Principles
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Energy production, storage and efficient usage are all crucial factors for environmentally sound and sustainable future technologies. One important question concerns the refrigeration industry, where the energy efficiency of the presently used technologies is at best 40% of the theoretical Carnot limit. Magnetic refrigerators offer a modern low-energy demand and environmentally friendly alternative. Iron phosphide based materials have been proposed to be amongst the most promising candidates for working body of magnetic refrigerators. Hydrogen is one of the central elements on the most promising sources of renewable energy. Considerable international research focuses on finding good solid state materials for hydrogen storage. On the other hand, hydrogen gas is obtained from hydrogen containing chemical compounds, which after breaking the chemical bounds usually yield to a mixture of different gases. Palladium-silver alloys are frequently used for hydrogen separation membranes for producing purified hydrogen gas. All these applications need a fundamental understanding of the structural, magnetic, chemical and thermophysical properties of the involved solid state materials. In the present thesis ab initio electronic structure methods are used to study the crystallographic and magnetic properties of Fe2P based magneto-caloric compounds and the thermophysical properties of Pd-Ag binary alloys. Lattice stability of pure Fe2P and the effect of Si doping on the phase stability are presented. In contrast to the observation, for the ferromagnetic state the body centered orthorhombic structure (bco, space group Imm2) is predicted to have lower energy than the hexagonal structure (hex, space group P62m). The zero-point spin fluctuation energy difference is found to be large enough to stabilize the hex phase. For the paramagnetic state, the hex structure is shown to be the stable phase and the computed total energy versus composition indicates a hex to bco crystallographic phase transition with increasing Si content. The magneto-structural effects and the mechanisms responsible for the structural phase transition are discussed in details. The magnetic properties of Fe2P can be subtly tailored by Mn doping. It has been shown experimentally that Mn atoms preferentially occupy one of the two different Fe sites of Fe2P. Theoretical results for the Mn site occupancy in MnFeP1-xSix are presented. The single crystal and polycrystalline elastic constants and the Debye temperature of Pd1-xAgx binary alloys are calculated for the whole range of concentration, 0≤x≤1. It is shown that the variation of the elastic parameters of Pd-Ag alloys with chemical composition strongly deviates from the simple expected trend. The complex electronic origin of these anomalies is demonstrated. Within the present thesis, all relaxed crystal structures are obtained using the Projector AugmentedWave full-potential method. The chemical and magnetic disorder is treated using the Exact Muffin-Tin Orbitals method in combination with the Coherent Potential Approximation. The paramagnetic phase is modeled by the Disordered Local Magnetic Moments approach.
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