| 1. |
- Ghorai, Sagar, et al.
(författare)
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Giant magnetocaloric effect in the (Mn,Fe)NiSi-system
- 2023
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The search for energy-efficient and environmentally friendly cooling technologies is a key driver for the development of magnetic refrigeration based on the magnetocaloric effect (MCE). This phenomenon arises from the interplay between magnetic and lattice degrees of freedom that is strong in certain materials, leading to a change in temperature upon application or removal of a magnetic field. Here we report on a new material, Mn1−xFexNiSi0.95Al0.05, with an exceptionally large isothermal entropy at room temperature. By combining experimental and theoretical methods we outline the microscopic mechanism behind the large MCE in this material. It is demonstrated that the competition between the Ni2In-type hexagonal phase and the MnNiSi-type orthorhombic phase, that coexist in this system, combined with the distinctly different magnetic properties of these phases, is a key parameter for the functionality of this material for magnetic cooling.
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| 2. |
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| 3. |
- Martinho Vieira, Rafael
(författare)
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Exploring magnetocaloric materials by ab-initio methods
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis explores the characterization of magnetocaloric materials from first-principles calculations, emphasizing entropy variation associated with the magnetocaloric effect. The study happens in the context of the search for new magnetocaloric materials to be applied in domestic magnetic refrigerators, as environmentally friendly and energy-efficient alternatives to conventional vapor-compression devices.The study involves benchmarking entropy calculations in systems like FeRh, which exhibits a first-order metamagnetic transition, and Gd, with a second-order ferromagnetic-paramagnetic transition. Different levels of approximations are examined and compared against experimental data, highlighting the need to distinguish between first-order and second-order transitions in the approach taken. The tests underscore the necessity of calculating vibrational and elastic properties for both phases to accurately calculate the entropy variation. This insight is applied in the study of Mn0.5Fe0.5NiSi0.9Al0.05, with results consistent with experimental data.Furthermore, the relationship between structural changes and magnetic properties is investigated, in particular for pressure-induced polymorphs in Gd and the phase transition in Mn0.5Fe0.5NiSi0.95Al0.05. In the case of Gd, it was shown that variations in magnetic ordering temperature under pressure could be explained through a model based on the formation and accumulation of stacking faults. For the Mn0.5Fe0.5NiSi0.95Al0.05 system, the adoption of a magnetic composite model, in conjunction with experimental data, allowed to determine that the magnetostructural transition in these compounds is predominantly driven by the lattice subsystem. The results positively confirm the feasibility of using first-principles entropy estimates as an effective screening tool in high-throughput studies for magnetocaloric materials. A promising workflow is proposed, demonstrating potential in its initial results. Through comparison with experimental data, the derived routes offer valuable insights for the further refinement of the workflow. This approach aims to enhance accuracy and systematically manage complex systems, highlighting a path forward for future advancements.Lastly, the introduction of a novel scaling scheme in Monte Carlo simulations enhancing accuracy across various temperatures, represents a potential advancement in the field of magnetic simulations.
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| 4. |
- Vieira Martinho, Rafael, et al.
(författare)
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The role of pressure-induced stacking faults on the magnetic properties of gadolinium
- 2023
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Experimental data show that under pressure, Gd goes through a series of structural transitions hcp → Sm-type (close-packed rhombohedral)→ dhcp that is accompanied by a gradual decrease of the Curie temperature and magnetization till the collapse of a finite magnetization close to the dhcp structure. We explore theoretically the pressure-induced changes of the magnetic properties, by describing these structural transitions as the formation of fcc stackings faults. Using this approach, we are able to describe correctly the variation of the Curie temperature with pressure, in contrast to a static structural model using the hcp structure.
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| 5. |
- Vieira, Rafael Martinho, et al.
(författare)
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High-throughput compatible approach for entropy estimation in magnetocaloric materials : FeRh as a test case
- 2021
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Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 857
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Tidskriftsartikel (refereegranskat)abstract
- Aiming to predict new materials for magnetic refrigeration from high-throughput calculations asks for an accurate, transferable, and resource-wise balanced approach. Here, we analyze the influence of various approximations on the calculation of key properties of magnetocaloric materials, while revisiting the well-known FeRh system for benchmarking our approach. We focus on the entropy change and its contributions from the electronic, lattice, and magnetic degrees of freedom. All approximations considered are based on first-principles methods and have been tested, and compared for FeRh. In particular, we find that in this context, the Debye approximation for the lattice entropy fails, due to the presence of soft phonon modes in the AFM phase. This approximation is frequently used in the literature as a simple alternative to full phonon calculations. Since soft modes are likely to occur also among promising magnetocaloric materials where structural transformations are common, the use of the Debye approximation should be discarded for these systems treatment. This leaves the calculations of the lattice contribution the most demanding task from the computational point of view, while the remaining contributions can be approximated using more efficient approaches. The entropy change AS shows a peak around 370 K, for which the total entropy change is given by 24.8 JK(-1) kg(-1) (Delta S-ele = 7.38, Delta S-lat = 7.05, Delta S-mag = 10.36 JK(-1) kg(-1)) in good agreement with previous theoretical and experimental findings.
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| 6. |
- Vieira, Rafael Martinho, et al.
(författare)
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Realistic first-principles calculations of the magnetocaloric effect : applications to hcp Gd
- 2022
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Ingår i: Materials Research Letters. - : Taylor & Francis. - 2166-3831. ; 10:3, s. 156-162
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Tidskriftsartikel (refereegranskat)abstract
- We present an efficient computational approach to evaluate field-dependent entropy of magnetocaloric materials from ab-initio methods. The temperature dependence is reported for the entropy change, specific heat and magnetization for hcp Gd. To obtain optimal accuracy in the calculations, a mixed-scheme for magnetic Monte Carlo simulations is proposed and found to be superior to using pure quantum or classic statistics. It is demonstrated that lattice and magnetic contributions play a role in the entropy change and that the dominating contribution comes from the magnetic contribution. The total calculated entropy change agrees with measurements at room temperature.IMPACT STATEMENT: Demonstration of the accuracy of ab-initio theory, coupled to statistical methods, for accurate calculations of the total entropy variation associated with the magnetic transition of Gd. Reproduction of experimental data of entropy change.
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