| 1. |
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- 2019
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Tidskriftsartikel (refereegranskat)
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| 2. |
- Kehoe, Laura, et al.
(författare)
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Make EU trade with Brazil sustainable
- 2019
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 364:6438, s. 341-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 3. |
- Locht, Inka L. M., et al.
(författare)
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Standard model of the rare earths analyzed from the Hubbard I approximation
- 2016
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Ingår i: PHYSICAL REVIEW B. - : American Physical Society. - 2469-9950. ; 94:8
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Tidskriftsartikel (refereegranskat)abstract
- In this work we examine critically the electronic structure of the rare-earth elements by use of the so-called Hubbard I approximation. From the theoretical side all measured features of both occupied and unoccupied states are reproduced, without significant deviations between observations and theory. We also examine cohesive properties like the equilibrium volume and bulk modulus, where we find, in general, a good agreement between theory andmeasurements. In addition, we have reproduced the spin and orbital moments of these elements as they are reflected from measurements of the saturation moment. We have also employed the Hubbard I approximation to extract the interatomic exchange parameters of an effective spin Hamiltonian for the heavy rare earths. We show that the Hubbard I approximation gives results which are consistent with calculations where 4f electrons are treated as core states for Gd. The latter approach was also used to address the series of the heavy/late rare earths. Via Monte Carlo simulations we obtained ordering temperatures which reproduce measurements within about 20%. We have further illustrated the accuracy of these exchange parameters by comparing measured and calculated magnetic configurations for the heavy rare earths and the magnon dispersion for Gd. The Hubbard I approximation is compared to other theories of the electronic structure, and we argue that it is superior. We discuss the relevance of our results in general and how this makes it possible to treat the electronic structure of materials containing rare-earth elements, such as permanent magnets, magnetostrictive compounds, photovoltaics, optical fibers, topological insulators, and molecular magnets.
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| 4. |
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| 5. |
- Keshavarz, Samara, et al.
(författare)
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Exchange interactions of CaMnO3 in the bulk and at the surface
- 2017
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 95
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Tidskriftsartikel (refereegranskat)abstract
- We present electronic and magnetic properties of CaMnO3 (CMO) as obtained from ab initio calculations. We identify the preferable magnetic order by means of density functional theory plus Hubbard U calculations and extract the effective exchange parameters (Jij ' s) using the magnetic force theorem. We find that the effects of geometrical relaxation at the surface as well as the change of crystal field are very strong and are able to influence the lower-energymagnetic configuration. In particular, our analysis reveals that the exchange interaction between the Mn atoms belonging to the surface and the subsurface layers is very sensitive to the structural changes. An earlier study [A. Filippetti and W. E. Pickett, Phys. Rev. Lett. 83, 4184 (1999)] suggested that this coupling is ferromagnetic and gives rise to the spin-flip (SF) process on the surface of CMO. In our work, we confirm their finding for an unrelaxed geometry, but once the structural relaxations are taken into account, this exchange coupling changes its sign. Thus, we suggest that the surface of CMO should have the same G-type antiferromagnetic order as in the bulk. Finally, we show that the suggested SF can be induced in the system by introducing an excess of electrons.
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| 6. |
- Rodrigues, Debora C. M.
(författare)
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Electronic Structure and Atomistic Spin Dynamics of Nanostructured Materials
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The theoretical studies of several magnetic materials are presented in this thesis. To each of them, it was investigated the electronic structure, by means of density functional theory calculations, and/or magnetization dynamics, in the context of atomistic spin dynamics (ASD). For bulk properties, we evaluate the magnon spectra of the heavy rare earths (Gd, Tb, Dy, Ho, Er, and Tm), using the exchange parameters and magnetic moments from first-principles calculations in ASD simulations. Additionally, we performed Monte Carlo simulations that nicely reproduced the qualitative trend of lowering of the critical temperatures across the series. Next, we discuss about the microscopic mechanism of the vanishingly low magnetic anisotropy of Permalloy using the concept of the orbital moment anisotropy for Fe and Ni atoms in the alloy. Turning to surface magnetism, we discuss the use of exchange parameters computed by a noncollinear formalism for 6 monolayers of Fe on the Ir(001) substrate, in order to have a more accurate description of magnons at finite temperature and to obtain good comparison with experimental data. Besides that, we also studied surface magnons on 3 and 9 Ni monolayers on Cu(001) and Cu(111) in order to track the significant surface and/or interface effects and contrast it to properties that are fcc Ni bulk-like. Likewise, we used the Monte Carlo method to estimate the critical temperatures of Ni surfaces and compared with experimental data. Finally, in the field of low dimensional magnetism, we present the ab-initio calculations for the electronic structure of Cr nanostructures of diverse geometries adsorbed on the Pd(111) surface, with focus on the formation of non-collinear spin configurations, either due to geometric frustration or the spin-orbit coupling provided by the substrate.
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| 7. |
- Rodrigues, Debora C. M., et al.
(författare)
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Finite-temperature interatomic exchange and magnon softening in Fe overlayers on Ir(001)
- 2016
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 94:1
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Tidskriftsartikel (refereegranskat)abstract
- We evaluate how thermal effects soften the magnon dispersion in 6 layers of Fe(001) on top of Ir(001). We perform a systematic study considering noncollinear spin arrangement and calculate configuration-dependent exchange parameters J(ij)(nc) following the methodology described by Szilva et al. [Phys. Rev. Lett. 111, 127204 (2013)]. In addition, Monte Carlo simulations were performed in order to estimate the noncollinear spin arrangement as a function of temperature. Hence the J(ij)(nc)'s related to these configurations were calculated and used in an atomistic spin dynamics approach to evaluate the magnon spectra. Our results show good agreement with recent room-temperature measurements, and highlights how thermal effects produce magnon softening in this, and similar, systems
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| 8. |
- Rodrigues, Debora C. M., et al.
(författare)
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Magnetic anisotropy in permalloy : hidden quantum mechanical features
- 2018
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 97:22
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Tidskriftsartikel (refereegranskat)abstract
- By means of relativistic, first principles calculations, we investigate the microscopic origin of the vanishingly low magnetic anisotropy of Permalloy, here proposed to be intrinsically related to the local symmetries of the alloy. It is shown that the local magnetic anisotropy of individual atoms in Permalloy can be several orders of magnitude larger than that of the bulk sample and 5–10 times larger than that of elemental Fe or Ni. We furthermore show that locally there are several easy axis directions that are favored, depending on local composition. The results are discussed in the context of perturbation theory, applying the relation between magnetic anisotropy and orbital moment. Permalloy keeps its pronounced soft ferromagnetic nature due to the exchange energy to be larger than the magnetocrystalline anisotropy. Our results shine light on the magnetic anisotropy of permalloy and of magnetic materials in general, and in addition enhance the understanding of pump-probe measurements and ultrafast magnetization dynamics.
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| 9. |
- Szilva, Attila, et al.
(författare)
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Theory of noncollinear interactions beyond Heisenberg exchange : Applications to bcc Fe
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:14
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Tidskriftsartikel (refereegranskat)abstract
- We show for a simple noncollinear configuration of the atomistic spins (in particular, where one spin is rotated by a finite angle in a ferromagnetic background) that the pairwise energy variation computed in terms of multiple-scattering formalism cannot be fully mapped onto a bilinear Heisenberg spin model even in the absence of spin-orbit coupling. The non-Heisenberg terms induced by the spin-polarized host appear in leading orders in the expansion of the infinitesimal angle variations. However, an Eg-T2g symmetry analysis based on the orbital decomposition of the exchange parameters in bcc Fe leads to the conclusion that the nearest-neighbor exchange parameters related to the T2g orbitals are essentially Heisenberg-like: they do not depend on the spin configuration, and can, in this case, be mapped onto a Heisenberg spin model even in extreme noncollinear cases.
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