| 1. |
- Borisov, Vladislav, et al.
(författare)
-
Heisenberg and anisotropic exchange interactions in magnetic materials with correlated electronic structure and significant spin-orbit coupling
- 2021
-
Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 103:17
-
Tidskriftsartikel (refereegranskat)abstract
- The Dzyaloshinskii-Moriya (DM) interaction, as well as symmetric anisotropic exchange, are important ingredients for stabilizing topologically nontrivial magnetic textures, such as, e.g., skyrmions, merons, and hopfions. These types of textures are currently in focus from a fundamental science perspective and they are also discussed in the context of future spintronics information technology. While the theoretical understanding of the Heisenberg exchange interactions is well developed, it is still a challenge to access, from first principles theory, the DM interaction as well as the symmetric anisotropic exchange, which both require a fully-relativistic treatment of the electronic structure, in magnetic systems where substantial electron-electron correlations are present. Here, we present results of a theoretical framework which allows to compute these interactions in any given system and demonstrate its performance for several selected cases, for both bulk and low-dimensional systems. We address several representative cases, including the bulk systems CoPt and FePt, the B20 compounds MnSi and FeGe as well as the low-dimensional transition metal bilayers Co/Pt(111) andMn/W(001). The effect of electron-electron correlations is analyzed using dynamical mean-field theory on the level of the spin-polarized T -matrix + fluctuating exchange (SPTF) approximation, as regards the strength and character of the isotropic (Heisenberg) and anisotropic (DM) interactions in relation to the underlying electronic structure. Our method can be combined with more advanced techniques for treating correlations, e.g., quantum Monte Carlo and exact diagonalization methods for the impurity solver of dynamical mean-field theory. We find that correlation-induced changes of the DM interaction can be rather significant, with up to fivefold modifications in the most distinctive case.
|
|
| 2. |
- Borisov, Vladislav, et al.
(författare)
-
Tuning skyrmions in B20 compounds by 4d and 5d doping
- 2022
-
Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 6:8
-
Tidskriftsartikel (refereegranskat)abstract
- Skyrmion stabilization in novel magnetic systems with the B20 crystal structure is reported here, primarily based on theoretical results. The focus is on the effect of alloying on the 3d sublattice of the B20 structure by substitution of heavier 4d and 5d elements, with the ambition to tune the spin-orbit coupling and its influence on magnetic interactions. State-of-the-art methods based on density functional theory are used to calculate both isotropic and anisotropic exchange interactions. Significant enhancement of the Dzyaloshinskii-Moriya interaction is reported for 5d-doped FeSi and CoSi, accompanied by a large modification of the spin stiffness and spiralization. Micromagnetic simulations coupled to atomistic spin-dynamics and ab initio magnetic interactions reveal the spin-spiral nature of the magnetic ground state and field-induced skyrmions for all these systems. Especially small skyrmions similar to 50 nm are predicted for Co0.75Os0.25Si, compared to similar to 148 nm for Fe0.75Co0.25Si. Convex-hull analysis suggests that all B20 compounds considered here are structurally stable at elevated temperatures and should be possible to synthesize. This prediction is confirmed experimentally by synthesis and structural analysis of the Ru-doped CoSi systems discussed here, both in powder and in single-crystal forms.
|
|
| 3. |
- Kons, Corisa, et al.
(författare)
-
Influence of Hard/Soft Layer Ordering on Magnetization Reversal of Bimagnetic Nanoparticles : Implications for Biomedical/Theranostic Applications
- 2023
-
Ingår i: ACS Applied Nano Materials. - : American Chemical Society (ACS). - 2574-0970. ; 6:13, s. 10986-11000
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the spatial distribution of spin orientationinmagnetic nanoparticles consisting of hard and soft magnetic layers.The nanoparticles are synthesized in a core-shell sphericalmorphology where the target stoichiometry of the magnetically hard,high anisotropy layer is CoFe2O4 (CFO), whilethe synthesis protocol of the lower anisotropy material is known toproduce Fe3O4. The nanoparticles have a meandiameter of similar to 9.2-9.6 nm and are synthesized as two variants:a conventional hard/soft core-shell structure with a CFO core/FOshell (CFO@FO) and the inverted structure FO core/CFO shell (FO@CFO).High-resolution electron microscopy confirms the coherent spinel structureacross the core-shell boundary in both variants, while magnetometryindicates the nanoparticles are superparamagnetic at 300 K and developa considerable anisotropy at reduced temperatures. Low-temperature M vs H loops suggest a multistep reversal process. Smallangle neutron scattering (SANS) with full polarization analysis revealsa considerable alignment of the spins perpendicular to the field evenat fields approaching saturation. The perpendicular magnetizationis surprisingly correlated from one nanoparticle to the next, thoughthe interaction is of limited range. More significantly, the SANSdata reveal a pronounced difference in the reversal process of themagnetization parallel to the field for the two nanoparticle variants.For the CFO@FO nanoparticles, the core and shell magnetizations appearto track each other through the coercive region, while in the FO@CFOvariant, the softer Fe3O4 core reverses beforethe higher anisotropy CoFe2O4 shell, consistentwith expectations from mesoscale magnetic modeling. These resultshighlight the interplay between interfacial exchange coupling andanisotropy as a means to tune the composite properties of the nanoparticlesfor tailored applications including biomedical/theranostic uses.
|
|
| 4. |
- Ntallis, Nikolaos, et al.
(författare)
-
Connection between magnetic interactions and the spin-wave gap of the insulating phase of NaOsO3
- 2021
-
Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:13
-
Tidskriftsartikel (refereegranskat)abstract
- The scenario of a metal-insulator transition driven by the onset of antiferromagnetic order in NaOsO3 calls for a trustworthy derivation of the underlying effective spin Hamiltonian. To determine the latter we rely on ab initio electronic-structure calculations, linear spin-wave theory, and comparison to experimental data of the corresponding magnon spectrum. We arrive this way to Heisenberg couplings that are less than or similar to 45 to less than or similar to 63% smaller than values presently proposed in the literature and Dzyaloshinskii-Moriya interactions in the region of 15% of the Heisenberg exchange J. These couplings together with the symmetric anisotropic exchange interaction and single-ion magnetocrystalline anisotropy successfully reproduce the magnon dispersion obtained by resonant inelastic x-ray scattering measurements. In particular, the spin-wave gap fully agrees with the measured one. We find that the spin-wave gap is defined from a subtle interplay between the single-ion anisotropy, the Dzyaloshinskii-Moriya exchange, and the symmetric anisotropic exchange interactions. The results reported here underpin the local-moment description of NaOsO3, when it comes to analyzing the magnetic excitation spectra. Interestingly, this comes about from a microscopic theory that describes the electron system as Bloch states, adjusted to a mean-field solution to Hubbard-like interactions.
|
|
| 5. |
- Ntallis, Nikolaos, et al.
(författare)
-
Macrospin model of an assembly of magnetically coupled core-shell nanoparticles
- 2022
-
Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 106:10
-
Tidskriftsartikel (refereegranskat)abstract
- Highly sophisticated synthesis methods and experimental techniques allow for precise measurements of mag-netic properties of nanoparticles that can be reliably reproduced using theoretical models. Here, we investigate the magnetic properties of ferrite nanoparticles by using theoretical techniques based on Monte Carlo methods. We introduce three stages of sophistication in the macromagnetic model. First, by using tailor-made Hamil-tonians we study single nanoparticles. In a second stage, the internal structure of the nanoparticle is taken into consideration by defining an internal (core) and external (shell) region, respectively. In the last stage, an assembly of core-shell nanoparticles is considered. All internal magnetic couplings such as interatomic and intra-atomic exchange interactions or magnetocrystalline anisotropies have been estimated. Moreover, the hysteresis loops of the aforementioned three cases have been calculated and compared with recent experimental measurements. In the case of the assembly of nanoparticles, the hysteresis loops together with the zero-field-cooling and field-cooling curves are shown to be in a very good agreement with the experimental data. The current model provides an important tool to understand the internal structure of the nanoparticles together with the complex internal spin interactions of the core-shell ferrite nanoparticles.
|
|
| 6. |
- Ntallis, Nikolaos, et al.
(författare)
-
Majority gate for two-dimensional ferromagnets lacking inversion symmetry
- 2021
-
Ingår i: Physical Review Research. - : American Physical Society. - 2643-1564. ; 3:2
-
Tidskriftsartikel (refereegranskat)abstract
- The manipulation of topologically protected field configurations such as skyrmions, merons, and antimerons, already predicted and experimentally observed in noncentrosymmetric magnets, could definitely have potential applications in logic gate operations as carriers of information. Here, we present and elaborate a proof of concept on how to construct a three-input noncanonical majority gate on a kagome ferromagnet lacking inversion symmetry. By taking advantage of the existence of edge modes in a kagome magnet, it is possible to create topological excitations such as merons and antimerons at the edge of the material. Using atomistic spin dynamics simulations, we determine the precise physical conditions for the creation and annihilation of merons and antimerons and, in a second stage, we describe the majority gate functionality.
|
|
| 7. |
- Vasilakaki, Marianna, et al.
(författare)
-
Effect of albumin mediated clustering on the magnetic behavior of MnFe2O4 nanoparticles : experimental and theoretical modeling study
- 2020
-
Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 31:2
-
Tidskriftsartikel (refereegranskat)abstract
- Over the last two decades, iron oxide based nanoparticles ferrofluids have attracted significant attention for a wide range of applications. For the successful use of these materials in biotechnology and energy, surface coating and specific functionalization is critical to achieve high dispersibility and colloidal stability of the nanoparticles in the ferrofluids. In view of this, the magnetic behavior of clusters of ultra-small MnFe2O4 nanoparticles covered by bovine serum albumin, which is known as a highly biocompatible and environmentally friendly surfactant, is investigated by magnetization measurements, and numerical simulations at an atomic and mesoscopic scale. The coating process with albumin produces a change in the structure, actual size and shape distribution of clusters of exchange coupled particles, giving rise to a distribution of blocking temperatures. The coated system exhibits a superspin glass (SSG) behavior with the SSG freezing temperatures similar to the uncoated ones, providing evidence that the strength of the dipolar interactions is not affected by the presence of the albumin. The DFT calculations show that the albumin coating reduces the surface anisotropy and the saturation magnetization in the nanoparticles leading to lower values of the coercive field in agreement with the experimental findings. Our results clearly demonstrate that the albumin coated clusters of MnFe2O4 particles are ideal systems for energy and biomedical applications since colloidal and thermal stability as well as biosafety is obtained through the albumin coating.
|
|
