| 1. |
- Bergqvist, Lars, 1976-, et al.
(författare)
-
Magnetic properties and disorder effects in diluted magnetic semiconductors
- 2005
-
Ingår i: Physical Review B Condensed Matter. - : American Physical Society (APS). - 0163-1829 .- 1095-3795. ; 72:19, s. 195210-
-
Tidskriftsartikel (refereegranskat)abstract
- We present calculations of the exchange interactions and critical temperatures for several diluted magnetic semiconductor systems. It is shown that the exchange interactions are dominated by short-ranged interactions that have a strong directional dependence. Using a combination of first-principles calculations of the exchange interactions together with Monte Carlo simulations of the classical Heisenberg model, in which the positional disorder and spin fluctuations are properly included, the calculated critical temperatures are in good agreement with experimantal observations. It is shown that agreement between theory and experiment, as regards ordering temperatures, is obtained only when the magnetic atoms are randomly positioned in a simulation cell which proves that disorder effects play a very important role. The effect of strong electron-electron interaction has been studied by means of the LSDA+U scheme. We investigate in detail the nature of the anisotropic exchange interactions by means of a Fermi surface analysis.
|
|
| 2. |
- Carva, Karel, et al.
(författare)
-
Out-of-plane spin-transfer torques : first-principles study
- 2010
-
Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 322:9-12, s. 1085-1087
-
Tidskriftsartikel (refereegranskat)abstract
- Spin-transfer torques in layered structures are studied with particular attention to their direction. Their direction is in spin valves determined mainly by the properties of the free layer and its interfaces. Situations which lead to a significant component of the torque in the direction perpendicular to both magnetizations present in the spin valve (out-of-plane torque) are described and analyzed. This component may arise from the transmitted spin current, as is demonstrated for a Cu vertical bar Ni vertical bar Cu(001) based system, or from the reflected spin current, which is found to be particularly high for systems with Cr vertical bar Co2MnSi interface. Ab initio calculations were performed for these systems employing the local spin density approximation (LSDA) and the linear muffin-tin orbital (LMTO) method. Information about the direction of the torque was obtained from the spin-mixing conductance.
|
|
| 3. |
- Carva, Karel, et al.
(författare)
-
Spin-mixing conductances : The influence of disorder
- 2008
-
Ingår i: Physica status solidi A, Applications and materials science. - : Wiley. - 1862-6300. ; 205:8, s. 1805-1808
-
Tidskriftsartikel (refereegranskat)abstract
- Spin transfer torque exerted on a magnetic layer can be viewed as a linear response to the spin accumulation inside an adjacent non-magnetic layer, information about their response coefficient is provided by the complex spin-mixing conductance C-mix. Substitutional disorder is known to affect the spin-dependent charge conductances and often reduces strongly the magnetoresistance. Here, we examine its impact on C-mix of several selected realistic systems. Recently predicted oscillations of C-mix as a function of ferromagnetic layer thickness in Ni based junctions might be suppressed by interface interdiffusion, but presented ab initio calculations disprove this possibility. Halfmetallic character of the Heusler compound Co(2)Mnsi is destroyed by often encountered antisite disorder; however the impact of this disorder to the predicted C-mix is rather weak. Diluted magnetic semiconductor (Ga,Mn)As is an intrinsically disordered system the analysis of calculations shows that the variation of C-mix with substitutional Mn content can be understood in terms of the associated change of the number of carriers, whereas the variation with lattice defects is more complex.
|
|
| 4. |
- Havela, Ladislav, et al.
(författare)
-
Strong 5f Ferromagnetism in UH3-Based Materials
- 2016
-
Ingår i: MRS Advances. - : Springer Science and Business Media LLC. - 2059-8521. ; 1:44, s. 2987-2992
-
Tidskriftsartikel (refereegranskat)abstract
- Several diverse types of UH3-based hydrides can be prepared by hydrogenation of bcc U-based alloys. Pair Distribution Function (PDF) analysis using high-energy X-rays identified that the (UH3)1-xMo x hydrides are nanocrystalline, with the structure motif based mainly on the β-UH3 structure. α-UH3 represents a minority component. On the other hand, PDF of the (UH3)1-xZr x hydrides corresponds well to the α-UH3 crystal structure. All the hydrides are ferromagnetic, with the Curie temperature T C reaching up to 203 K.
|
|
| 5. |
- Holmström, Erik, 1974-
(författare)
-
Magnetism and Structure in Metallic Multilayers
- 2003
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The interplay between magnetism and structure has been studied in magnetic multilayers by electronic structure calculations based on density functional theory and analyzed in terms of models. The main ideas behind the Korringa-Kohn-Rostocker Green’s function method are described and the implementation of the coherent potential approximation is outlined.A simple model for the bilinear magnetic interlayer coupling in metallic multilayers is derived that elucidates the main characteristics of the effect such as coupling period and origin of damping. An analysis of two exotic effects on the magnetic interlayer coupling, Fermi surface nesting and magnetic enhancement is also performed. The Fermi surface nesting in CuPd for the (110) direction is shown to induce a sharp peak in the magnetic interlayer coupling amplitude for a Fe/CuPd/Fe system when the Cu concentration is 60% in the CuPd alloy. The high magnetic susceptibility in Pd is shown to have strong influence on the magnetic interlayer coupling in a Fe/Pd/Fe (100) system where it changes the amplitude, phase and induces an offset.The relation between surface structure and magnetic properties in metallic multilayers is investigated in terms of a theory that is based on a symbiosis between experiment and theory. By calculating the total magnetic moment of a sample for a large range of possible interface structures and comparing to experimental results for equivalent samples a parameter that describes the interface structure is determined. This parameter is then shown to be universal for the particular combination of elements in the structure both as regards the calculated total magnetic moment as well as the magnetic interlayer coupling and the critical temperatures.
|
|
| 6. |
- Kudrnovsky, Josef, et al.
(författare)
-
Exchange interactions and critical temperatures in diluted magnetic semiconductors
- 2004
-
Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 16:48, s. S5571-S5578
-
Tidskriftsartikel (refereegranskat)abstract
- A first-principles approach to magnetic properties of diluted magnetic semiconductors (DMS) is presented that is based on the local spin-density approximation (LSDA) as implemented in the framework of the tight-binding linear muffin-tin orbital method, while the effect of randomness is described by the coherent potential approximation. Application of a real-space Green-function formalism yields the exchange pair interactions between distant magnetic atoms that are needed for quantitative studies of magnetic excitations including the Curie temperatures. We have found that the pair exchange interactions exhibit a strong directional dependence and are exponentially damped with increasing distance between magnetic atoms due to disorder and the half-metallic character of the DMS. As a case study we consider (Ga, Mn) As, (Ga, Mn)N, and (Zn, Cr)Te alloys. The calculations demonstrate that inclusion of disorder and, in particular, realistic distances among magnetic impurities, are needed to obtain critical temperatures which are in good agreement with available experiments.
|
|
| 7. |
- Kudrnovsky, Josef, et al.
(författare)
-
Exchange interactions and magnetic percolation in diluted magnetic semiconductors
- 2005
-
Ingår i: Local-Moment Ferromagnets. - Berlin/Heidelberg : Springer Berlin/Heidelberg. - 3540272860 ; , s. 131-145
-
Konferensbidrag (refereegranskat)abstract
- We propose a theory that combines first-principles evaluations of inter-atomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations. Exchange interactions are determined using the magnetic force theorem and the one-electron Green functions. The magnetic properties of diluted magnetic semiconductors are dominated by short ranged interatomic exchange interactions that; have a strong directional dependence. We show that; critical temperatures of a broad range of diluted magnetic semiconductors, involving Mn-doped GaAs and GaN as well as Cr-doped ZnTe, arc reproduced with a good accuracy only when the magnetic atoms arc randomly positioned on the Ga (Zn) sites, whereas in ordered structure of the magnetic atoms results in critical temperatures that are too high. This suggests that the ordering of diluted magnetic semiconductors is heavily influenced by magnetic percolation, and that the measured critical ternperatures should be very sensitive to the details of the sample preparation, in agreement with observations.
|
|
| 8. |
|
|
| 9. |
- Rusz, Jan, et al.
(författare)
-
Exchange interactions and Curie temperatures in Ni2−xMnSb alloys : First-principles study
- 2006
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - Charles Univ, Fac Math & Phys, Dept Elect Struct, CR-12116 Prague 2, Czech Republic. Univ Uppsala, Dept Phys, S-75121 Uppsala, Sweden. Acad Sci Czech Republ, Inst Phys, Prague 18221 8, Czech Republic. : American Physical Society (APS). - 1098-0121 .- 1550-235X. ; 73:21, s. 214412-
-
Tidskriftsartikel (refereegranskat)abstract
- We present a first-principles study of the physical properties of the disordered Ni2−xMnSb alloys which form a continuous series connecting two typical members of the Heusler alloy family: namely, the half-metallic semi-Heusler alloy NiMnSb (x=1) and related metallic Heusler alloy Ni2MnSb (x=0). Magnetic moments, exchange interactions, magnon spectra, and Curie temperatures at ambient and elevated pressures are determined and compared with available experimental data. The spin-spin correlation functions at the critical temperature are also calculated. The pair exchange interactions and corresponding classical Heisenberg Hamiltonian are derived from self-consistent electronic structure calculations using a magnetic force theorem. Heusler alloys NiMnSb and Ni2MnSb exhibit strikingly different asymptotic behavior of exchange interactions with the distance between magnetic atoms. The Curie temperatures are estimated using multisublattice versions of the mean-field approximation, random-phase approximation, and Monte Carlo simulations. The robustness of the results with respect to the effect of correlations beyond the local density approximation, the selected reference state used for mapping to the Heisenberg Hamiltonian, and the applied mapping procedure are also discussed.
|
|
| 10. |
- Xie, Ruiwen, 1993-
(författare)
-
Defects in Austenitic Steels and Hard Metals - A DFT-based Study
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Materials are never 100% pure due to the limitation of purification method or manufacturing process. Nor are they perfect, especially under deformation. The present work aims to explore different roles played by the defects in austenitic steels and hard metals.The first focus is iron-manganese (Fe-Mn) based twinning induced plasticity(TWIP) steels which are a category of austenitic materials showing a good combination of high strength and ductility. The planar fault is fundamental for the TWIP mechanism. First, the γ-surface of pure γ-Fe (fcc-Fe) is calculated for different magnetic states. Next, the effects of alloying elements, including Mn,interstitial carbon (C) and nitrogen (N), are addressed. The γ-surface includes several prominent stacking fault energies that are fundamental for, e.g, predicting critical twinning stress and twinnability. The present work compares the γ-surface obtained at different magnetic states, including nonmagnetic (NM), paramagnetic(PM), antiferromagnetic single-layer (AFMI) and double-layer (AFMD) states. The local magnetism significantly influences the γ-surface. In addition, the existing antiferromagnetic (AFM) order results in two different deformation paths inγ-Fe, leading to the generations of superlattice intrinsic stacking fault (SISF) and complex stacking fault (CSF), respectively. The intrinsic stacking fault energy corresponding to SISF is relatively low while its corresponding unstable stacking fault energy is relatively high. The magnetic structures are investigated in the unstable stacking fault and the intrinsic stacking fault configurations via Monte Carlo (MC) simulations. The MC results show that only SISF configuration is favourable, and the two distinctive unstable stacking fault configurations may coexist.The Mn effect on the γ-surface of γ-Fe is studied at AFMI state and the crystal tetragonality is considered. The comparison with experimentally measured stacking fault energy (SFE) dependence on Mn composition shows that the AFMI results reproduce better the experimental trend in high-Mn Fe-Mn alloys than the PM results. Further, the interstitial alloying effects of C and N on the γ-surface of γ-Fe are investigated and no remarkable difference is observed betweenthe C and N impacts. The interaction between dislocation and interstitial atoms, which is fundamental to understand the phenomenon like dynamic strain ageing (DSA), is studied using the generalized stacking fault as an approximation of the partial dislocation core. The minimum migration energy path (MEP) and migration energy surface (MES) of C in the dislocation core of AFMD γ-Fe are calculated. In contrast to the common assumption that the interstitial atoms are stationary during the passage of fast-moving dislocations, the present work suggests that a pair of dislocation partials are capable of moving C atoms forward on the slip plane by one full Burgers vector. Moreover, at the stacking fault ribbon and especially near the dislocation core, the in-plane diffusion energy barriers of C are significantly reduced compared to that in the bulk, rendering a fast diffusion channel for C. The proposed mechanisms for C transport and diffusion are not decided by local magnetic order and can be used to explain the strain rate dependent formation kinetics of twinning or hexagonal close-packed (hcp) martensite in C-alloyed TWIP steels or high entropy alloys. Similarly, the ab initio results show that the diffusion energy barrier of N in the dislocation core is approximately 14.9% of that in the bulk. According to experimental observations, carbon promotes while N suppresses the DSA. However, the different C and N effects on the DSA cannot be understood from current thermodynamic investigations.The defects in the binder phase of hard metals (cemented carbides) are another important topic in this thesis. The interstitial tungsten (W) and C defects in hard metals come from the sintering process during industrial manufacturing. The cemented carbides are composite materials made of tungsten carbide (WC) grains glued together by a binder phase. Typically, the binder phase consists of ductile cobalt (Co) and some amount of dissolved W and C. The measurement ofthe magnetic saturation is one method employed for quality control of cemented carbides. Despite the great success of Co, a substitute of Co is needed due to its rising price and health threats. The substitution of a material in production processes can be complex. Ideally, manufacturing processes and quality controls should be used as usual or at least new ones have to be devised in a simpleway. The present work selects 85Ni-15Fe (85 at.% of Ni and 15 at.% of Fe) to demonstrate the relation between the magnetic saturation and the components of the binder phase of cemented carbides using ab initio method, which providesa non-destructive quality control method in cemented carbides.
|
|
