
5felectron Magnetism in Intermetallic Uranium Compounds
Vališka, Michal ; Sechovský, Vladimír (advisor) ; Šob, Mojmír (referee) ; Veis, Martin (referee)
Tato práce je zaměřená na studium magnetických vlastností tří r·zných sloučenin na bázi uranu obsahujících 5f elektrony (U4Ru7Ge6, UAu2Si2 a UIrGe). V rámci této práce byly, za pomoci r·zných metod, připraveny vysoce kvalitní krystaly těchto sloučenin. Vlastnosti těchto systém· byly studovány r·znými objemovými meto dami (magnetizace, měrné teplo, teplotní roztažnost, elektrický transport) a také neu tronovým a rentgenovým rozptylem při širokém spektru vnějších podmínek (nízká teplota, vysoké magnetické pole, vysoký tlak). Kombinace těchto metod odhalila komplexní chování těchto systém· a pomohla sestrojit jejich magnetické fázové diagramy. První studovanou sloučeninou je feromagnet U4Ru7Ge6vykazující velmi nízkou mag netokrystalovou anizotropii, která je neočekávaná pro sloučeninu na bázi uranu. To se promítá do izotropní závislosti téměř všech měřených fyzikálních vlastností. Bylo zjištěno, že osa snadné magnetizace se mění v uspořádaném stavu a tento jev je spojen s anomáliemi v teplotní roztažnosti ukazující na možnou rhomboedrickou distorzi. Ta vede k vytvoření dvou odlišných pozic uranu s odlišným magnetickým momentem. Tato před pově¤ byla potvrzena teoretickými výpočty a pomocí polarizované neutronové difrakce. Rozdíl magnetických moment· na dvou odlišných uranových pozicích je zp·soben...


Quantummechanical study of magnetic properties of superalloy nanocomposite phase Fe2AlTi
Slávik, Anton ; Miháliková, Ivana ; Friák, Martin ; Všianská, Monika ; Šob, Mojmír
The L21structure Fe2AlTi intermetallic compound is one of the two phases identified in FeAlTi superalloy nanocomposites. Experimental data related to lowtemperature magnetic properties of this Heusler compound indicate that magnetic moment is about 0.1 Bohr magneton per formula unit. In contrast, previous quantummechanical calculations predicted Fe2AlTi to have much higher magnetic moment, 0.9 Bohr magneton per formula unit. In order to solve this discrepancy between the theory and experiment we have performed a series of quantummechanical fixspinmoment calculations and compared our results with those for nonmagnetic state. It turns out that the total energy of the nonmagnetic state is only by 10.73 meV/atom higher than that of the magnetic state. When applying Boltzmann statistics to this very small energy difference we predict that the nonmagnetic state appears at nonzero temperatures with significant probabilities (for instance, 22.36 % at T = 100 K) and reduces the overall magnetic moment. As another mechanism lowering the magnetization we studied selected shape deformations, in particular trigonal shearing. Fe2AlTi exhibits a compressiontension asymmetry with respect to these strains and, for example, the strain 0.08 destabilizes the spinpolarized state, leaving the nonmagnetic state as the only stable one.


Firstprinciples study of interface energies in FeAlbased superalloy nanocomposites
Miháliková, Ivana ; Slávik, Anton ; Friák, Martin ; Všianská, Monika ; Koutná, N. ; Holec, David ; Šob, Mojmír
FeAlbased nanocomposites with a superalloytype of microstructure constitute a very promising class of materials. They possess a great potential as an alternative to the currently used steel grades in high temperature applications. Intermetallicscontaining nanocomposites, such as those with the Fe3Al compound being one of the phases, may open a way towards future automotive and energyconversion technologies with lower fuel consumption and reduced environmental impact. We employ quantummechanical calculations to analyze relations between ordering tendencies of Al atoms in the disordered Fe18.75at.%Al phase on one hand and thermodynamic, structural and magnetic properties of FeAlbased nanocomposites on the other. When comparing supercells modeling disordered FeAl phase with different atomic distribution of atoms we find out that the supercell without 1st and 2nd nearest neighbor AlAl pairs has a lower energy than that mimicking a perfect disorder (a special quasirandom structure, SQS). Further, coherent interfaces with (001), (110) and (110) crystallographic orientations between Fe3Al compound and SQS FeAl phase have higher energies than those exhibiting atomic distribution without 1st and 2nd nearest neighbor AlAl pairs.


Quantummechanical study of tensorial elastic and hightemperature thermodynamic properties of grain boundary states in superalloyphase Ni3Al
Friák, Martin ; Všianská, Monika ; Holec, D. ; Šob, Mojmír
Grain boundaries (GBs), the most important defects in solids and their properties are crucial for many materials properties including (in)stability. Quantummechanical methods can reliably compute properties of GBs and we use them to analyze (tensorial) anisotropic elastic properties of interface states associated with GBs in one of the most important intermetallic compounds for industrial applications, Ni3Al. Selecting the Sigma 5(210) GBs as a case study because of its significant extra volume, we address the mechanical stability of the GB interface states by checking elasticitybased Born stability criteria. One critically important elastic constant, C 55, is found nearly three times smaller at the GB compared with the bulk, contributing thus to the reduction of the mechanical stability of Ni3Al polycrystals. Next, comparing properties of Sigma 5(210) GB state which is fully relaxed with those of a Sigma 5(210) GB state when the supercell dimensions are kept equal to those in the bulk we conclude that lateral relaxations have only marginal impact on the studied properties. Having the complete elastic tensor of Sigma 5(210) GB states we combine Green'sfunction based homogenization techniques and an approximative approach to the Debye model to compare thermodynamic properties of a perfect Ni3Al bulk and the Sigma 5(210) GB states. In particular, significant reduction of the melting temperature (to 7981% of the bulk value) is predicted for nanometersize grains.


Theoryguided design of novel FeAlbased superalloys
Friák, Martin ; Holec, D. ; Jirásková, Yvonna ; Palm, M. ; Stein, F. ; Janičkovič, D. ; Pizúrová, Naděžda ; Dymáček, Petr ; Dobeš, Ferdinand ; Šesták, Pavel ; Fikar, Jan ; Šremr, Jiří ; Nechvátal, Luděk ; Oweisová, S. ; Homola, V. ; Titov, Andrii ; Slávik, Ondrej ; Miháliková, Ivana ; Pavlů, Jana ; Buršíková, V. ; Neugebauer, J. ; Boutur, D. ; Lapusta, Y. ; Šob, Mojmír
Our modern industrialized society increasingly requires new structural materials\nfor hightemperature applications in automotive and energyproducing industrial\nsectors. Ironaluminides are known to possess excellent oxidation and sulfidation\nresistance as well as sufficient strength at elevated temperatures. New FeAlbased\nmaterials will have to meet multiple casting, processing and operational criteria\nincluding hightemperature creep strength, oxidation resistance and roomtemperature\nductility. Such desirable combination of materials properties can be achieved in multiphase\nmulticomponent superalloys with a specific type of microstructure (the matrix contains\ncoherent particles of a secondary phase  a superalloy microstructure). In order to design\nnew FeAlbased superalloys, we employ a stateoftheart theoryguided materials design\nconcept to identify suitable combinations of solutes.


AB INITIO STUDY OF EFFECT OF SEGREGATED SPIMPURITIES AT GRAIN BOUNDARIES IN NICKEL
Všianská, Monika ; Šob, Mojmír
The embrittling/strengthening effects of segregated spelements in the 3rd 4th and 5th period (Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb and Te) at the Sigma 5(210) grain boundary (GB) in fcc nickel have been investigated using density functional theory. We predict Si as a GB cohesion enhancer, Al and P have none or minimal strengthening effect and S, Ga, As, Se, In, Sn, Sb and Te are GB embrittlers in Ni. We also analyze the segregation enthalpy of all impurities. It turns out that AI, Ga, In, Sn, Sb and Te are substitutional and Si, P, S, Ge, As and Se interstitial impurities at the GB in Ni.


Relativistic Theory of Electron Transport in Magnetic Layers
Sýkora, Rudolf ; Turek, Ilja (advisor) ; Šob, Mojmír (referee) ; Drchal, Václav (referee)
Title: Relativistic Theory of Electron Transport in Magnetic Layers Author: Rudolf Sýkora Department / Institute: Institude of Theoretical Physics Supervisor of the doctoral thesis: doc. RNDr. Ilja Turek, DrSc., Department of Condensed Matter Physics Abstract: We review the densityfunctional theory (DFT) in detail using the Levy Lieb ap proach. The Kohn Sham scheme is discussed, starting from the simplest spinless non relativistic case, then including spin and considering potential spin magnetism, and finally deriv ing the full Kohn Sham Dirac relativistic scheme. The Linear MuffinTin Orbital (LMTO) method for electronicstructure calculation is presented, together with mentioning the necessary changes to include the spinorbit (SO) interaction effects to an otherwise scalarrelativistic (SR) theory. Derivation of an electronicconductance formula for a layered system is given, based on the Landauer scattering picture and using simple nonequilibrium Green functions. The formal ism is applied to layered metallic systems of light elements Co, Ni, Cu elements, and to layered systems with a tunnelling barrier, Fe/MgO/Ag and Fe/GaAs/Ag. The effects of the SO interac tion on the Giant Magnetoresistance (GMR) ratio and/or the Tunnelling Anisotropy Magnetore sistance (TAMR) for these systems are discussed....


Magnetic anisotropies in (Ga,Mn)As and metallic multilayers with strong spinorbit coupling
Zemen, Jan ; Jungwirth, Tomáš (advisor) ; Diviš, Martin (referee) ; Šob, Mojmír (referee)
The thesis presents a numerical study of magnetocrystalline anisotropies in dilute ferromagnetic semiconductors and transition metal systems intended to advance the current understanding of the microscopic origins of this relativistic effect and to contribute to the development of spintronic devices with new functionalities. The major part of the work surveys magnetocrystalline anisotropies in (Ga,Mn)As epilayers and compares the calculations to available experimental data. Our model is based on an envelope function description of the valence band holes and a spin representation for their kineticexchange interaction with localised electrons on Mn2+ ions, treated in the meanfield approximation. For epilayers with growth induced latticematching strains we study inplane to outofplane easy axis reorientations as a function of Mn localmoment concentration, hole concentration, and temperature. Next we focus on the competition of inplane cubic and uniaxial anisotropies. We add an inplane shear strain to the effective Hamiltonian in order to capture measured data in bare, unpatterned epilayers, and we provide microscopic justification for this approach. The model is then extended by an inplane uniaxial strain and used to directly describe experiments with magnetisation direction controlled by...

 
 