Martin Zelený

On spin glass-like behaviour of UGaNi

Abstract The isofield magnetization curves, time relaxation of magnetization, temperature dependence of susceptibility and coercivity at low temperatures of a UGaNi single crystal have been investigated. It has been shown that there exists a spin glass-like behaviour of UGaNi which is associated with the simultaneous presence of anisotropy and exchange fluctuations at U atom sites originated by the local environment effect in regions with fluctuating Ni and Ga atom concentration.

THE EFFECTS OF INDIVIDUAL COMPONENTS ON MAGNETIC BEHAVIOUR OF UNIGA AND UCOGA INTERMETALLIC COMPOUNDS

Abstract The paper gives a review of differences in magnetic behaviour of UNiGa and UCoGa found at investigations of the existence of magnetic double-transitions in single and polycrystalline samples of the compounds. The influence of alloying components on magnetic properties of the compounds is analysed. The existing model of the equiatomic ternary UTX (T = transition metals, X = Al, Ga, Sn) system is discussed on the basis of the results. The importance of RKKY indirect exchange interactions in the compounds is shown.

Ab initio prediction of stable nanotwin double layers and 4O structure in Ni2MnGa

The ab initio electronic structure calculations of the

Magnetic properties of UFe8Al4 and UFe10X2 (X = Mo, Si)

{\mathrm{Ni}}_{2}\mathrm{MnGa}

A peculiar bonding of sulphur at the Nb(001) surface

alloy indicate that the orthorhombic 4O structure exhibits the lowest energy compared to all known martensitic structures. The 4O structure is formed by nanotwin double layers, i.e., oppositely oriented nanotwins consisting of two (101) lattice planes of nonmodulated martensitic structure. It exhibits the lowest occupation of density of states at the Fermi level. The total energy 1.98 meV/atom below the energy of nonmodulated martensite is achieved within structural relaxation by shifting Mn and Ga atoms at the nanotwin boundaries. The same atomic shift can also be found in other martensitic nanotwinned or modulated structures such as 10M and 14M, which indicates the importance of the nanotwin double layer for the stability of these structures. Our discovery shows that the nanotwinning or modulation is a natural property of low-temperature martensitic phases in Ni-Mn-Ga alloys.

Tensorial elastic properties and stability of interface states associated with Σ5(210) grain boundaries in Ni3(Al,Si)

Abstract Magnetic hysteresis curves at 4.2 K and temperature dependence of ac initial susceptibility of UFe8Al4, UFe10Mo2 and UFe10Si2 have been measured. The influence of annealing on magnetic properties of UFe10Mo2 has also been investigated.

Different Ab Initio Approaches for Doping Descriptions: Tetragonal Deformation of Ni-Mn-Ga Alloys

The morphological, crystallographical and electronic structure of a well-ordered pseudomorphic sulphur monolayer (S ML) bonded at the (001) surface of a niobium substrate was investigated experimentally and theoretically. The system exhibits strong sulphur-niobium bonds with fourfold symmetry which do not exist in bulk niobium chalcogenides. These bonds enhance the localization of occupied bands near the Fermi energy in two Nb MLs under the S ML and, due to changed symmetry and stoichiometry, push the p-states of the S ML to deeper binding energies as compared to Nb chalcogenides. The S ML shows strong iono-covalent bonding to Nb(001) with bonding states mainly due to S-3p and antibonding states due to Nb-4d orbitals.

Low temperature a / b nanotwins in Ni 50 Mn 25+x Ga 25−x Heusler alloys

Graphical Abstract Grain boundaries (GBs) represent one of the most important types of defects in solids and their instability leads to catastrophic failures in materials. Grain boundaries are challenging for theoretical studies because of their distorted atomic structure. Fortunately, quantum-mechanical methods can reliably compute their properties. We calculate and analyze (tensorial) anisotropic elastic properties of periodic approximants of interface states associated with GBs in one of the most important intermetallic compounds for industrial applications, Ni3Al, appearing in Ni-based superalloys. Focusing on the Σ5(210) GBs as a case study, we assess the mechanical stability of the corresponding interface states by checking rigorous elasticity-based Born stability criteria. The critical elastic constant is found three-/five-fold softer contributing thus to the reduction of the mechanical stability of Ni3Al polycrystals (experiments show their GB-related failure). The tensorial elasto-chemical complexity of interface states associated with the studied GBs exemplifies itself in high sensitivity of elastic constants to the GB composition. As another example we study the impact caused by Si atoms segregating into the atomic layers close to the GB and substituting Al atoms. If wisely exploited, our study paves the way towards solute-controlled design of GB-related interface states with controlled stability and/or tensorial properties.

Transformation Paths from Cubic to Low-Symmetry Structures in Heusler Ni 2 MnGa Compound

The influence of Co and Cu doping on Ni-Mn-Ga alloy is investigated using the different ab initio methods for description of chemical disorder. The exact muffin-tin orbital method in combination with the coherent-potential approximation provides almost identical profiles of total energies along the tetragonal deformation path compared to the supercell approach used within the projector-augmented wave method. On the other hand, the simple virtual crystal approximation exhibits different results and thus it is not able to describe doping in Ni-Mn-Ga alloy properly.

On magnetic properties of the U(Fe,Ti)2 system

We have found low temperature a/b nanotwins having (110) twinning plane in a five-layered modulated martensite phase of Ni50Mn25+xGa25−x (at. %) Heusler alloys and identified the particular region in phase diagram where the nanotwinning occurs. Evolution of the structure with decreasing temperature was studied by X-ray diffraction using single crystals exhibiting magnetic shape memory effect. The merging of (400) and (040) lines upon cooling for 2.6 < x < 3.5 indicated a/b nanotwinning originating from the refinement of initially coarse a/b twins. Refinement of the twins with decreasing temperature was observed directly using scanning electron microscopy. The prerequisite for nanotwinning is an extremely low twin boundary energy, which we estimated using first-principles calculations to be 0.16 meV/Å2. As the nanotwinning distorts the relation between the crystal lattice and the X-ray diffraction pattern, it should be taken into consideration in structural studies of Ni-Mn-Ga Heusler alloys.