Samuel Piovarči

Structural and Magnetic Characterization of Half-Metallic Co 2 MnAl Heusler Alloy

Magnetic and structural characterization of full Heusler Co2MnAl alloys prepared by rapid quenching from pure elements was studied. This technique allows a fast production of relatively a large amount of Heusler alloys without the necessity of further thermal treatment. The Heusler alloys have been prepared by the melt-spinning method in the form of ribbons with homogeneous chemical composition (Co = 46.1%, Mn = 29.4%, and Al = 24.5%). The X-ray diffraction shows a single-phase B2 crystalline structure with lattice parameter α = 5.671 Å. Magnetic measurements reveal an anisotropic character of the Co2MnAl ribbon with easy magnetization axis well defined in parallel direction with respect to the ribbon plane. Curie temperature was estimated from magnetization measurement to be 710 K.

Transport properties of variously doped SmB6

Abstract SmB6 is a heavy fermion semiconductor with a narrow energy gap at the Fermi level, often denoted also as Kondo insulator, which has recently been considered to be a topological Kondo insulator, the first strongly correlated electron system to exhibit topological surface conduction states. In this contribution we have investigated the electrical transport properties of single crystalline SmB6 samples having various surfaces, of single crystalline SmB6 doped with various concentrations of lanthanum, europium, ytterbium and strontium, and of samarium deficient (i.e. doped with metal vacancies) Sm1-xB6 sintered samples with the aim to study the influence of miscellaneous doping and sample surfaces on the electronic states in the narrow energy gap of this material. The received results show rather complex resistivity vs. temperature ρ(T) and resistance vs. magnetic field ρ(H) behaviours as a function of wide range La doping and as a function of doping with different elements and vacancies. These results are discussed within the framework of the electronic structure of SmB6 observed in recent angle resolved photoemission spectroscopy as well as in other (above all transport properties) investigations, which exhibit surface electronic states in the narrow energy gap of this material.

Microstructure and Superconducting Properties of YBCO Bulk Superconductors With RE Substitutions

The influence of RE (RE = Yb, Sm) substitutions on the microstructure and superconducting properties of Y-Ba-Cu-O (YBCO) bulk superconductors has been studied for different RE concentrations and their combinations. Precursor powders YBa₂Cu₃O_7-δ (Y-123), Y₂O₃ and CeO₂ were enriched with different amounts of Yb₂O₃ and (or) Sm₂O₃ powders with the aim to increase critical current density, i.e., Jc, in self-field as well as in higher magnetic fields, by introducing additional pinning centers. YBCO bulk superconductors with RE₂O₃ addition were prepared by the optimized top-seeded melt-growth process in the form of single grains. Microstructure analysis revealed that RE₂O₃ addition leads to a higher amount of smaller Y₂BaCuO₅ (Y-211) particles, which is related to high critical current density (J_c ~ 7 × 10⁴ A/cm²) of the samples with RE₂O₃ addition in low magnetic fields. In this paper, the effect of RE₂O₃ addition on Y-211 particle size, critical temperature T_c, and critical current density J_c is reported.

The Influence of Sintering Conditions on Microstructure of La-Ca-Sr-Mn-O Magnetocaloric Ceramics

A La0.67Ca0.33-xSrxMnO3 (x=0.33; 0.03; 0) perovskite magnetocaloric powders were prepared by solid state synthesis in air. Phase transformations were characterized by thermal analyses and the phase composition was confirmed by X-ray powder diffraction. Pressed pellets were sintered at different temperatures. Microstructure analyses were done by polarized light microscopy and scanning electron microscopy. The influence of sintering conditions on porosity, grain size and content of secondary phases is shown.

Microstructure of the NiMnGa Heusler Alloys Prepared by Suction Casting and Melt-Spinning

The microstructure of the Ni50Mn25Ga25 Heusler alloy (HA) rod cast to a copper mould and melt-spun ribbon was studied. According to X-ray diffraction analyses in both cases, the solidified alloy is a single-phase. In suction casting, the radial columnar crystals grow by cellular or dendritic growth, which leads to chemical inhomogeneity of the whole cast rod on the microscopic scale. The melt-spun ribbon exhibits a homogeneous alloy at the wheel side and an inhomogeneous alloy on the free surface side of the ribbon formed by cellular and dendritic growth. At both types of casting, the columnar crystals growth along the <100> crystal direction. According to texture analyses by EBSD, the fine grain structure at the wheel side of the ribbon exhibits no texture, while the columnar grain structure on the free surface side exhibits the <100> fibre texture with a declination by about 10 degrees in the spinning direction. Melt-spinning leads to significant refinement of the HA microstructure.

Study of Synthesis and Crystal Structure of Barium Cerate at High Temperatures

Crystalline barium cerate was synthesized by oxalate coprecipitation from nitrates of barium and cerium [1]. The oxalate precursor prepared by chemical methods was calcined at different temperatures up to 950°C. The barium cerate was studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). X-ray diffraction investigation enables the determination of the phases that originate at different stages of synthesis and the crystal structure of final barium cerate, as well. From XRD patterns the average size of coherent regions was estimated by using Halder-Wagner method [2]. Both size and shape of crystallites were also studied by scanning electron microscopy. It was found that crystallites of barium cerate arise within the initial particles of the oxalate precursor.

Top-Seeded Infiltration Growth and Structure of YBCO Bulk Superconductors

The conditions for epitaxial growth of single-grain YBCO bulk superconductors at isothermal conditions were studied. The size of epitaxially grown grain and the size and number of parasitic grains as a function of growth temperature were measured. The way the CeO2 addition influences the microstructure of the bulk single crystal was also studied. It is shown that CeO2 has more positive effect on the size of Y211 particles when added to the Y211 pellet than when added to the source of the melt.

Influence of BaCeO3 and BaO2 Addition on the Microstructure of Y-123/Y-211 Bulk Superconductors

YBCO (YBa2Cu3O7/ Y2BaCuO5) bulk single-grain superconductors were prepared by Top-Seeded Melt-Growth (TSMG) process. CeO2, BaCeO3 and BaO2 powders were added to nominal composition Y1.5Ba2Cu3Oy (Y-123) with the aim to refine Y2BaCuO5 (Y-211) secondary particles, which provide pining. Added powders were refined by friction milling. The system behaviour was characterized by thermal analyses. Microstructure of prepared samples was studied by polarized light microscopy and the influence of additions is described.

The Influence of Y211 Milling Conditions on the Microstructure of Y123/Y211 Bulk Superconductors

The YBCO bulk single-grain superconductors were prepared by Top-Seeded Melt-Growth (TSMG) process. Y2BaCuO5 (Y211) fine powder was added to YBa2Cu3O7-δ (Y123) nominal composition. Powder refinement was characterized by X-ray powder difractometry, laser granulometry and scanning electron microscopy. The system behaviour was characterized by thermal analysis. Microstructure of prepared samples was studied by polarized light microscopy. The influence of Y211 addition the sample microstructures are shown.