Gy. Kovács

Stress-induced magnetic anisotropy in nanocrystalline alloys

Abstract Stress-annealing experiments were extended to both nanocrystalline alloy families, Finemet and Nanoperm (Hitperm), and, for comparison, to amorphous Fe 62 Nb 8 B 30 alloy. For both Finemet and bulk amorphous, stress-annealing results in a strong induced transversal anisotropy (flattening of hysteresis loop) but yields longitudinal induced anisotropy (square hysteresis loop) in Nanoperm and Hitperm. These results are interpreted in terms of back-stress theory.

High Tc superconductivity of a TlBaCaCuO compound

Abstract A TlBaCaCuO 4.5+ x compound has been investigated from the point of view of superconductivity. Depending on the heat treatment, one part of the samples exhibits superconductivity with an onset of 121 K and a zero resistivity of 106 K, while the other part of them shows only a sharp drop in resistivity at 130 K which hints at the existence of superconducting domains. This picture was confirmed by magnetic and ESR measurements giving possibility for estimation of the critical magnetic fields.

Preparative chemistry consequences on YBaCuO superconducting compounds

Abstract A new way of two-stage synthesis has been elaborated for preparation of Y Ba Cu O superconductors. In the first step, a new Ba 2 Cu 3 O 5 double-metal oxide has been established from which a wide composition range of Y 1 Ba 2 Cu 3 O 7−d Y 1 Ba 17 Cu 25.5 O x (x ≈ 44±d) superconductors have been prepared by addition of Y 2 O 3 . Resistivity and microwave measurements hint at an existence of two superconducting phases.

Spin splitting of the Landau levels and exchange interaction of a non-ideal two-dimensional electron gas in heterostructures

Shubnikov - de Haas oscillations corresponding to the spin-resolved Landau levels of the two-dimensional electron gas at an heterointerface were studied in wide ranges of magnetic fields (up to 22 T) and tilt angles. Dependences of the spin splitting on the parallel component of magnetic field were investigated for half-filled Landau levels , , , and . The exchange interaction was shown to be strongly dependent on the broadening and overlapping of adjacent Landau levels. On the basis of this model, the experimental magnetic field dependences of were described. As a result, the absolute value of bare g-factor , exchange energy and Landau level broadening were obtained.

Preparation dependent superconductivity in TlCaBa2Cu3O7.5±d above 100 K

Different heat treatment procedures were applied during sample preparation, which resulted in different superconducting properties in samples of the same nominal composition, TlCaBa2Cu3O7.5±d. This manifests itself in different critical temperatures: 104 K, 107 K and 93 K. The highestTc belongs to the best-crystallized samples in which the tetragonal Tl2Ca1Ba2Cu2O8±d phase is responsible for the superconductivity. The effect of heat treatments is reflected in the structural and magnetic properties as well.

Non-bulk superconductivity induced by neutron irradiation in multiphase YBa2Cu3O7 and Tl2Ca1Ba2Cu2O8

Abstract Irradiation effects of low-dose (10 14 n/cm 2 ) neutrons on the resistance and AC susceptibility of multiphase YBa 2 Cu 3 O 7 and Tl 2 Ca 1 Ba 2 Cu 2 O 8 have been studied. Experimental results show that dispersed local superconducting regions in the samples may be unified to form a continuous percolating network using neutron irradiation. If the concentration of oxygen vacancies and interstitials is below or above an optimum value, the irradiation increases or decreases, respectively, the superconductive volume fraction and - via Cu-O bond length - the T c value.

Structure and superconductivity of variously prepared Tl-Ca-Ba-Cu-O compounds

Abstract Starting materials, compositions and heat treatments were varied to study the influence of preparation conditions on the structure and superconductivity of Tl-Ca-Ba-Cu-O compounds. Twenty-four different groups of samples were synthesized, and their phases, structures, superconducting parameters and inter-relations were investigated. Most of the samples are superconducting above 100 K. The critical temperatures occupy a rather wide interval of 61–121 K. A few specimens have no superconductivity down to liquid helium temperatures. The number, type, quantity and proportion of phases produced strongly depend on the chemical quality and mixture of the starting materials and on the heat treatment. We searched for all known phases which may be responsible for superconductivity. It has been observed that better superconductivity generally occurs for the more highly crystalline structures. In our experiments, the presence of non-reacted barium cuprates in the samples does not obstruct the establishment of superconductivity, but Tl2O3, CaO, BaO, CuO and other impurities disturb and limit the quality of superconductivity. The superconducting parameters are also affected by the oxygen stoichiometry. The resistivity-temperature curves of some samples indicate the presence of a two-phase system. A change with time of the microstructure of some samples was observed, and was reflected in macroscopic change of the superconducting parameters. This is supported by NMR investigations. Scanning tunneling microscopy indicates an oriented surface structure of the samples.

Construction and parameters of the first high Tc, superconducting ceramic magnets for small fields

Abstract High T c superconducting magnets have been constructed and built of individual ceramic rings. For the elimination of problems of power dissipation on the resistance of electrical contacts, energy was supplied by a contactless magnetic induction method. The magnetic fields produced by two systems of rings were 16.1 and 20.7 mT. Other types of high T c magnets have been built based on superconducting ceramic spirals supplied by d.c. current, which produces a magnetic field of 0.15 and 3.2 mT.

Experimental investigation of anomalous magnetoresistance in monocrystalline lead sulphide

Investigation of galvanomagnetic effects was carried out in the temperature range of 1.5–350 K. The samples were made ofn- andp-type PhS, cut off in different orientations and rotated in the magnetic field.The reproductibility of the measurement and the stability of the temperature was better than 0.1%.The results show in the entire temperature range that the magnetoresistance contrary to accepted assumptions cannot be expressed in terms of only even powers of the magnetic field strength and a negative change appears as well.Calculating the electron effective mass more precisely considering the electron motion in thek-space the experimental results could be explained in a simple way in the whole temperature range.

Effect of starting materials on the critical parameters of superconducting Tl Ca Ba Cu O compounds

Abstract Using different batches of starting materials (samples I, TINO 3 , CaO, Ba 2 Cu 3 O 5 ; II, TINO 3 , CaO, BaO, CuO; III, TINO 3 , CaO, Ba(NO 3 ) 2 , CuO; IV, TINO 3 , CaO, BaCuO 2 ; V, TINO 3 , CaO, Ba(NO 3 ) 2 , CuO, O 2 ) and the same heat treatment process, high T c superconducting systems of nominal composition TICaBaCuO 4.5 were prepared. Different superconducting phases were formed depending on the starting compounds used. Specimens I and IV had a single-phase (2,1,2,2) character. In samples II and III the dominant (2,1,2,2) phase was accompanied by (2,0,2,1) and by (1,2,2,3) and (2,2,2,3) phases in small quantity, respectively, while specimens V can be characterized by the existence of CuO and Cu 2 O beside the chief (2,1,2,2) phase. The superconducting properties were affected by starting materials, which manifests itself in the difference in onset and critical temperatures, lower and upper critical magnetic fields, paramagnetic limits, coherence length, penetration depth and Ginzburg - Landau parameter of specimens of different kinds. Due to the weak coupling created by interstitial material, different intergrain, intragrain and direct transport critical currents were determined, depending on the type of samples. The main aim of this work was to compare the experimental data with the corresponding theories to draw conclusions on the role of the starting compounds.