M. Baran

The influence of fast neutron irradiation on the intra- and intergrain properties of the polycrystalline BiPbSrCaCuO system

Abstract The influence of irradiation by fast neutrons with fluences from 3.3 x 10 16 n / cm 2 up to 3 x 10 18 n / cm 2 on the physical properties of polycrystalline Bi 0.7 Pb 0.3 SrCaCu 1.8 O x was examined. Studies of DC magnetization, AC susceptibility, transport and thermoelectric power were performed. The irradiation caused a decrease of T c , determined from the onset of diamagnetism, by as much as 31 K for a fluence of 3 x 10 18 n / cm 2 . A strong influence of neutron irradiation on both intra- and intergranular properties was observed. The defects within the superconducting grains created by neutrons caused an increase of the pinning forces which enhanced the critical magnetization current. A gradual decoupling of Josephson weak links with increasing neutron fluence was observed in transport and low field magnetization measurements. From the AC susceptibility measurements the irreversibility lines between the flux-creep and flux-flow regions were determined. An increase of the absolute values of thermoelectric power with rising fluence was noticed.

A new High-Tc ferromagnet: Manganese-doped CdGeAs2 chalcopyrite

New ferromagnets with high Curie temperatures, reaching 355 K, have been obtained by doping a GdGeAs2 semiconductor compound with manganese. The obtained compounds are magnetically inhomogeneous, comprising a mixture of ferromagnetic (FM) and paramagnetic phases. The volume fraction of the FM phase increases with the degree of doping. The development of ferromagnetism in this system is probably related to the presence of vacancies of the (Cd, VC, Mn)GeAs2 type or to a nonstoichiometry of the (Cd, Ge, Mn)GeAs2 type, since theoretical estimates show that the FM state is energetically more favorable than the spin glass state only in these cases.

Magnetotransport properties of flux melt grown single crystals of Co-substituted manganites with perovskite structure

Single crystals of La1−xBaxMn1−yCoyO3 (0.26 ≤ x ≤ 0.37; 0 ≤ y ≤ 0.31) perovskites were grown by the flux melt technique using the BaO–B2O3–BaF2 ternary system as a solvent. It was found that the cobalt content in the crystals is determined by the average oxidized state of the manganese ions and depends nonlinearly on the molar ratio of Co/Mn in the initial mixture. All the crystals exhibit ferromagnetic behaviour at low temperatures as well as a jump of resistivity and a magnetoresistance peak around the Curie temperature, gradually decreasing as the cobalt content increases. The low-temperature ferromagnetic phase transforms from the metallic to the semiconducting state with increase in cobalt content.

Synthesis and magnetic properties of Cu3B2O6 single crystals

The temperature dependence of the magnetic susceptibility of Cu3B2O6 single crystals grown by spontaneous crystallization from a melt consisting of a mixture of CuO and B2O3 and the behavior of their magnetization are investigated in magnetic fields up to 55 kOe. A broad susceptibility maximum is observed near 39 K, and a sharp drop in susceptibility is observed at T<10 K. The paramagnetic Néel temperatures for all orientations of the magnetic field in the crystal investigated are negative, attesting to the predominantly antiferromagnetic character of the exchange interactions. The effective magnetic moment of the Cu2+ ion is anisotropic and lies in the range from 1.054μB to 1.545μB. The magnetization depends linearly on magnetic field at T>10 K, whereas at temperatures below 10 K a discontinuity is observed at fields of the order of 40 kOe. At room temperature, electron magnetic resonance characterized by an almost isotropic g factor (g=2.165) is detected at 36.22 GHz. The exchange interactions in Cu3B2O6 are analyzed on the basis of the Goodenough-Kanamori rules. The possibility of the establishment of a singlet magnetic state in the crystal is analyzed.

Effects of irradiation — fast neutrons and implantation on sintered YBaCuO superconductors

Abstract Effects of irradiation damage by fast neutrons, and nitrogen implantation on magnetic and electrical properties of Y Ba Cu O materials have been studied. The samples were subjected to a fluence of 2 × 10 17 and 1 × 10 17 neutrons / cm 2 . In the implantation studies dosages of 10 15 and 10 16 ions/ cm 2 of N + were used. Here we will report on the detailed changes in the magnetic hysteresis loop due to irradiation. It is found that the transport inter-grain critical current decreases with irradiation, while the critical current inside the grains is enhanced.

Enhancement of critical current density in fast neutron irradiated melt-textured YBa2Cu3O7-x☆

Abstract The critical current density in melt-textured samples obtained by the powder melting process was determined from magnetization measurements. Linear dependence between the width of the hysteresis loop and sample size was observed for both unirradiated and irradiated samples. This indicates that the critical current is circulating through the whole sample and is not disconnected by weak links, even when a magnetic field is applied in the irradiated sample. After fast neutron irradiation with fluences from 5 × 10 16 to 6 × 10 17 n cm −2 ( E > 0.5 MeV), significant enhancement of the critical current density, j c , was observed. Critical current density, determined from magnetization measurements, for magnetic field perpendicular to the a–b plane, j c ab , reaches − 10 5 A cm 42 at 77 K in 1 T. For H parallel to the a–b plane, j c c along the c-axis reaches 5 × 10 3 A cm −2 . An increase in the anisotropy of the critical current was observed after fast neutron irradiation in the temperature range 60 – 80 K.

Transport and magnetic properties of the new cerium ternary Ce-Pt-Ge compounds

The new cerium ternary compounds Ce/sub 3/Pt/sub 4/Ge/sub 6/ and Ce/sub 2/Pt/sub 7/Ge/sub 4/ with novel crystal structure types have been prepared and characterized. We present transport and magnetic properties of these compounds in temperature range (2-300) K. Behavior of resistivity and magnetisation is similar, both compounds demonstrate Kondo-like features and possible magnetic transition near the helium temperature. >

A new method for growing Ba1−xKxBiO3 single crystals and investigation of their properties

Large single crystals of the copper-free Ba 1-x K x BiO 3 (BKBO) superconductor have been successfully grown using modified method of electrochemical deposition. For the first time we employed the method of growth where a BaBiO 3 single crystal plate was used as a seed. A BKBO layer with (100) orientation and ∼ 1 mm thick grown on the substrate was cut off and thereafter used as a secondary seed. We used a dynamic method, i.e., a crystal holder with reciprocating motion to homogenize the potassium distribution over the crystal volume. The maximum volume of the as-grown crystals reached ∼ 2 cm 3 with the FWHM of the Bragg reflection curves not exceeding 1°. The lattice structure of the Abrikosov vortices as well as the character of defects of the Ba 0.6 K 0.4 BiO 3 single crystal, caused by the growth technique, has been studied by small-angle neutron scattering. From SQUID magnetisation measurements, the growth-induced anisotropy of hysteresis loops and pinning force of the second-order superconductor are reported.

Flux pinning in HgBa2Ca2Cu3O8+x single crystals

Single crystals of HgBa2Ca2Cu3O8+x, with Te onset=120 K, were grown using a gas high pressure system. Magnetization studies were performed in a wide range of temperature and in magnetic fields up to 5 T using a dc SQUID magnetometer. The irreversibility line was determined from the magnetization measurements. On a log(Hirr) versus log(1-Tirr/Tc) plot the irreversibility line shows two slopes. At higher temperatures the irreversibility line has rather three-dimensional character, whereas at lower temperatures the irreversibility line exhibits two-dimensional character.

Field and temperature dependences of anisotropic magnetic susceptibility of CaNdAlO/sub 4/ crystal

The measurements of temperature dependences of magnetic susceptibility both in (a-b) plane, and along c-axis of CaNdAlO/sub 4/ single crystal have been performed. The strong uniaxial anisotropy of magnetic properties has been found. The low field susceptibility along c-axis is well described by the Curie law, while the susceptibility in (a-b) plane can be described by Curie-Weiss law suggesting the presence of antiferromagnetic interactions between neodymium ions. However, the anisotropy of magnetic susceptibility can also appear to be due to crystal field acting on magnetic ions in a system without any exchange interaction. The successful description of experimental data was done in frames of the crystal field approximation. >