[en] This work first deals with problems concerning accurate measurements on SQUID magnetometers and offers solutions relevant to a quick and reproducible orientation method of tiny single crystals that might be done even under liquid nitrogen. Given models to obtain the critical current density J_c(B) from magnetization measurements had to be reconstructed due to the sample shape and anisotropy effects. Taking into account the average induction B inside the sample a new evaluation procedure has been developed. Fast neutron irradiation of YBa₂Cu₃O_7-δ single crystals has proved to result in large increases of critical current densities and pinning forces. In addition the very anisotropic shift of the irreversibility line and the dependence of the critical temperature T_c on the neutron fluence have been observed. One single crystal was exposed to fast neutrons at low temperatures and its magnetization measured without warming up to more than 95 K. The increase of critical currents in this crystal was significantly larger than in those irradiated at ambient reactor temperature (ca. 50 deg C). Only part of this difference was removed during annealing at room temperature. This experiment referred to possible applications in future fusion reactors. The flux pinning mechanism in neutron-irradiated YBa₂Cu₃O_7-δ is deduced to depend on an interaction between the pre-irradiation defect structure and the intrinsic pinning mechanism on one side, and the neutron-induced cascades and point defect clusters on the other side

[en] Fast neutron irradiation of YBCO single crystals has proved to result in significant increases of magnetization hysteresis and consequently an enhancement of critical current densities. Shifts of the irreversibility line have been observed. Both of these observations are discussed in terms of an interaction of three different flux pinning mechanisms. The results near T_c prove directly the existence of strong intrinsic pinning, when H is applied parallel to the Cu-O layers. Significant changes of the magnetization curves following irradiation with fast neutrons have been observed in various high-T_c single crystals. The large enhancements of hysteresis in YBCO single crystals as well as considerable shifts of the irreversibility line in YBCO and BiSSCO single crystals have been investigated to some extent. In order to explain the experimental results, an interaction of radiation-induced pinning centers with the original pinning structures had to be taken into account. More detailed information on these effects can be expected from samples irradiated to lower neutron fluences. Hence, a new set of experiments was started, where a single crystal of YBCO was irradiated with fast neutrons, starting at a fluence of 5x10²⁰ m^-2 and gradually increasing the fluence between each measurement. In this work the authors report on these detailed experiments, in which magnetization curves as well as irreversibility lines have been measured with the field oriented both parallel and perpendicular to the ab-planes of the crystal. From these data and from a new TEM study, a qualitative picture of flux pinning effects as a function of neutron fluence could be developed. To explain the highly anisotropic magnetization curves, especially at high temperatures, and the anisotropic shifts of the irreversibility lines with neutron fluence, the existence of intrinsic pinning along the Cu-O planes has to be invoked

[en] The irreversibility line of high-T_c superconductors is shifted considerably by irradiating the material with fast neutrons. The anisotropic and non-monotonous shift is qualitatively explained by a simple model based on an interaction between three pinning mechanisms, the intrinsic pinning by the ab-planes, the weak pinning by the pre-irradiation defect structure, and strong pinning by neutron induced defect cascades. A correlation between the cascade density and the position of the irreversibility line is observed. (author)

[en] YBCO single crystals with very sharp transitions at T_c were subjected to reactor neutron irradiation at ambient temperature. The magnetization curves, which were measured in a SQUID magnetometer in fields up to 8 T, show increasing irreversibility and decreasing anisotropy with neutron fluence. Bean-model evaluations of critical current densities are presented and discussed in terms of induced flux pinningpinning centers

[en] The irreversibility line of high-T_c superconductors is shifted considerably by irradiating the material with fast neutrons. The anisotropic and non-monotonous shift is qualitatively explained by a simple model based on an interaction between three pinning mechanisms, the intrinsic pinning by the ab-planes, the weak pinning by the pre-irradiation defect structure, and strong pinning by neutron induced defect cascades. A correlation between the cascade density and the position of the irreversibility line is observed

[en] The critical current densities of a single crystal of YBCO were evaluated from magnetization measurements with the field parallel and perpendicular to the c-axis. The dependence of J_c^ab;H//c and of J_c^c;H//c on neutron fluence is found to be very anisotropic. This is discussed in a qualitative manner in terms of the summation of the elementary pinning forces to the macroscopic pinning force, which defines the critical current densities. We find that they are affected by the competitive influence of the anisotropic properties of the flux line lattice and the pin density. (orig.)

[en] Neutron irradiation of high-T_c single crystals in the fluence range up to 2 x 10²² m² (E > 0.1 MeV) has been found to lead to the following property changes: 1. The critical temperature decreases only moderately at a rate of approximately 4 K per 10²² neutrons per m²; 2. The critical current densities J_c as deduced from magnetization measurements on the basis of an extended Bean model increase significantly; 3. These J_c-enhancements are larger at higher temperatures; 4. The anisotropy of J_c decreases significantly with neutron fluence; and 5. The irreversibility line in BiSCCO-2212 (Bi₂Sr₂CaCu₂O₈) is shifted to much higher fields and temperatures. (author)

[en] Flux-pinning effects were studied by sequentially irradiating a YBa₂Cu₃O_7-δ single crystal to fast neutron fluences of 8x10²¹ m^-2. From superconducting quantum interference device magnetometry, the transition temperature T_c and the magnetic moments were obtained for fields parallel and perpendicular to the crystallographic c axis. Irreversibility lines and critical current densities were evaluated as a function of neutron fluence. In order to explain the data, a qualitative model is developed that is based on the layered structure of the system and the nature of the various defects introduced by irradiation. Because of the dominating influence of large defect cascades on flux pinning, different summation models have to be employed, depending on temperature, field orientation, field strength, and pin density

[en] In the present contribution we will report on a systematic investigation of fast neutron irradiation effects carried out in the fluence range from 2x10²¹ to 2x10²² m^-2 (E>0.1 MeV) on two high-quality single crystals of YBa₂Cu₃O_7-δ. Furthermore, we will present results of an irradiation experiment on a Bi₂Sr₂CaCu₂O₈ single crystal, which has demonstrated the feasibility of flux pinning at temperatures and magnetic fields beyond the 'irreversibility' line for the first time. (orig./BHO)

[en] We report on the influence of fast neutron irradiation on the critical currents in single crystalline YBa₂Cu₃O_7-δ and Bi₂Sr₂CaCu₂O_8+γ. From magnetization measurements and especially from an analysis of the irreversibility lines, models for the defect structure, effective for flux pinning in these systems, are derived. A comparison of fast neutron irradiation of YBa₂Cu₃O_7-δ at 5 K (and subsequent measurement without warming the sample above 95 K) and at ambient reactor temperature allows us to distinguish qualitatively between the contributions of defects of different size to flux pinning in these systems. (author)