Study Of YBa₂Cu₃O_7-? Composite Superconducting Films Fabricated By Magnetron Sputtering And Pulsed Laser Co-deposition Method

Transport properties of high temperature superconducting materials in magnetic field are strongly related to the defects in superconducting materials.Doping of non-supconducting materials can introduce artificial pinning centers in the superconductors,which can effectively improve the transport property.The BaZrO₃:YBa₂Cu₃O_7-??BZO:YBCO?,MgO:YBa₂Cu₃O_7-??MgO:YBCO?,and BiFeO₃:YBa₂Cu₃O_7-??BFO:YBCO?superconducting films have been successfully prepared on?001?SrTiO₃?STO?substrates using magnetron sputtering and pulsed laser co-deposition method?MSPLC?,and the structural and physical properties have been investigated.Ar gas has to be introduced into the MSPLC chamber in order to provide Ar ions used for sputtering bombing.YBCO thin films with T_c⁰⁹0 K have been fabricated epitaxially on?001?STO substrates by pulsed laser deposition method in O₂/Ar mixture atmosphere.It is found that the introduction of Ar gas can effectively reduce the surface particle density of the YBCO film.BZO:YBCO,MgO:YBCO composite films with different volume percents are fabricated epitaxially on?001?STO substrates by MSPLC method.It is found that less doping does not impact T_c⁰ of the BZO:YBCO,MgO:YBCO composite films.Based on the dependence of critical current density J_c and flux pinning force density F_p on magnetic field at various temperatures,it is found that BZO,MgO dopants,which act as pinning centers,can effectively improve the critical current densities of the YBCO films.It is shown that at temperatures of 40 K and 65 K,the dominating pinning mechanism should be normal core surface pinning with the characteristic parameters of p⁰.5,q² for the low doping samples,and normal core point pinning with the characteristic parameters of p¹,q² for the high doping samples.The dominating pinning mechanism is normal core surface pinning at temperature of 77 K.TEM results show the epitaxially relation of YBCO[00l]//STO[00l].The defects acting as surface pinning effect are mainly stacking faults in the pure YBCO film,and the defects acting as point pinning effect are the randomly distributed BZO nanoparticles in BZO:YBCO composite films.Based on the decrease in T_c at higher doping concentrationofnon-supconductingmaterialinYBCOfilm,multi-layer?BFO:YBCO/YBCO?_N composite films were prepared on?001?STOsubstrates in O₂:Ar=1:3 mixture atmosphere by MSPLC,providing an approach for improving T_c and J_c of supconducting composite films under magnetic fields.Epitaxial BaFe_1.8Co_0.2As₂ superconducting films were prepared on?00l?STO single crystal substrates by pulsed laser deposition.The maximum T_c⁰¹6 K was obtained for the epitaxial film prepared at 670?.The hysteresis loops at room temperature?H//c?indicate antiferromagnetic behavior for the BaFe_1.8Co_0.2As₂ target,and ferromagnetic behavior for the BaFe_1.8Co_0.2As₂ films deposited at different temperatures.With the increase of deposition temperature,the saturation magnetization M_s increased from 7.88 emu/cm³ to 240.5 emu/cm³ monotonously,however,coercive field H_s increase first and then decreases.This can be attributed to the effect of size,shape,and stress of the Fe grains in the BaFe_1.8Co_0.2As₂ film.XPS analysis shows Fe⁺22 exists in BaFe_1.8Co_0.2As₂ target,but Fe and Fe⁺22 with almost the same content found in the BaFe_1.8Co_0.2As₂ film deposited at 670?.This study provides the evidence of coexistence of both ferromagnetism and superconductivity.