Fabrication Of BaHfO₃ Doped Y_0.5Gd_0.5Ba₂Cu₃O_7-x Superconducting Films By Pulsed Laser Deposition

REBa₂Cu₃O_7-?(RE:rare element,REBCO)high temperature superconductor has a wide range of applications due to its high irreversibility field(H_irr)and high critical current density(J_c)in high magnetic field.However,for high magnetic field applications,it remains a problem how to improve the in-field superconducting properties.The introduction of artificial flux pinning centers is an effective method.In this paper,we introduced Ba Hf O₃(BHO)artificial flux pinning centers and fabricated BHO doped YGBCO superconducting films by pulsed laser deposition(PLD)technique.We first investigated the effect of BHO doping concentration on the structure and superconducting properties of YGBCO films,and then adopted a seed layer method to see how it improves the property of the superconducting films.(1)The effect of BHO doping concentration on YGBCO superconducting layerBHO doped Y_0.5Gd_0.5Ba₂Cu₃O_7-x(YGBCO)films with a series of doping concentration(1 mol%,3 mol%,5 mol%and 10 mol%)were fabricated on Ce O₂buffer layer by PLD,and un-doped YGBCO film was also deposited at the same conditions for comparison.The X-Ray diffraction(XRD)results showed that un-doped and doped YGBCO films had pure c-axis orientation.While surface morphology and crystal quality of doped YGBCO films deteriorated as the BHO content increased,as SEM results showed.The critical current density J_c at 77 K and self-field and the critical temperature T_c of BHO doped YGBCO films exhibited varying degrees of decline in comparison with un-doped YGBCO film.However,5mol%BHO doped YGBCO film exhibited greatly enhanced in-field J_c and the highest pinning force densities F_p at both 4.2 K and 65 K which showed the maximum value of 5776 GN/m³(4.2 K,H//c)and 117 GN/m³(65 K,H//c),about 1.5 times that of the un-doped YGBCO film.Based on the results,the 5 mol%BHO doped YGBCO films had the most effective flux pinning centers,which are responsible for strongly improved critical current densities in magnetic fields.(2)Improving the property of YGBCO superconducting by a seed layertechniqueWe then introduced the seed layer technique on the basis of the optimal 5 mol%doping concentration.We inserted a seed layer with different thickness(1 nm and 2nm)and different chemical composite(5 mol%BHO doped YGBCO and pure YGBCO)and compared with samples without a seed layer.XRD patterns show that all samples have a consistent c-axis orientation,moreover,samples with a 2 nm seed layer almost have no internal stress.SEM images indicate that the surface becomes rougher when the seed layer thickness increases.The introduction of seed layer increased J_c^77,sf.It is worthy to mention that the optimal thickness of the seed layer is2 nm which is nearly equivalent to the root mean square(RMS)of Ce O₂buffer layer(RMS=1.87 nm).Besides,the self-seed layer is more effective than pure YGBCO seed layer.At 4.2 K,the 5 mol%BHO doped YGBCO film with 2 nm thick self-seed layer had a very high F_p of 860 GN/m³ for B//c under a 9 T field,and more importantly,the peak of the F_p curve was not observed.