Temperature dependence of magnetic-field angle dependent critical current density and the flux pinning in YBa2Cu3O7 thin films

Abstract

The magnetic-field angle dependence of the critical current density Jc(H, θ) was measured over a wide temperature range (20–85K) in high-Jc YBa2Cu3O7 (YBCO) and DyBa2Cu3O7 (DyBCO) epitaxial films belonging to two categories. The films in the first category were prepared by thermal co-evaporation and were characterized by a high density of nanometer-sized precipitates (mostly<7nm), which act as small random (point) pins. The films in the second category were prepared by fluorine-free metalorganic deposition (FF-MOD), whose primary pinning centers were recently revealed to be dislocations (linear pins) associated with stacking faults parallel to the ab-plane. Relatively broad Jc(θ) peaks around H//ab in the shape of a stratovolcano were observed at 77–80K in the co-evaporated YBCO and DyBCO films in the first category, and the Jc(H, θ) data were scaled to be a single curve when plotted against scaled fields using an effective anisotropy parameter. However, the Jc(θ) curves became more gradual at 60–70K and the scaling behavior was broken because the radius of nano-precipitates was not sufficiently smaller than the temperature dependent Ginzburg–Landau coherence length ξ(T) at these temperatures. In contrast, sharp Jc(θ) peaks around H//ab were observed in the FF-MOD YBCO films in the second category at all temperatures in the range 20–85K. Anisotropic scaling analysis showed that the pinning was apparently due to small random pins and pins correlated with the ab-plane, and the linear pins whose radius is sufficiently smaller than ξ(T) can cause the observed Jc(H, θ). The temperature dependence of Jc due mainly to the random pinning was approximately ∼(1−T/Tc)2(1+T/Tc)2 for the FF-MOD thin films (dislocation pins), which is consistent with a simple theoretical model based on core pinning interactions. The T dependence of Jc for the co-evaporated films (nano-precipitate pins) was ∼(1−T/Tc)m (1+T/Tc)2 (m=2.1–2.6), which deviates from m=2.5 expected from the simple theoretical model because nano-precipitates are not sufficiently smaller than ξ(T) at T⩽70K.