Chemically Introduced Disorder Effects on the Critical Current Density and Pinning Mechanisms of $\text{YBa}_{2-x}\text{Sr}_{x}\text{Cu}_{3}\text{O}_{7-\delta}$

We report on isotherm $(T=77.5\ {K})$ dc magnetization hysteresis loops of a series of $\text{YBa}_{2-x}\text{Sr}_{x}\text{Cu}_{3}\text{O}_{7-\delta}$ ( $x=0$ , 0.02, 0.1, 0.25, and 0.37) single crystals with the purpose of studying the influence of chemically introduced site disorder on the $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ critical current density $J_{c}(B,T)$ and normalized pinning force density $f(h)$ . The Sr ion chemical disorder is inserted at $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ structure by lattice distortion. Transmission electronic microscopy of the doped samples shows a structure constituted by a high density of twins, which is, probably, decorated by many small precipitates. The $J_{c}(B,T)$ transported by the doped samples with $x\le 0.10$ is higher than that transported by the $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ pure sample. In contrast, the $J_{c}(B,T)$ transported by the doped samples with $x$ > 0.10 is significant and systematically lower than that transported by the undoped sample. The analysis of the $f(h)$ plots indicates, in the light of the Dew–Hughes model, that the preponderant pinning mechanism of $\text{YBa}_{2-x}\text{Sr}_{x}\text{Cu}_{3}\text{O}_{7-\delta}$ single crystals with $x\le 0.10$ is the core normal point type. However, to the samples with $x > 0.10$ , the $f(h)$ analyses indicate that the preponderant pinning mechanism undergoes a crossover from the core normal point type to the core normal surface type. This crossover of pinning mechanism dynamics could be appointed as the possible reason of the decrease of the $J_{c}(B,T)$ observed in the samples with $x > 0.10$ .