Change of strength of vortex pinning in YBCO due to BaZrO_3 inclusions
We probe the short-range pinning properties with the application of microwave currents at very high driving frequencies (47.7 GHz) on YBa$_2$Cu$_3$O$_{7-δ}$ films with and without sub-micrometer BaZrO$_3$ inclusions. We explore the temperature and field ranges 60 K$<T<T_c$ and 0$<μ_0H<$0.8 T, with the field applied along the c-axis. The magnetic field induces a much smaller increase of the microwave resistivity, $Δρ_1(H)+\mathrm{i}Δρ_2(H)$, in YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$ with respect to pure YBa$_2$Cu$_3$O$_{7-δ}$. $Δρ_1(H)$ is slightly superlinear in pure YBa$_2$Cu$_3$O$_{7-δ}$ (suggesting a possible contribution of thermal activation), but linear or sublinear in YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$ (suggesting a possible suppression of thermal activation as a consequence of BaZrO$_3$ inclusions). These features persist up to close to $T_c$. We discuss our data in terms of the ratio $r=ΔX_s'(H)/ΔR_s'(H)$ in the framework of the models for the microwave surface impedance in the mixed state. Large $r$ are found in YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$, with little field dependence. By contrast, smaller values and stronger field dependences are found in pure YBa$_2$Cu$_3$O$_{7-δ}$. We discuss the different field dependence of the pinning constant.