Anisotropy of the irreversibility field for Zr-doped (Y,Gd)Ba₂Cu₃O_(7-x) thin films up to 45T

The anisotropic irreversibility field B$_{Irr}$ of two $YBa_2Cu_3O_{7-x}$ thin films doped with additional rare earth (RE)=(Gd,Y) and Zr and containing strong correlated pins (splayed BaZrO$_{3}$ nanorods, and $RE_2O_3$ nanoprecipitates), has been measured over a very broad range up to 45T at temperatures 56 K<T<$T_c$. We found that the experimental angular dependence of $B_{Irr}(\theta)$ does not follow the mass anisotropy scaling $B_{Irr}(\theta)=B_{Irr}(0)(cos^2\theta+\gamma^{-2}sin^2\theta)^{-1/2}$, where $\gamma=(m_c/m_{ab})^{1/2}=5-6$ for the RE-doped $YBa_2Cu_3O_{7-x}$ (REBCO) crystals, m$_{ab}$ and m$_{c}$ are the effective masses along the ab plane and the c-axis, respectively, and $\theta$ is the angle between B and the c-axis. For B parallel to the ab-planes and to the c-axis correlated pinning strongly enhances B$_{Irr}$, while at intermediate angles, $B_{Irr}(\theta)$ follows the scaling behavior $B_{Irr}(\theta)\propto(cos^2\theta+\gamma_{RP}^2sin^2\theta)^{1/2}$ with the effective anisotropy factor $\gamma_{RP}\approx3$ significantly smaller than the mass anisotropy would suggest. In spite of the strong effects of c-axis BaZrO$_{3}$ nanorods, we found even greater enhancements of B$_{Irr}$ for fields along the ab-planes than for fields parallel to the c-axis, as well as different temperature dependences of the correlated pinning contributions to B$_{Irr}$ for B//ab and B//c. Our results show that the dense and strong pins, which can now be incorporated into REBCO thin films in a controlled way, exert major and diverse effects on the measured vortex pinning anisotropy and the irreversibility field over wide ranges of B and T. In particular, we show that the relative contribution of correlated pinning to B$_{Irr}$ for B//c increases as the temperature increases due to the suppression of thermal fluctuations of vortices by splayed distribution of BaZrO$_{3}$ nanorods.