The anisotropic irreversibility field $B_{Irr}$ of two YBa${}_2$Cu${}_3$O${}_{7-\mathrm{x}}$ thin films doped with additional rare earth (RE) $=$ (Gd, Y) and Zr and containing strong correlated pins (splayed BaZrO${}_3$ nanorods and RE${}_2$O${}_3$ nanoprecipitates) has been measured over a very broad range up to 45 T at temperatures 56 K < T < T${}_{\mathrm{c}}$. We found that the experimental angular dependence of $B_{Irr}$ (θ) does not follow the mass anisotropy scaling $B_{Irr}\left(\theta \right)=B_{Irr}\left(0\right){\left({\cos }^2\theta +{\gamma }^{-2}{\sin }^2\theta \right)}^{-1/2}$, where $\gamma ={(m_c/m_{ab})}^{1/2}=5-6$ for the RE-doped YBa${}_2$Cu${}_3$O${}_{7-\mathrm{x}}$ (REBCO) crystals, $m_{ab}$ and $m_c$ are the effective masses along the $ab$ plane and the $c$-axis, respectively, and θ 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, $B_{Irr}$(θ) follows the scaling behavior $B_{Irr}\left(\theta \right)\propto {\left({\cos }^2\theta +{\gamma }_{RP}^{-2}{\sin }^2\theta \right)}^{-1/2}$ with the effective anisotropy factor ${\gamma }_{RP}\approx 3$ 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.

• Received 15 November 2011

DOI:https://doi.org/10.1103/PhysRevB.84.224514