Abstract
The anisotropic irreversibility fieldBIrr of twoYBa2Cu3O7−x thin films dopedwith additional rare earth (RE)=
(Gd, Y) and Zr and containing strong correlated pins (splayed BaZrO3 nanorods and RE2O3 anoprecipitates) has been measured over a very broad range up to 45 T at temperatures 56 K < T < Tc. We found that the experimental angular dependence of BIrr (θ) does not follow the mass anisotropy scaling BIrr (θ) = BIrr (0)(cos2 θ + γ−2 sin2 θ)−1/2, where γ = (mc/mab)1/2 = 5−6 for the RE-doped Ba2Cu3O7−x (REBCO) crystals, mab and mc 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 BIrr , while at intermediate angles, BIrr (θ) follows the scaling behavior BIrr (θ) ∝ (cos2 θ + γ−2 RP sin2 θ)−1/2 with the effective anisotropy factor γRP ≈ 3 significantly smaller than the ass anisotropy would suggest. In spite of the strong effects of c-axis BaZrO3 nanorods, we found even greater enhancements of BIrr 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 BIrr 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 BIrr for B//c increases as the temperature increases due to the suppression of thermal fluctuations of vortices by splayed distribution of BaZrO3 nanorods.