Anisotropy of the irreversibility field for Zr-doped $(Y,Gd)B{a}_{2}C{u}_3{O}_{7-x}$ thin films up to 45T

Abstract

The anisotropic irreversibility field B${}_{Irr}$ of two $YB{a}_{2}C{u}_3{O}_{7-x}$ thin films doped with additional rare earth (RE)=(Gd,Y) and Zr and containing strong correlated pins (splayed BaZrO${}_3$ nanorods, and $R{E}_2{O}_3$ 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 $B_{Irr}(\theta )=B_{Irr}(0)(co{s}^2\theta +{\gamma }^{-2}si{n}^2\theta {)}^{-1/2}$, where $\gamma =(m_c/m_{ab}{)}^{1/2}=5-6$ for the RE-doped $YB{a}_{2}C{u}_3{O}_{7-x}$ (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 $B_{Irr}(\theta )\propto (co{s}^2\theta +{\gamma }_{RP}^{2}si{n}^2\theta {)}^{1/2}$ 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.

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