Magnetoresistance anisotropy and scaling in textured high-temperature superconductor Bi$_{1.8}$Pb$_{0.3}$Sr$_{1.9}$Ca$_2$Cu$_3$O$_x$

The magnetoresistance of the textured high-temperature superconductor (HTSC) Bi$_{1.8}$Pb$_{0.3}$Sr$_{1.9}$Ca$_2$Cu$_3$O$_x$ + Ag has been studied at different directions of the transport current $\mathbf{I}$ and external magnetic field $\mathbf{H}$ with respect to crystallographic directions of HTSC crystallites. When $\mathbf{I}$ and $\mathbf{H}$ are oriented along the $ab$ planes of crystallites and $\varphi$ is the angle between $\mathbf{H}$ и $\mathbf{I}$, the anisotropic part of the magnetoresistance follows the functional dependence $\sin^2\varphi$, which is characteristic of vortex flows under Lorentz forces. The magnetoresistance $R$ at $\mathbf{H}$ parallel to the $c$ axis of crystallites ($\mathbf{H}\parallel c$) is higher than $R$ at $\mathbf{H}\parallel ab$ for both cases of $\mathbf{I}\parallel c$ and $\mathbf{I}\parallel ab$. The anisotropy coefficient $\gamma\approx$ 2.3 has been estimated from the scaling of the dependences $R(H)$ measured at $\mathbf{H}\parallel c$ and $\mathbf{H}\parallel ab$. The inclusion of the magnetic field induced by the transport current allows scaling of the dependences $R(H)$ at different values of $I$. A qualitative picture of the current flow along the $c$ axis of crystallites in the textured HTSC has been proposed.