Linear Crack Arrays and Resistive Anisotropy in Nd$_{0.2}$Sr$_{0.8}$Mn$O_3$ Thin Films Under Tensile Strain$^{\ast}$

The structure, morphology, and electrical properties of epitaxial $a$-axis oriented thin films of Nd$_{0.2}$Sr$_{0.8}$Mn$O_3$ are reported for thicknesses $10\leq t\leq 150$~nm. Films with $t\geq 20$~nm grown under tensile stress on NdGaO$_3$ (100) and LSAT (110) substrates develop uniform linear crack arrays (cracks running along film $c$ axis) with a crack spacing (0.3-10~$\mu$m) that decreases with increasing thickness. Films grown under compression on LaAlO$_3$(110) substrates exhibit no cracks. The room-temperature in-plane electrical resistance ratio, $\rho_b/\rho_c$, increases approximately exponentially with increasing film thickness to values of $\sim 1000$ in the thickest films studied. The temperature dependencies for $\rho_b$ and $\rho_c$ are essentially identical, suggesting that very long effective transport paths perpendicular to the cracks are responsible for enhanced values of $\rho_b$. \vskip .1in \noindent $^{\ast}$ This material is based upon work supported by the National Science Foundation under grants DMR-0072276 (Univ. Miami) and DMR-0504769 (Montana State Univ.), the Research Corporation (Univ. Miami), and the U.S. DOE Office of Basic Energy Sciences (Grant No. DE-FG-06ER46269).