Magnetic-field dependence of the reversible axial-strain effect in Y-Ba-Cu-O coated conductors

The critical-current density J_c of an yttrium-barium-copper-oxide (YBCO) coated conductor deposited on a biaxially-textured Ni-5at.%W substrate was measured at 76.5 K as a function of axial tensile strain ε and magnetic field B applied parallel to the YBCO (a,b) plane. Reversibility of J_c with strain was observed up to ε≃0.6% over the entire field range studied (from 0.05 to 16.5 T), which confirms the existence of an intrinsic strain effect in YBCO coated conductors. J_c vs. ε depends strongly on magnetic field. The decrease of J_c(ε) grows systematically with magnetic field above 2-3 T, and, unexpectedly, the reverse happens below 2 T as this decrease shrinks with increasing field. The pinning force density F_p=J_c×B scaled with field for all values of strain applied, which shows that F_p can be written as K(T,ε)b^p(1-b)^q, where p and q are constants, K is a function of temperature and strain, b=B/B_c2^* is the reduced magnetic field, and B_c2^* is the effective upper critical field at which F_p(B) extrapolates to zero.