DEPENDENCE OF THE CRITICAL TEMPERATURE OF YBCO THIN FILMS ON SPINPOLARIZED QUASIPARTICLE INJECTION

Heterostructures consisting of ferromagnetic La_2/3Ca_1/3MnO₃ and superconducting YBa₂Cu₃O_7-x layers were fabricated by conventional pulsed laser deposition in conjunction with shadow mask techniques to define appropriate sample geometries. Using standard dc- four probe measurements we observe a 2% decrease of T_c for injection current densities of 5.10^-2 A/cm² in the case of unpolarized quasiparticles whereas highly spin polarized quasiparticles show a much stronger decrease of 14%. This result clearly demonstrates that the spin plays a crucial role in the T_c reduction. The results are discussed within the frame of a theoretical model based on the Owen-Scalapino approach for nonequilibrium effects in superconductors.