Superconducting and normal-state interlayer exchange coupling in La_0.67Sr_0.33MnO₃-YBa₂Cu₃O₇-La_0.67Sr_0.33MnO₃ epitaxial trilayers

Abstract

The issue of interlayer exchange coupling in magnetic multilayers with a superconducting spacer is addressed in La_0.67Sr_0.33MnO₃ (LSMO)-YBa₂Cu₃O₇ (YBCO)-La_0.67Sr_0.33MnO₃ (LSMO) epitaxial trilayers through resistivity, ac susceptibility, and magnetization measurements. The ferromagnetic LSMO layers possessing in-plane magnetization suppress the critical temperature (T_c) of the c-axis oriented γBCO thin film spacer. The superconducting order, however, survives even in very thin layers (thickness d_Y~50 λ, ~ four unit cells) at T lt;25k. A predominantly antiferromagnetic exchange coupling between the moments of the LSMO layers at fields lt;200 Oe is seen in the normal as well as the superconducting states of the γBCO spacer. The exchange energy J₁ (~0.08 erg/cm² at 150 d&k for d_Y=75 λ) grows on cooling down to T_c, followed by truncation of this growth on entering the superconducting state. The coupling energy J₁ at a fixed temperature drops exponentially with the thickness of the γBCO layer. The temperature and δ_γ dependencies of this primarily nonoscillatory J₁ are consistent with the coupling theories for systems in which transport is controlled by tunneling. The truncation of the monotonic T dependence of J₁ below T_c suggests inhibition of single-electron tunneling across the CuO₂ planes as the in-plane gap parameter acquires a nonzero value.