Study of ferromagnetism and superconductivity in layered YBCO/LCMO thin films.

摘要

The superconductivity and ferromagnetism of YBCO/LCMO layered films have been examined, in which YBCO (YBa₂Cu₄O_δ) exhibits high superconducting transition temperature (T_c), and LCMO (La_2/3Ca_1/3MnO_δ) exhibits colossal magnetoresistance (CMR).; The coexistence of superconductivity (S) and ferromagnetism (F) is shown by CMR effect and superconducting transition in R ∼ T curves and the broadened hysteresis loops at T T_c. Growth and magnetic proximity effects play the important roles for the T_c depression in LCMO/YBCO/LCMO sandwiches. An indirect exchange interaction between adjacent LCMO layers through the spacer YBCO was suggested by magnetic anisotropy and the oscillatory behaviors of metal-semiconductor transition temperature, resistance and MR with a large period. The spin fluctuations in YBCO and LCMO layers may play a role for this behavior. On the other hand, the interlayer coupling of YBCO layers in YBCO/LCMO/YBCO sandwiches results in the oscillatory T_c with increasing LCMO layer thickness due to the interface roughness, the inelastic electron scattering, the magnetic spin-flip scattering, or the appearance of so-called π-phase. Furthermore, single layer LCMO and LCMO/YBCO/LCMO sandwiches show easy in-plane anisotropy. A nearly sinusoidal dependence of MR on the magnetic field direction Θ and Φ ( = (H,Θ,Φ)) is observed. A positive MR is present with lower fields applied not only out-of-plane but also in-plane in sandwich films. Although LCMO can destroy the superconductivity of YBCO, the aging tests of YBCO films fully covered by LCMO show that LCMO film forms a good passivation layer, protecting YBCO film from reacting with water.