Proximity Effects in YBa2Cu3O7 /La0.7Sr0.3MnO3 Heterostructures

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CHINESE JOURNAL OF PHYSICS VOL. 42, NO. 5 OCTOBER 2004

J. G. Lin,1, 2, ∗ _{S. L. Cheng,}1, 2 _{P. C. Kuo,}2, 3 _{C. R. Chang,}2, 4 _{and D. Y. Xing}5 1_{Centre of Condensed Matter Science, National Taiwan University, Taipei 106, Taiwan}

2_{Nanostorage Research Center, National Taiwan University, Taipei 106, Taiwan} 3_{Department of Material Sciences and Engineering, National Taiwan University,}

Taipei 106, Taiwan

4_{Department of Physics, National Taiwan University, Taipei 106, Taiwan} 5_{National Laboratory of Solid State Microstructures and Department of Physics,}

Nanjing University, Nanjing 210093, China (Received December 30, 2003)

YBa2Cu3O7/La0.7Sr0.3MnO3(YBCO/LSMO) heterostructures with different thicknesses

of LSMO (dLSMO) were fabricated, and their electrical transport properties were investigated.

It was found that for dLSMO ≤40 nm the LSMO films are non-magnetic insulators, and the

YBCO/LSMO systems remain superconducting; while for dLSMO ≥50 nm the LSMO films

become ferromagnetic metals, and the YBCO/LSMO systems lose their superconductivity, due to the strong proximity effect of ferromagnetism in LSMO. A new approach based on the proximity effect is proposed to measure the intrinsic normal-state resistivity below Tc

for high-Tc cuprates.

PACS numbers: 75.30.Vn, 75.70.Ak

Superconductor/ferromagnet (SC/FM) heterostructures were fabricated early in the eighties of the twentieth century [1]. As an SC film was grown on top of an FM film, the proximity effect governs the physical properties of the SC/FM structure [2, 3]. Pre-vious studies [4] showed that a physical condition for the coexistence of ferromagnetism and superconductivity within a bulk material requires comparable magnitudes of the fer-romagnetic exchange energy Eex and the superconducting energy gap ∆. However, this

condition can not be realized in a bulk FM, where Eex is typically at least 2 orders of

magnitude larger than ∆. Recently, it was suggested [5] that this condition could be less critical if Cooper pairs in the SC diffuse across the FM/SC interface into the FM in an FM/SC proximity structure. The density of states with superconducting characteristics has been observed in the FM region near the PdNi/Nb interface [6], implying a coexis-tence of weak ferromagnetism and s-wave superconductivity [6]. On the other hand, the pair-breaking effect, due to the diffusion of spin-polarized electrons from the FM into the SC, will be enhanced with an increase in the spin polarization of the conduction elec-trons in the FM, which has seldom been discussed. Cuprate/manganite heterostructure research has received much attention, since the discoveries of high temperature supercon-ductivity in doped cuprates and then later the colossal magnetoresistance (CMR) effect

http://PSROC.phys.ntu.edu.tw/cjp 643 c 2004 THE PHYSICAL SOCIETY OF THE REPUBLIC OF CHINA