We show magnetoresistance in excess of 1000% in trilayers containing highly spin-polarized La 0:7 Ca 0:3 MnO 3 and high-T c superconducting YBa 2 Cu 3 O 7 . This large magnetoresistance is reminiscent of the giant magnetoresistance (GMR) in metallic superlattices but with much larger values, and originates at spin imbalance due to the injection of spin-polarized carriers. Furthermore, in contrast to ordinary GMR, the magnetoresistance is intimately related to the superconductivity in the YBa 2 Cu 3 O 7 layer and vanishes in the normal state. This result, aside from its fundamental importance, may be of interest for the design of novel spintronic devices based on ferromagnet/superconductor structures.

We study the interplay between magnetism and superconductivity in high-quality YBa 2 Cu 3 O 7 (YBCO)/La 0.7 Ca 0.3 MnO 3 (LCMO) superlattices. We find evidence for the YBCO superconductivity depression in the presence of the LCMO layers. We show that due to its short coherence length, superconductivity survives in the YBCO down to a much smaller thickness in the presence of the magnetic layer than in low T c superconductors. We also find that for a fixed thickness of the superconducting layer, superconductivity is depressed over a thickness interval of the magnetic layer in the 100 nm range. This is a much longer length scale than that predicted by the theory of ferromagnetic/superconducting proximity effect. DOI: 10.1103/PhysRevB.67.214511 PACS number͑s͒: 74.78.Fk, 74.50.ϩr, 75.70.Cn The ferromagnetic (F)/superconducting ͑S͒ proximity effect has been a subject of intense research in recent years due to the rich variety of phenomena resulting from the competition between both long range orderings. In this context F/S superlattices have been extensively used in the past because they offer the possibility of tailoring individual thicknesses or modulation length to match characteristic length scales governing ferromagnetism, superconductivity, or their interaction. Most research in this field has involved single element or alloy-based metallic superlattices.1-9 The extension of concepts of the F/S proximity effect to the high-T c superconductors ͑HTS͒ or colossal magnetoresistance ͑CMR͒ oxides is of primary interest since peculiarities like the short superconducting coherence length and full spin polarization could open the door to interesting new effects. Although there has been a theoretical effort recently to examine the F/S interface in oxides, 10 to the best of our knowledge experimental results on the F/S proximity effect are lacking in the literature. In this paper we examine the interplay between magnetism and superconductivity in YBa 2 Cu 3 O 7 (YBCO)/La 0.7 Ca 0.3 MnO 3 (LCMO) superlattices and provide evidence for superconductivity depression due to the presence of magnetic layers. YBCO and LCMO have oxide perovskite structure with very similar in-plane lattice parameters, which allows the growth of superlattices with sharp interfaces, thus strongly reducing extrinsic ͑structural͒ effects which otherwise could obscure the F/S interplay.At the F/S interface, Cooper pairs entering the ferromagnet from the superconductor experience the exchange interaction, which favors one of the spin orientations. This causes the superconducting order parameter to decay in the F layer faster than in a normal metal, within a length scale F ϭបv F /⌬E ex ͑where v F is the Fermi velocity and ⌬E ex is the exchange splitting͒. In single element or alloy ferromagnets, for typical values of ⌬E ex ϭ1 eV and v F of 10 8 cm/s, F is of the order of 1 nm ͑Ref. 3͒, which is shorter than the superconducting coherence length of the low-temperature superconductors ͑usually larger than 10 nm͒. Superconductivity is also depressed in the...

Here we examine the ferromagnetic/superconducting proximity effect in half-metallic ferromagnetic La 0.7 Ca 0.3 MnO 3 and high-T c superconducting YBa 2 Cu 3 O 7 artificial structures. We have found experimental evidence for the coupling between superconducting layers through ferromagnetic spacers in superlattices. This is consistent with a long-range proximity effect in half-metal ferromagnet/d-wave superconductor structures. It is well known that in superconductor (S)/normal (N) structures superconducting pairing may occur deep into the normal metal.1 If the normal metal is a ferromagnet (F), its exchange field reduces drastically the length scale for the proximity effect, 2 and it should be completely suppressed 3 in the limiting case of a fully spin polarized ferromagnetic/ singlet superconductor structure. Here we investigate this issue using a high-temperature superconductor YBa 2 Cu 3 O 7 ͑YBCO͒ and a spin polarized ferromagnet La 0.7 Ca 0.3 MnO 3 ͑LCMO͒. The interplay between magnetism and superconductivity in hybrid structures involving colossal magnetoresistance and high-T c superconducting oxides has gathered considerable interest in recent years.4 Scanning tunneling spectroscopy 5 and tunneling magnetoresistance 6 have shown that the LCMO is essentially half metallic (HM). LCMO and YBCO have similar in-plane lattice parameters (0.3% mismatch) which allows heteroepitaxial growth with little interface disorder. [7][8][9] We find a long-range proximity effect, which yields coupling between superconducting layers through 10-nm thick HM ferromagnetic layers. These LCMO/YBCO coupled superlattices represent a class of artificially layered materials showing "coexistence" of spinpolarized ferromagnetism and superconductivity over macroscopic length scales.In F / S structures the transfer of Cooper pairs into the ferromagnet occurs via the Andreev reflection.10 Electrons with an energy lower than the superconducting gap are reflected back as holes with opposite spin orientation. The interference between electron and hole wave functions gives rise to the Andreev bound states which carry the supercurrent. Energy conservation requires that Cooper pairs entering a ferromagnet with an exchange field energy h acquire a finite momentum ⌬p = v F / h where v F is the Fermi velocity.This causes the superconducting wave function to be oscillating and to decay with a characteristic length scale F = ͑D /2h͒ 1/2 , where D is the diffusion coefficient. 2,11 This length is in the nanometer range for common single element or alloy ferromagnets and is typically one to three orders of magnitude smaller than the normal metal coherence length in N/S junctions. 12,13 In a ferromagnet with different number of spin-up n↑ and spin-down n↓ conduction channels only a fraction n ↓ / n↑ of the majority channels can be Andreev reflected. 2 Thus, Andreev reflection is completely suppressed for a fully spin polarized ferromagnet (HM) and accordingly the F / S proximity effect, i.e., superconductivity and magnetism should not mix. This is not...

The structure of high quality ͓YBCO N ͞PBCO M ͔ 1000 ± A superlattices, with N ranging between 1 and 12 unit cells and M 5 unit cells, grown by high oxygen pressure sputtering, is analyzed. Intracell atomic structure of the layers along the c axis and disorder at interfaces is investigated using an x-ray refinement technique. Negligible roughness, step disorder, and interdiffusion are found at the interfaces. Epitaxial mismatch strain results in a surprising reorganization of interatomic distances for the thinnest YBCO layers, which seems to correlated with the decrease in the critical temperature. Intracell structure is invoked as an additional source of T c changes in very thin YBCO layers. PACS numbers: 74.76.Bz, 61.10.Nz, 68.65. + g Since the discovery of the high T c superconductivity, structure has been recognized to play a crucial role towards the understanding of its nature and mechanisms. It has been known for years that distortions arising from cation substitution can produce significant changes in T c [1], and recent experiments on doped La 2 CuO 4 superconductors at constant carrier concentration show a clear dependence of T c on lattice strains [2]. A great effort has been put in structure determination under hydrostatic pressure [3]. Epitaxial stress in thin films offers a simple way to arrive at a strain pattern not attainable under hydrostatic pressure [4]: According to the Poisson effect, film growth on a substrate with slightly smaller (larger) in-plane lattice parameters may lead to a compression (expansion) in the ab plane that can result in an expansion (contraction) in the out-ofplane direction. Uniaxial epitaxial strain, together with Poisson's ratios, has been addressed before [5]. However, the general applicability of the Poisson effect to thin films is still doubtful [6], especially in these highly anisotropic materials. Anyway, Locquet et al. [7] have been able to double the critical temperature in the La 1.9 Sr 0.1 CuO 4 high T c superconductor using mismatch strain. They show that compressive epitaxial strain in-plane can generate much larger increases in T c than those obtained by comparable hydrostatic pressures, and their claim is that the distance relevant to the mechanism of the superconductivity being modified is the separation between consecutive CuO 2 planes. Mismatch strain constitutes an alternative way to change the intracell distances which may be "relevant" to the mechanism of superconductivity, but a quantitative structure analysis of strained films is necessary. X-ray diffraction is a widely used technique to analyze structure, which supplies structural information averaged over a length scale (structural coherence length) which may be around a hundred angstroms. The extraction of quantitative information requires the fit of the diffraction pattern to a structure model containing a large number of parameters in these complex materials, and, therefore, results may not be very reliable for single epitaxial films, which usually show a reduced number of diffraction peak...

We report on the depression of the superconducting critical temperature of ultra superlattices. The thickness of the manganite layer is kept at 15 unit cells and the YBCO thickness is varied between N=12 and N=1 unit cells. The structural analysis using x-ray diffraction and electron microscopy shows sharp interfaces with little structural disorder. While a critical temperature, Tc=85 K is found for 12 YBCO unit cells, superconductivity is completely suppressed for YBCO layer thickness below 3 unit cells. The possible interaction between superconductivity and magnetism is investigated.2 Ferromagnetic (F) / superconductor (S) heterostructures have recently attracted much interest for applications in spin injection (three terminal) devices [1]. High Tc superconductors (HTS) and colossal magnetoresistance (CMR) oxides are interesting candidate materials because the low carrier density of the HTS and the almost full spin polarization of the CMR oxides can be combined to yield high sensitivity (gain) fast devices. A reduction of the critical current consistent with suppression of superconductivity by spin polarized quasiparticle injection has been reported by several groups in recent years [2,3], opening the door to practical devices based on complex oxides. The samples reported so far involve quite thick (50-100 nm ) YBCO layers, thus shadowing interface effects. However, interface properties are expected to play a dominant role in the physics of CMR/HTS F/S heterostructures, and extrinsic (interface alloying or roughness) or intrinsic factors (proximity effect) may deeply influence the performance of the devices. The use of superlattices (instead of bilayers) allows an in depth characterization of the interfaces with conventional structure probes like x-ray diffraction (XRD) or transmission electron microscopy (TEM). The presence of magnetism and superconductivity in this kind of samples has been reported before [4,5].In this letter we report on the growth of LCMO/YBCO superlattices with ultrathin (1 to 12 unit cells) YBCO layers and fixed LCMO thickness (15 unit cells), to investigate how robust is the superconductivity of the YBCO when its thickness is reduced in presence of magnetic layers. We have found that superconductivity is depressed in presence of the adjacent LCMO layers. A structural analysis with XRD and TEM is used to explore the influence of interface disorder on the depression of the superconductivity.Samples were grown in a high pressure (3.4 mbar) pure oxygen sputtering system at high temperatures (900 ºC). Individual YBCO films on STO (100) given the small thickness of the layer is consistent with the absence of interdiffusion.At this point it seems very unlikely that the systematic depression of the critical temperature when the YBCO thickness is reduced might result of extrinsic factors like deoxygenation or roughness. Another source of Tc depression in ultrathin layers is the reduced dimensionality [9]. In this context, we compare the depression of the critical temperature when the Y...