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...

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Ag-doped MgB2 bulk superconductors have been prepared using a standard solid state processing. The addition of Ag to MgB2 powders during the sintering process has been found to result in an important advantage, namely, the prevention/reduction of loss of Mg, a problem most commonly observed in the sintering of MgB2 bulk samples at elevated temperature and ambient pressures. The Ag-doped MgB2 sample has a distinct superconducting transition temperature around 39 K, while the undoped MgB2 undergoes only a very feeble transition to a diamagnetic superconducting state at around 39 K. The normal conducting silver regions in the MgB2 matrix act as pinning centres resulting in the realization of high critical currents in the presence of magnetic fields.

Complex-oxide materials exhibit physical properties that involve the interplay of charge and spin degrees of freedom. However, an ambipolar oxide that is able to exhibit both electron-doped and hole-doped ferromagnetism in the same material has proved elusive. Here we report ambipolar ferromagnetism in LaMnO3, with electron–hole asymmetry of the ferromagnetic order. Starting from an undoped atomically thin LaMnO3 film, we electrostatically dope the material with electrons or holes according to the polarity of a voltage applied across an ionic liquid gate. Magnetotransport characterization reveals that an increase of either electron-doping or hole-doping induced ferromagnetic order in this antiferromagnetic compound, and leads to an insulator-to-metal transition with colossal magnetoresistance showing electron–hole asymmetry. These findings are supported by density functional theory calculations, showing that strengthening of the inter-plane ferromagnetic exchange interaction is the origin of the ambipolar ferromagnetism. The result raises the prospect of exploiting ambipolar magnetic functionality in strongly correlated electron systems.

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