Coupling of superconductors through a half-metallic ferromagnet: Evidence for a long-range proximity effect

Abstract

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...

Spin diffusion versus proximity effect at ferromagnet/superconductorLa0.7Ca0.3MnO3∕YBa2Cu3OPeña1, Visani2, García‐Barriocanal3 et al. 2006Phys. Rev. BSelf Cite541251View full textAdd to dashboardCite

“…19,[27][28][29] In particular transmission electron microscopy shows flat interfaces over nanometer length scales. 29 X-ray refinement techniques using the SUPREX 9.0 software were used to obtain quantitative information about the interface roughness at longer length scales. 30 T c was measured from four-contact resistance measurements as the zeroresistance temperature in linear scale resistance plots.…”

Induced triplet pairing in cleans-wave superconductor/ferromagnet layered structures

“…The observed effects are over length scales much larger than those of the usual SF proximity effect and more like the much longer length scales associated with the standard proximity effect between a superconductor and a normal nonmagnetic metal. These observations include measurements in superlattices 13 with ferromagnetic spacers and SQUIDs 14 with ferromagnetic interlayers. Superconducting characteristics, such as a critical temperature and enhanced sub-gap conductance, have been observed in point contact conductance measurements on s-wave superconductor/half metallic systems.…”

Paramagnetic Meissner effect inYBa2Cu3O7∕La0.7Torre1, Peña2, Sefrioui3 et al. 2006Phys. Rev. BSelf Cite353171View full textAdd to dashboardCite

“…For the samples studied in this work, x-ray diffraction ͑XRD͒ and transmission electron microscopy ͑TEM͒ experiments were performed showing atomically sharp interfaces with little step disorder and negligible interdiffusion. 15,16 For every sample the transition temperature was determined from results of supplementary electrical resistivity measurements performed in a closed cycle refrigerator. Hysteresis loops, dc magnetization and ac magnetization versus temperature were measured using a Quantum Design MPMS superconducting quantum interference device magnetometer.…”