Inverse proximity effect in superconductors near ferromagnetic material

– We study the electronic density of states in a mesoscopic superconductor near a transparent interface with a ferromagnetic metal. In our tunnel spectroscopy experiment, a substantial density of states is observed at sub-gap energies close to a ferromagnet. We compare our data with detailed calculations based on the Usadel equation, where the effect of the ferromagnet is treated as an effective boundary condition. We achieve an excellent agreement with theory when non-ideal quality of the interface is taken into account. Proximity effect (PE) is a phenomenon where the order parameter of a material in an ordered phase leaks to another material which has no such order. A number of experiments have investigated the PE in superconducting normal (S-N) mesoscopic structures [1]. Here a finite pairing amplitude is formed in the N side, with the decay length ξN = √ h̄D/2πkBT set by the dephasing of correlations between electrons and Andreev-reflected holes having slightly different momenta. At typical experimental conditions at low temperatures, the effect can survive up to large distances of several hundred nanometers. The experiments in non-magnetic metals have been generally well understood within the framework of the quasiclassical theory of inhomogeneous superconductivity [2]. Another example of a proximity effect occurs in systems composed of ferromagnetic (F) and nonmagnetic materials, where the spin polarization leaks from F [3], but its decay length is typically very small. Going further, one may also consider the combination of two differently ordered materials, such as superconductors and ferromagnets. In this Letter, we study the mutual proximity effects of superconducting and ferromagnetic materials by measuring the local density of states in the superconducting side of the SF interface formed by superconducting Al and ferromagnetic Ni. Since the ferromagnetic and singlet superconducting order parameters try to exclude each other, the direct proximity effect is expected to get suppressed [4] such that the pairing amplitude diffusing into a ferromagnet should decay over microscopic distances ξF = √ h̄D/2πkBTCurie due to the considerable difference in the momenta of spin-up and spin-down quasiparticles. A spectroscopy experiment (∗) E-mail: [email protected]