Superconducting loops containing a π-junction are predicted to show a spontaneous magnetic moment in zero external magnetic field. In order to confirm this longstanding prediction experimentally, we performed magnetization measurements on individual mesoscopic superconducting niobium loops with a ferromagnetic (PdNi) π-junction. The loops are prepared on top of the active area of a micro Hall-sensor based on high mobility GaAs/AlGaAs heterostructures. We observe switching of the loop between different magnetization states at very low-magnetic fields, which is asymmetric for positive and negative sweep direction. This is evidence for a spontaneous current induced by the intrinsic phase shift of the π-junction. In addition, the presence of the spontaneous current at zero applied field is directly revealed by an increase of the magnetic moment with decreasing temperature, which results in a half integer flux quantization in the loop at low temperatures.
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This work is dedicated to H. von Löhneysen on the occasion of his 60th birthday.
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Bauer, A., Bentner, J., Aprili, M. et al. Half-Integer Flux Quantization in a Superconducting Loop with a Ferromagnetic π-Junction. J Low Temp Phys 147, 431–439 (2007). https://doi.org/10.1007/s10909-007-9328-7
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DOI: https://doi.org/10.1007/s10909-007-9328-7