Switching at small magnetic fields in Josephson junctions fabricated with ferromagnetic barrier layers

Nb-based Josephson junctions have been fabricated, which can select one of two states depending on the relative magnetization of their ferromagnetic barrier layers. To minimize the free-layer switching energy, while maintaining adequate thermal stability at 4.2 K, a dilute Cu-permalloy alloy [Cu _0.7(Ni₈₀Fe₂₀)_0.3] with a low magnetic saturation (M_s ∼ 80 emu/cm³) is used. The optimal thickness of the permalloy (Ni₈₀Fe₂₀) fixed-layer is shown to be 2.4 nm. Such devices exhibit switching at magnetic fields as low as 5 Oe, demonstrating their potential use in low power non-volatile memory for superconductor digital circuits.