Critical current oscillations of elliptical Josephson junctions with single-domain ferromagnetic layers

Abstract

Josephson junctions containing ferromagnetic layers are of considerable interest for the development of practical cryogenic memory and superconducting qubits. Such junctions exhibit a ground-state phase shift of π for certain ranges of ferromagnetic layer thickness. We present studies of Nb based micron-scale ellipticallyshaped Josephson junctions containing ferromagnetic barriers of Ni 81 Fe 19 or Ni 65 Fe 15 Co 20 . By applying an external magnetic field, the critical current of the junctions are found to follow characteristic Fraunhofer patterns, and display sharp switching behavior suggestive of single-domain magnets. The high quality of the Fraunhofer patterns enables us to extract the maximum value of the critical current even when the peak is shifted significantly outside the range of the data due to the magnetic moment of the ferromagnetic layer. The maximum value of the critical current oscillates as a function of the ferromagnetic barrier thickness, indicating transitions in the phase difference across the junction between values of zero and π. We compare the data to previous work and to models of the 0-π transitions based on existing theories.

Spin-valve Josephson junctions for cryogenic memory

“…The value of a is only 15% smaller than the value of 5.0 mT/nm obtained in Ref. 17, while the latter is about 15% smaller than we would estimate by using the Ni thin film magnetization data shown in Fig. 1(a) of Ref.…”

“…Measurements of current-voltage characteristics (I-V curves) were performed using a battery-operated current source and a SQUID-based self-balancing potentiometer with a voltage noise of a few pV/ √ Hz. 17,32 As shown in the inset to Fig. 2(a), the SQUID-based potentiometer has a voltage limit of less than 1µV ; hence the full shapes of the I-V curves for I > I c are not visible for junctions with large values of critical current (I c ) and normal-state resistance (R N ), such as those in this study.…”

“…First, the Crouzy theory implicitly assumes that the length scales governing the 0-π oscillations and the exponential decay of the supercurrent with F-layer thicknesses are the same, whereas our data on S/F/S junctions containing only NiFe show a difference between those lengths. 17 Given the rather small range of NiFe thicknesses in our junctions, this constraint in the Crouzy theory should not present a major problem. Second, it is instructive to compare our results with those of Baek et al 13,14 Those authors studied spin valve junctions containing a Ni layer of variable thickness and a Nb-doped NiFe layer of fixed thickness.…”

“…Those workers were the first to confirm the theoretical prediction 3,4 that the ground-state phase difference across an S/F/S Josephson junction could be either zero or π, depending on the thickness of the ferromagnetic F-layer inside the junction. Oscillations in the magnitude of the critical current as a function of F-layer thickness, signifying oscillations between 0 and π junctions, have now been demonstrated with a wide variety of ferromagnetic materials, including CuNi alloy, 1,[5][6][7] PdNi, 2,8 Ni, 9-14 Ni 3 Al, 15 Co, 11 Fe, 11 NiFe, 11,16,17 NiFeMo, 18 PdFe, 19 and NiFeCo.…”

“…17 In addition, most of the electron-pair phase accumulation in the Baek samples occurs in the Ni layer, while the weakly-magnetic NiFeNb layer only shifts the positions of the 0-π transitions slightly to the left or right along the Ni thickness axis. In our work, the strong magnet NiFe contributes substantially to the accumulated phase shift.…”