This project investigates the fabrication and analysis of superconducting-ferromagnetic-superconducting (SFS) Josephson junctions with antiferromagnetic chromium (Cr) spacers. The study compares structures with and without copper (Cu) layers to explore the role of spin-triplet Cooper pairs in enabling non-dissipative spin currents for superconducting spintronics. Experimental results reveal symmetric phase shifts in critical current under magnetic fields, unresolved magnetization dynamics in Cr/Fe layers, and the mitigating effect of Cu spacers on spin-orbit coupling suppression.
- Two Junction Groups:
- Group A: Includes Cu layers encapsulating Cr (
Nb/Cu/Cr/Fe/Cr/Cu/Nb). - Group B: No Cu layers (
Nb/Cr/Fe/Cr/Nb).
- Group A: Includes Cu layers encapsulating Cr (
- Fabrication Methods: Photolithography, sputtering (Nb, Al, Fe, Cr, Au), ion milling, and e-beam lithography.
- Measurements:
- SQUID-based critical current ((I_c)) vs. magnetic flux ((\Phi)) at 4.2 K.
- Fraunhofer patterns modeled using Bessel functions for elliptical junctions.
- Key Observations:
- Symmetric phase shift in (I_c) during field sweeps.
- Absence of critical current suppression in upper sweeps (unresolved magnetization dynamics).
- Cu spacers enhance triplet coherence by reducing direct coupling to Nb.
- Substrate: Silicon wafer with Nb/Al base electrodes.
- Sputtering: Sequential deposition of superconducting (Nb), ferromagnetic (Fe), and antiferromagnetic (Cr) layers.
- Lithography: Photolithography for patterning and e-beam lithography for fine features.
- Ion Milling: Etching to define junctions and isolate electrodes.
- SQUID Setup: Custom cryogenic system for (I_c)-(\Phi) dependence.
- Fraunhofer Analysis: Airy function modeling for elliptical junctions.
- Critical Current Behavior:
- Symmetric phase shifts indicate spin-triplet dominance.
- Lack of suppression in upper sweeps suggests spin-glass interfacial dynamics.
- Cu Layer Impact:
- Group A (with Cu) shows larger (I_c) and preserved triplet coherence.
- Group B (without Cu) exhibits suppressed (I_c) due to spin-orbit coupling.
The study demonstrates unconventional spin-rotation at F/AF/S interfaces, highlighting the potential of spin-triplet currents for superconducting spintronics. Future work could explore domain-resolved magnetization measurements and atomistic modeling of interfacial dynamics.
- Gingrich et al., Nature Physics 12, 564–567 (2016).
- Komori et al., Science Advances 7, eabe0128 (2021).
- Jeon et al., Nature Materials 17, 499–503 (2018).
- Korucu et al., Applied Physics Letters 124, 242603 (2024).
Special thanks to Prof. Dr. Birge, Dr. Alex Madden, and the MSU Condensed Matter Physics Department for guidance and facility access. Cleanroom training support from Josh Willard, Robert M. Klaes, and Dr. Bi Baokang is gratefully acknowledged.