The superconductor/ferromagnet/heterostructure owns exotic physical properties, which provides a platform to explore the interaction between superconductivity and ferromagnetism. What’s more, it is predicted to as huge application prospect in the field of superconducting spintronic and superconducting quantum computing. Especially, due to the Fermi vector mismatch of electrons, the wave functions of Cooper pairs oscillate in the ferromagnets (Fig. A). Thus, the ferromagnetic Josephson junctions based on the SC/FM heterostructures were thought as the effective way to realize the unconventional π-Josephson junctions. Meanwhile, the 0-π Josephson junction transitions can be modulated via the accuracy control of the thickness of ferromagnetic layer (Fig. B and C). However, previous reports about ferromagnetic Josephson junctions were mainly focused on the electronic transport measurement. For the spin dynamics, it has not been explored yet.

In order to furtherly investigate the spin dynamics of 0- and π-Josephson junctions, recently, ICQM Prof. Wei Han group collaborated with Prof. Xincheng Xie, and Prof. See-Hun Yang of IBM Research, and Dr. Tao Yu of Max-Planck Institute, successfully observed the giant oscillatory Gilbert damping in ferromagnetic JJs with varying the thickness of ferromagnetic thin films (Fig. D). By furtherly experimental demonstration and theoretical analysis, the superconducting quasiparticles and spin triplet Cooper pairs are rule out for the possibility to dissipate spin currents. And the zero-energy Andreev bound states are proposed as the dominated spin carriers in the system. It also demonstrates that the efficiency of Andreev bound states to dissipate spin currents are modulated effectively by the 0-π transitions of Josephson junctions.

The results provide a new perspective for understanding the spin dissipation in superconductors. Furthermore, it could pave the way for the application of the quantum devices based on the ferromagnetic Josephson junctions in the field of superconducting spintronics and flux qubit quantum computing.

These results are published online in Science Advances tilted “Giant oscillatory Gilbert damping in superconductor/ferromagnet/superconductor junctions” on November 26, 2021. This work was done by two PhD students Yunyan Yao and Ranran Cai in ICQM, who serves as the co-first authors. Prof. Wei Han and See-Hun Yang are the corresponding authors of the paper. This work has been supported by the National Science Foundation of China, the Ministry of science and technology, the Chinese Academy of Sciences, and Beijing National Science Foundation.

Paper Link: https://www.science.org/doi/10.1126/sciadv.abh3686

Fig. A: Schematic of the wave function of Cooper pair near the SC/FM interface oscillates in the FM layer, where the zero-phase and π-phase are formed. Fig. B-C: Illustration of the spin currents excited by FMR inject into the Andreev Bound States in the zero- and π-Josephson junctions. Fig. D: The Gilbert damping difference (∆α) oscillates giant as the thickness of NiFe increasing.