Magnetism and superconductivity out of equilibrium: Towards ultrafast superconducting spintronics

The integration of superconductors in spintronic devices can enhance their performance. For instance, superconducting spintronics may enable the development of suitable memories for cryogenic computing systems, which ideally should operate at a higher speed than current technologies. Femtosecond laser pulses are natural candidates to attain such ultrafast functioning. This thesis takes a crucial initial step towards this goal by asking: "how does superconductivity evolve upon an ultrashort laser excitation in proximity to magnetism?". In order to address this matter, first, we investigate the photo-induced melting and recovery of superconductivity in NbN. Our optical pump-THz probe experiments show that, at low temperatures and laser fluences, the dynamics can be adequately described by considering the phonon-bottleneck effect through the Rothwarf-Taylor model. Moreover, we observe that the picosecond melting process is critically slowed down when the photoexcitation intensity is commensurate with the superconductivity condensation energy, which we elucidate through the time-dependent Landau theory of phase transitions. On the other hand, the sub-nanosecond recovery time displays signs of a divergence as the critical temperature is approached that are indicative of a relaxation governed by anharmonic phonon decay. Subsequently, we study analogous non-equilibrium superconductivity dynamics in bilayers comprising NbN and a thin paramagnetic or ferromagnetic metallic cap. By carefully mapping the photoexcitation intensity-temperature phase space, we determine that the behaviour of the isolated superconductor and the bilayers is qualitatively similar when accounting for differences in critical temperatures and laser absorption. We attribute the quantitative differences to quasiparticle transport, in the case of the melting, and, for the recovery, to the interplay of heat transfer, acoustic matching, and electron-phonon scattering in cap. Thus, our results indicate that NbN/ferromagnet devices can operate at speeds comparable to the timescales intrinsic to superconductivity. Finally, we investigate the response of magnetic order to an ultrashort optical stimulus. Specifically, through THz emission spectroscopy, we characterize the picosecond spin current generation from ferrimagnetic GdCo. Our measurements reveal that the spin emission vanishes at the compensation point when a magnetic field is applied, but it remains sizeable at zero field. Thus, compensated GdCo may be employed as a spin source less disruptive for superconductivity due to its antiferromagnetic-like character. In addition, we scrutinize the Gd and Co contributions to the spin current and show how the latter may be tailored by varying the alloy composition and temperature.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Nava Antonio, Guillermo

Advisor dc:contributor.advisor

• Ciccarelli, Chiara

• Magnetism
• Superconductivity
• Spintronics
• Ultrafast spectroscopy

Licence

• https://www.repository.cam.ac.uk/bitstreams/f33aa233-b6d3-45b4-89eb-b08293e2a727/download
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

eng

Author Identifier

0000-0003-2813-2841

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/399486