Kharkiv Quantum Seminar: March 17

Fast Fluxons Excite Short Magnons

Oleksandr Dobrovolskiy
(Cryogenic Quantum Electronics, EMG and LENA, Technische Universität Braunschweig, 38106 Braunschweig, Germany)

Ferromagnetism and superconductivity are fundamental states in condensed-matter physics. Recent discoveries in these topical areas enable the exploration of new physical phenomena at their interfaces. In this regard, interactions between the fundamental quasiparticles – magnetic flux quanta (fluxons) in superconductors and quanta of spin waves (magnons) in magnets – are especially fascinating [1]. While short-wavelength magnons are indispensable building blocks for magnonic nanodevices it is challenging to excite them coherently. Previous efforts have combined magnetic nanogratings [2] or spin textures [3] with microwave induction to push the limit of the shortest wavelength down to about 50 nm. But it was not possible to go beyond this value in the past several years.

In our recent work [4], coherent excitation of exchange magnons has been demonstrated experimentally, with a wavelength down to 36 nm by driving fast-moving fluxons in a superconductor. The successful generation of monochromatic magnons at a record short wavelength of 36 nm represents a milestone in coherent magnonics [5]. It provides an alternative means for coherent magnon excitation beyond conventional inductive microwave methods by leveraging magnetic flux quanta moving at a speed beyond 1 km/s. Once the velocity of the vortices in the superconductor reaches the phase velocity of magnons in the ferromagnetic CoFe film, the condition for coherent coupling is met, see Fig. 1. The dynamic stray fields of the vortices, which vary both in time and space, excite magnons unidirectionally along the direction of vortex motion. The generated magnons are monochromatic and the period of the vortex lattice, which can be tuned by varying the external magnetic field, defines the magnon wavelength.

[1] P. Pirro et al. Nat. Rev. Mater. 6, 1114 (2021).
[2] C. Liu et al. Nat. Commun. 9, 738 (2018).
[3] V. Sluka et al. Nat. Nanotechnol. 14, 328 (2019).
[4] O. Dobrovolskiy et al. Nat. Nanotechnol. 20 (2025) 1764.
[5] H. Yu, Nat. Nanotechnol. 20 (2025) 1719.