Electromagnetic Approach to Cavity Spintronics

Abstract

The fields of cavity quantum electrodynamics and magnetism have recently merged into 'cavity spintronics', investigating a quasiparticle that emerges from the strong coupling between standing electromagnetic waves confined in a microwave cavity resonator and the quanta of spin waves, magnons. This phenomenon is now expected to be employed in a variety of devices for applications ranging from quantum communication to dark matter detection. To be successful, most of these applications require a vast control of the coupling strength, resulting in intensive e↵orts to understanding coupling by a variety of di↵erent approaches. Here, the electromagnetic properties of both resonator and magnetic samples are investigated to provide a comprehensive understanding of the coupling between these two systems. Because the coupling is a consequence of the excitation vector fields, which directly interact with magnetisation dynamics, a highly-accurate electromagnetic perturbation theory is employed which predicts the resonant hybrid mode frequencies for any field configuration within the cavity resonator. The coupling is shown to be strongly dependent not only on the excitation vector fields and sample's magnetic properties but also on the sample's shape. These findings are illustrated by applying the theoretical framework to two distinct experiments: a magnetic sphere placed in a three-dimensional resonator, and a rectangular, magnetic prism placed on a two-dimensional resonator. The theory provides comprehensive understanding of the overall behaviour of strongly coupled systems and it can be easily modified for a variety of other systems.

Nonreciprocity in millimeter wave devices using a magnetic grating metamaterial

“…techniques which can be used to cut, slice, and drill magnetic samples [41,42]. For effective medium theory to be appropriate, one wants the overall change in the fields to be small over the length of a unit cell.…”

Advances in Magnetics Roadmap on Spin-Wave Computing

“…The lifetime of magnons in YIG can reach microseconds, especially at cryogenic temperatures. Large tunability [436], intrinsic nonlinearities [437], excellent ability to connect to other systems (like optical and microwave photons, or acoustic phonons) [438], [439], [440], and easy creation of macroscopic quantum states like magnon Bose-Einstein Condensate (BEC) [422], [441], [161], [442], are some of the other main features of YIG which are already available for the advancement of quantum computing. The first steps for employing magnon states in YIG as quantum information carriers have already been taken by studying the possibility to strongly couple bulk YIG crystal samples (spheres) to superconducting circuits [418], [443].…”

“…This has also been investigated, and the time-dependent control of the coupling has been achieved in [451]. Finally, theoretical understanding of such coupling, developed in [439], has allowed predicting the strength of coupling using a perturbation approach. This method works even for ultra-strong coupling regimes, micro-structured planar samples, and can be employed at both high and low temperature regimes using simple analytical expressions.…”

Strong magnon–photon coupling with chip-integrated YIG in the zero-temperature limit

“…As discussed in the main text, a new approach has been proposed for modeling the magnon-photon coupling g using electromagnetic perturbation theory 27 . In these terms, the coupling g can be modeled as…”

“…The coupling g can also be theoretically derived using electromagnetic perturbation theory (see Ref. 27 and also the supplement). In this case, g is defined in terms of the magnetization rather than spin density.…”