Probing spin dynamics of ultra-thin van der Waals magnets via photon-magnon coupling

Abstract

Layered van der Waals (vdW) magnets can maintain a magnetic order even down to the single-layer regime 2,3,9 and hold promise for integrated spintronic devices 4,5 . While the magnetic ground state of vdW magnets was extensively studied, key parameters of spin dynamics, like the Gilbert damping, crucial for designing ultra-fast spintronic devices, remains largely unexplored. Despite recent studies covering the optical frequency range 6,7 , achieving spin wave control at microwave frequencies is highly desirable, a regime where modern integrated information technologies predominantly operate 8,9 . The intrinsically small numbers of spins, however, poses a major challenge to this. Here, we present a novel approach to detect spin dynamics mediated by photon-magnon coupling between high-Q superconducting resonators and ultrathin flakes of Cr 2 Ge 2 Te 6 (CGT) as thin as 11 nm. We test and benchmark our technique with 23 individual CGT flakes and extract an upper limit for the Gilbert damping parameter. These results are crucial in designing on-chip integrated circuits using vdW magnets and offer prospects for probing spin dynamics of monolayer vdW magnets.van der Waals (vdW) materials 5,10,11 consist of individual atomic layers bonded by vdW forces and can host different types of collective excitations such as plasmons, phonons and magnons. Strong coupling between these excitation modes and electromagnetic waves (i.e. photonic modes) creates confined light-matter hybrid modes, termed polaritons. Polaritons in vdW materials are an ideal model system to explore a variety of polaritonic states 12,13 , e.g. surface plasmon polaritons in graphene 14,15 and exciton polaritons in a monolayer MoS 2 embedded inside a dielectric microcavity 16 . These states can be further modified by electrostatic gating, as well as by hetero-structuring with dissimilar vdW layers.

Ferromagnetic Resonance in Two-dimensional van der Waals Magnets: A Probe for Spin Dynamics

“…This essentially means that when we aim to probe spin dynamics of 2D vdW monolayers (which are typically µm sized flakes) using microwave photons, the photon mode must have a correspondingly small mode size to maximize the coupling strength and hence its sensitivity. One of the promising approaches to this is to use on-chip superconducting (SC) resonators [176,186,187]. Strong coupling between photons in SC resonators and magnons in anti-ferromagnetic CrCl 3 bulk systems has been demonstrated [186,187].…”

“…Strong coupling between photons in SC resonators and magnons in anti-ferromagnetic CrCl 3 bulk systems has been demonstrated [186,187]. Zollitsch et al recently reported that it is possible to probe spin dynamics of Cr 2 Ge 2 Te 6 as thin as 11 nm using SC resonators [176]. Cr 2 Ge 2 Te 6 flakes were exfoliated and transferred onto magnetically-active parts of high-qualityfactor SC lumped element resonators with their small mode volume (≈ 6000 µm 3 ), as schematically shown in Fig.…”

“…As shown in Fig. 12c, this causes the observable linewidth broadening for the particular field range, which can be analysed by using a phenomenological model of coupled harmonic oscillators [176]. The photon mode frequency (ω res ) as well as the relaxation rate of the hybrid magnon-photon modes (κ eff ) were extracted by the spectral data as shown in Fig.…”

“…κ eff is strongly enhanced due to the large relaxation rate of CGT magnons and using the plot shown in Fig. 12e, the spin dynamics of the CGT magnon modes can be analysed (see details in [176]). They also demonstrated the photon-magnon coupling using a 11 nm CGT flake (Fig.…”

“…f. Effective loss rate κ eff as a function of magnetic field of a resonator loaded with the thinnest CGT sample (∼ 11nm). All panels are adapted from Ref [176]…”