Coherent Spin Pumping in a Strongly Coupled Magnon-Magnon Hybrid System

Abstract

We experimentally identify coherent spin pumping in the magnon-magnon hybrid modes of permalloy/yttrium iron garnet (Py/YIG) bilayers. Using broadband ferromagnetic resonance, an "avoided crossing" is observed between the uniform mode of Py and the spin wave mode of YIG due to the fieldlike interfacial exchange coupling. We also identify additional linewidth suppression and enhancement for the in-phase and out-of-phase hybrid modes, respectively, which can be interpreted as concerted dampinglike torque from spin pumping. Our analysis predicts inverse proportionality of both fieldlike and dampinglike torques to the square root of the Py thickness, which quantitatively agrees with experiments.Coherent information processing has recently become an emerging topic for the post-CMOS electronics era [1,2]. In spintronics, exchange-induced magnetic excitations, called spin waves, or magnons [3,4], are good candidates because information can be encoded by both the amplitude and the phase of spin waves. For example, the interference of coherent spin waves can be engineered for spin wave logic operations [5][6][7]; the coherent interaction of spin-torque oscillators leads to mutual synchronization [8][9][10][11][12][13], which can be applied in artificial neural networks [14,15]; and the coherent coupling between magnons and microwave cavities [16][17][18][19][20][21][22] opens up new opportunities for magnon-based quantum information science [23,24].Recently, strong coupling between two magnonic systems has been observed [25][26][27], which allows excitations of forbidden spin wave modes and high group velocity of propagating spin waves [28,29]. The coupling is dominated by the exchange interaction at the interface of the magnetic bilayers, providing a new pathway to coherently transfer magnon excitations between two magnetic systems possessing distinctive properties: from conductor to insulator, from uniform to nonuniform mode and from high-damping to low-damping systems. However, the underlying physical mechanisms of the coupling are still not fully understood. First, what are the key parameters that dictate the coupling efficiency and enable one to reach the strong-coupling regime? Second, with the interfacial exchange coupling acting as a fieldlike torque, is there a dampinglike torque associated with spin pumping [30][31][32][33]? The second question is particularly important for optimizing the coherence of spin wave transfer in hybrid systems. Furthermore, any parasitic effect on the incoherent spin current from the conduction band is well-removed [34-36] by using magnetic insulators such as yttrium iron garnet (Y 3 Fe 5 O 12 , YIG) [29,37,38], which facilitates the study of spin pumping coherency.In this work, we study YIG/permalloy (Ni 80 Fe 20 , Py) bilayers with varying Py thicknesses. By using a much thinner YIG film compared with previous work [25,27], we define well-separated perpendicular standing spin wave (PSSW) modes in YIG and create an avoided crossing much larger than the linewidths, allowing us to stud...

Probing magnon–magnon coupling in exchange coupled Y$$_3$$Fe$$_5$$O$$_{12}$$/Permalloy bilayers with magneto-optical effects

“…1c, the quadratic increase of H ex with the mode number n confirms the observation of the PSSWs. Fittings to the Kittel equation and the exchange field expression yield M s = 1.97 kOe and A ex = 3.76 pJ/m, which are in good agreement with the previously reported values [30][31][32][33] .…”

“…Our experimental configuration, similar to previously reported [30][31][32][33] , is relevant to the Schlömann excitation mechanism of spin wave 44 with a dynamic pinning at the interface 45 . The interfaces of two distinct, coupled magnetic layers have been recently recognized as an interesting source of spin dynamics generation and manipulation 46,47 .…”

“…The interfaces of two distinct, coupled magnetic layers have been recently recognized as an interesting source of spin dynamics generation and manipulation 46,47 . In particular, the critical role of the microwave susceptibilities of the distinct magnetic layers has been theoretically laid out 48,49 that are directly relevant to the magnon-magnon coupled experiments [30][31][32][33] .…”

Tunable magnon-magnon coupling in synthetic antiferromagnets

“…4(a) and 4(b). This linewidth averaging is similar to those discussed in spin-photon coupling systems [35,46,47] as well as magnon-magnon coupling at YIG/NiFe interfaces [17]. We went on to quantify the loss rates for both modes by using Eq.…”

“…On the other hand, magnon-magnon interaction does not suffer from this since two modes normally reside within the same host media, providing mode overlapping of 100% or close to. While magnon-magnon coupling has been studied in single magnets [11][12][13][14] and magnetic bilayers [15][16][17], magnon-magnon interaction in highly tunable material systems could offer unexplored parameter spaces on which to tailor the coupling phenomena. Here, we focus on synthetic antiferromagnets (SyAFs) as a host of magnon-magnon coupling and report clear hybridization of two distinct SyAF modes arising from interlayer exchange coupling between two magnetic layers.…”