Spin-Torque Excitation of Perpendicular Standing Spin Waves in Coupled YIG/Co Heterostructures

Abstract

We investigate yttrium iron garnet (YIG)/cobalt (Co) heterostructures using broadband ferromagnetic resonance (FMR). We observe an efficient excitation of perpendicular standing spin waves (PSSWs) in the YIG layer when the resonance frequencies of the YIG PSSWs and the Co FMR line coincide. Avoided crossings of YIG PSSWs and the Co FMR line are found and modeled using mutual spin pumping and exchange torques. The excitation of PSSWs is suppressed by a thin aluminum oxide (AlOx) interlayer but persists with a copper (Cu) interlayer, in agreement with the proposed model.

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

“…The linewidths can be understood from the Landau-Lifshitz-Gilbert (LLG) equation with interfacial exchange coupling and mutual spin pumping, which provide the fieldlike and dampinglike interlayer coupling torques, respectively. The analysis also explains the thickness dependence of the two coupling strengths and reconfirms the antiferromagnetic coupling between YIG and Py [25]. Our results provide important insights for improving the coupling strength and coherence in magnon-magnon hybrid systems and pave the way for coherent information processing with exchange coupled magnetic heterostructures.The samples consist of YIG(100 nm)/Py(t Py ) bilayers where t Py varies from 5 nm to 60 nm.…”

“…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.…”

“…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 study the linewidth evolution of the two individual hybrid modes. We find a pronounced suppression of the total linewidth for in-phase hybrid modes and a linewidth enhancement for out-of-phase hybrid modes.…”

“…In order to understand the physical meaning ofg c , we consider the coupled Landau-Lifshitz-Gilbert (LLG) equations of YIG/Py bilayer [25,31,33] in the macrospin limit:…”

“…A different interlayer exchange coupling from Ruderman-Kittel-Kasuya-Yosida interaction may generate a larger J [47-49] but a smaller g ↑↓ [50]. There could also be a fieldlike contribution of J from g ↑↓ [25,[51][52][53][54]. But since the exchange J dominates in the coupled dynamics, it is difficult to distinguish the spin mixing conductance contribution in our experiments.…”