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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...

Published by the AIP PublishingArticles you may be interested in Volume contribution of exchange-coupling-induced uniaxial anisotropy in Fe/CoO/MgO(001) system

The uncompensated spins on the surfaces of antiferromagnetic CoO films exhibit a thermoremanent magnetization after field cooling from T>TN that has the same temperature dependence as the exchange field of Ni81Fe19/CoO bilayers after field cooling. This suggests that these interfacial uncompensated spins are responsible for unidirectional anisotropy. A model based on a calculation of the density of these interfacial uncompensated spins predicts the correct magnitude of the exchange field, as well as the observed inverse dependence on average interfacial grain size.