In magnetic trilayer structures, a contribution to the Gilbert damping of ferromagnetic resonance arises from spin currents pumped from one layer to another. This contribution has been demonstrated for layers with weakly coupled, separated resonances, where magnetization dynamics are excited predominantly in one layer and the other layer acts as a spin sink. Here we show that trilayer structures in which magnetizations are excited simultaneously, antisymmetrically, show a spin-pumping effect roughly twice as large. The antisymmetric (optical) mode of antiferromagnetically coupled Ni 79 Fe 21 (8nm)/Ru/Ni 79 Fe 21 (8nm) trilayers shows a Gilbert damping constant greater than that of the symmetric (acoustic) mode by an amount as large as the intrinsic damping of Py (∆α 0.006). The effect is shown equally in field-normal and field-parallel to film plane geometries over 3-25 GHz. The results confirm a prediction of the spin pumping model and have implications for the use of synthetic antiferromagnets (SAF)-structures in GHz devices.Pumped spin currents 1,2 are widely understood to influence the magnetization dynamics of ultrathin films and heterostructures. These spin currents increase the Gilbert damping or decrease the relaxation time for thin ferromagnets at GHz frequencies. The size of the effect has been parametrized through the effective spin mixing conductance g ↑↓ r , which relates the spin current pumped out of the ferromagnet, transverse to its static (timeaveraged) magnetization, to its precessional amplitude and frequency. The spin mixing conductance is interesting also because it determines the transport of pure spin current across interfaces in quasistatic spin transport, manifested in e.g. the spin Hall effect.In the spin pumping effect, spin current is pumped away from a ferromagnet / normal metal (F 1 /N) interface, through precession of F 1 , and is absorbed elsewhere in the structure, causing angular momentum loss and damping of F 1 . The spin current can be absorbed through different processes in different materials. When injected into paramagnetic metals (Pt, Pd, Ru, and others), the spin current relaxes exponentially with paramagnetic layer thickness 3-5 . The relaxation process has been likened to spin-flip scattering as measured in CPP-GMR, where spin-flip events are localized to heavy-metal impurities 6 and the measurement reveals the spin diffusion length λ SD . When injected into other ferromagnets (F 2 in F 1 /N/F 2 ), the spin current is absorbed through its torque on magnetization 5,7 . A similar process appears to be relevant for antiferromagnets as well 8 .In F 1 /N/F 2 structures, only half of the total possible spin pumping effect has been detected up until now. For well-separated resonances of F 1 and F 2 , only one layer will precess with large amplitude at a given frequency ω, and spin current is pumped from a precessing F 1 into a static F 2 . If both layers precess symmetrically, with a) Electronic mail: Contact author. web54@columbia.edu the same amplitude and phase, equal and ...

The generation of oxygen microbubbles in catalytic microtubes has attracted tremendous attention towards the exploration of unidirectional and overloaded bubble ejection regimes, leading to simple and more complex motions of micromotors. While it is widely believed that a bubble's frequency in a unidirectional regime (i.e., a bubble ejected from a single tubular opening) is random, this study shall demonstrate that periodic oxygen bubble frequencies and sequences can be experimentally controlled using various concentrations of hydrogen peroxide fuel and surfactants. When released from a substrate, unidirectional micromotors self-propel in straight, circular, and helical trajectories, leading to a class of well-predictable or simple micromachines. Under overloaded conditions, micromotors generate bubbles at both tubular openings, which influence the trajectories of micromotor motion strongly. A one-dimensional reaction-diffusion equation is formulated to explain the possible mechanisms of mass transport in microtubes and the transition from the unidirectional to the overloaded regime of micromotors.

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