A long-ranged dynamic interaction between ferromagnetic films separated by normal-metal spacers is reported, which is communicated by nonequilibrium spin currents. It is measured by ferromagnetic resonance (FMR) and explained by an adiabatic spin-pump theory. In FMR the spin-pump mechanism of spatially separated magnetic moments leads to an appreciable increase in the FMR line width when the resonance fields are well apart, and results in a dramatic line-width narrowing when the FMR fields approach each other.PACS numbers: 75.40. Gb,75.70.Cn,76.50.+g,75.30.Et The giant magnetoresistance [1] accompanying realignment of magnetic configurations in metallic multilayers by an external magnetic field is routinely employed in magnetic read heads and is essential for high-density nonvolatile magnetic random-access memories. These typically consist of ferromagnetic/normal/ferromagnetic (F/N/F ) metal hybrid structures, i.e., magnetic bilayers which are an essential building block of the so called spin valves. The static Ruderman-Kittel-Kasuya-Yosida (RKKY) interlayer exchange between ferromagnets in magnetic multilayers [2] is suppressed in these devices by a sufficiently thick nonmagnetic spacer N or a tunnel barrier. The interest of the community shifts increasingly from the static to the dynamic properties of the magnetization [3]. This is partly motivated by curiosity, partly by the fact that the magnetization switching characteristics in memory devices is a real technological issue. A good grasp of the fundamental physics of the magnetization dynamics becomes of essential importance to sustain the exponential growth of device performance factors.In this Letter we study the largely unexplored dynamics of magnetic bilayers in a regime when there is no discernible static interaction between the magnetization vectors. Surprisingly, the magnetizations still turn out to be coupled, which we explain by emission and absorption of nonequilibrium spin currents. Under special conditions the two magnetizations are resonantly coupled by spin currents and carry out a synchronous motion, quite analogous to two connected pendulums. This dynamic interaction is an entirely new concept and physically very different from the static RKKY coupling. E.g., the former does not oscillate as a function of thickness and its range is exponentially limited by the spin-flip relaxation length of spacer layers and algebraically by the elastic mean free path. This coupling can have profound effects on magnetic relaxation and switching behavior in hybrid structures and devices.The unit vector m = M/M of the magnetization M(t) of a ferromagnet changes its direction in the presence of a noncollinear magnetic field. The motion of m in a single domain is described by the Landau-Lifshitz-Gilbert (LLG) equationwith γ being the absolute value of the gyromagnetic ratio. The first term on the right-hand side represents the torque induced by the effective magnetic field H eff = −∂F/∂M, where the free-energy functional F [M] consists of the Zeeman energy, ma...

Unique features of the Gilbert damping in magnetic multilayers were investigated by ferromagnetic resonance (FMR) using magnetic single and double layer structures prepared by molecular beam epitaxy. The FMR linewidth for the Fe films in the double layer structures was larger than the FMR linewidth in the single Fe films having the same thickness. The additional FMR linewidth scaled inversely with the film thickness, and increased linearly with increasing microwave frequency. These results demonstrate that a transfer of electron angular momentum between the magnetic layers leads to additional relaxation torques.

Micron-sized magnetic platelets in the flux-closed vortex state are characterized by an in-plane curling magnetization and a nanometer-sized perpendicularly magnetized vortex core. Having the simplest non-trivial configuration, these objects are of general interest to micromagnetics and may offer new routes for spintronics applications. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field core toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was established. At frequencies more than an order of magnitude higher vortex state structures possess spin wave eigenmodes arising from the magneto-static interaction. Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited. These results are confirmed by micromagnetic simulations, which clearly show the selection rules for this novel reversal mechanism. Our analysis reveals that for spin-wave excitation the concept of a critical velocity as the switching condition has to be modified.

A method for the calculations of the Gilbert damping parameter α is presented, which based on the linear response formalism, has been implemented within the fully relativistic Korringa-KohnRostoker band structure method in combination with the coherent potential approximation alloy theory. To account for thermal displacements of atoms as a scattering mechanism, an alloy-analogy model is introduced. This allows the determination of α for various types of materials, such as elemental magnetic systems and ordered magnetic compounds at finite temperature, as well as for disordered magnetic alloys at T = 0 K and above. The effects of spin-orbit coupling, chemical and temperature induced structural disorder are analyzed. Calculations have been performed for the 3d transition-metals bcc Fe, hcp Co, and fcc Ni, their binary alloys bcc Fe1−xCox, fcc Ni1−xFex, fcc Ni1−xCox and bcc Fe1−xVx, and for 5d impurities in transition-metal alloys. All results are in satisfying agreement with experiment.

Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with <27 fs time resolution using terahertz spectroscopy on bilayers of ferrimagnetic yttrium iron garnet and platinum. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current js arises on the same ~100 fs time scale on which the metal electrons thermalize. This observation highlights that efficient spin transfer critically relies on carrier multiplication and is driven by conduction electrons scattering off the metal–insulator interface. Analytical modeling shows that the electrons’ dynamics are almost instantaneously imprinted onto js because their spins have a correlation time of only ~4 fs and deflect the ferrimagnetic moments without inertia. Applications in material characterization, interface probing, spin-noise spectroscopy and terahertz spin pumping emerge.