High frequency spin currents in magnetic nanostructures

Multilayers composed of ferromagnetic and normal metals have been studied extensively, because they allow one to create and manipulate spin currents with the goal of information processing and storage. An example of the importance of these structures for applied and fundamental research is provided by the different magneto-resistive effects, such as the giant magneto-resistive effect (GMR). For high-frequency spin currents and rapid spin-current switching, the question arises how fast a signal that is encoded in the spin current can be transmitted through these structures. For the case of (collinear and non-collinear) spin transport though copper-cobalt heterostructures, we have shown recently that spin currents exhibit wave-like characteristics in addition to their diffusive character. Due to the wave-character of the spin currents, we were able to calculate the finite propagation velocity of signals encoded in spin currents in bulk metals. This result is in sharp contrast with conventional spin diffusion theory, which yields an infinite spin signal-velocity.  In general, the results of our spin wave-diffusion theory and the conventional spin diffusion equation are significantly different for fast switching or high-frequency modulation of spin currents.

This project combines a theoretical (Schneider) and an experimental (Hillebrands) approach to the investigation of spin-transport processes in heterostructures composed of ferromagnetic and normal metals. Using our spin wave-diffusion theory and Brillouin light-scattering techniques, we intend to investigate high-frequency spin currents in magnetic nanostructures. Of particular interest is the design of nanostructures, which may exhibit novel features, such as standing spin-current waves, which are based on the wave character of spin currents. This project also involves some interesting technical problems concerning the propagation of spin currents in the high frequency and small structure-size range of interest, because one has to deal with an anomalous skin effect and non-Ohmic behaviour of the spin currents.

Principal Investigators:

Prof. Dr. Hans Christian Schneider (Department of Physics, TU Kaiserslautern)

Prof. Dr. Burkard Hillebrands (Department of Physics, TU Kaiserslautern)