Spin current and two magnon scattering in nanoscale systems

Bret Heinrich
Simon Fraser University, Burnaby, BC, Canada

Research interest in magnetic nanostructures and spintronics has shifted increasingly from the static to dynamic properties of magnetic nanostructures. This is motivated by the fact that the switching time of magnetic hybrid multilayers used in mass data storage devices and magnetic random access memories (MRAM) is a real technological issue. The crystalline Fe/Au,Pd/Fe/Au (001) nano-structures were prepared by Molecular Beam Epitaxy (MBE) technique using 4x6 reconstructed GaAs(001) substrates. A gyrating magnetic moment creates a spin current in surrounding normal metal layers and leads to non-local interface spin damping. The precessing magnetization acts as a peristaltic spin pump, which transports the spin momentum and allows one to establish a transfer of information between the magnetic layers separated over thick nonmagnetic metallic spacers. Modified Landau-Lifshitz-Gilbert (LLG) equations of motion are modified by spin pumping and spin sink effects. Time Resolved Magneto-Optical Kerr effect (TRMOKE) is an ideal tool to investigate propagation of spin currents. The stroboscopic time-resolved measurements (with the time resolution of 1 ps and sub micron spatial resolution) were carried out using a slotted transmission line with repetitive ps magnetic pulses. Spin currents generated by spin pumping propagate across the Au spacer in ballistic manner and result in rf excitations of the surrounding magnetic films.

The Pd lattice has a large lattice mismatch with respect to Fe. The lattice strain is partially released by a self-assembled rectangular network of misfit dislocations. It will be shown that the nano-network of misfit dislocations leads to a strong extrinsic magnetic damping. This system provides an ideal opportunity to investigate the role of two magnon scattering in a wide range of microwave frequencies. FMR measurements were carried out from 4 GHz to 73 GHz. The contribution to the FMR linewidth from this two magnon scattering is strongly anisotropic and follows the rectangular symmetry of the glide planes of the misfit dislocation network. The angular dependence of the FMR linewidth is a consequence of channeling of the scattered spinwaves along the misfit dislocation glide planes .