X-ray detected Ferromagnetic Resonance studies of spin current propagation
R. J. Hicken (Department of Physics & Astronomy, University of Exeter, UK)

Precessional pumping offers a convenient means by which to generate pure spin currents within thin film structures without the need to pattern the film or define nanoscale electrical contacts. Application of a microwave magnetic field causes precession within a ferromagnetic source layer so that a spin current is pumped into an adjacent non-magnetic layer. The spin current may either induce a voltage by means of the inverse spin Hall effect (ISHE), or else propagate into a second ferromagnetic sink layer in which spin transfer torque (STT) induces a precessional response. Ordinarily it is difficult to separate the small amplitude precession of the sink from the much larger amplitude precession of the source. This problem may however be overcome in x-ray detected ferromagnetic resonance (XFMR) measurements where the element specific nature of x-ray magnetic circular dichroism (XMCD) can be used to study the source and sink layer response separately. Importantly, the sink layer precession has a different characteristic phase when excited by means of STT as opposed to interlayer exchange or dipolar coupling. In a first study of a Co/Cu/Py spin valve structure, spin propagation from the Py source to the Co sink was used to quantify the spin-mixing conductance of the structure [1]. Excitation of a Mn doping layer within the Cu spacer layer was then used to directly observe the presence of the spin current [2], before the dependence of spin current absorption upon sink layer thickness was explored within Ni81Fe19(05 nm)/Ag (6 nm)/Co2MnGe(5 nm) structures [3]. Finally, the results of a study performed upon epitaxial MgO(001)/FeCo/NiO/Fe/Ni81Fe19 structures will be presented. Spin current propagation through the NiO is thought to be mediated by evanescent antiferromagnetic spin waves [4] and is found to lead to a qualitatively different response of the sink layer.