Magnetoresistive measurements, through giant- or tunneling-magnetoresistance (GMR/TMR), have provided most of our understanding about the transport of spin-polarized electrons in heterostructures with ferromagnets. Ferromagnetic resonance (FMR), a microwave absorption measurement in the range 1-100 GHz, characterizes very low energy spin excitations in ferromagnets. The lifetime of these excitations can be used to measure the scattering of "pure spin" transport, i.e. chargeless flow of spin current, in different parts of the heterostructure, accessing some phenomena which are inaccessible to magnetoresistive measurements. In this talk I will describe our efforts to probe relaxation processes in ultrathin ferromagnets and paramagnets using FMR-pumped spin current. In ferromagnets, we have found that pure spin current relaxes over a very short range with distinct thickness power law, with surprisingly little anisotropy in spin direction and with an additional influence due to magnetization texture in finite-wavelength spin waves. Ultrathin paramagnets (Pd, Pt) are shown to be influenced by a induced magnetic order due to direct exchange, suggesting a configuration-dependent relaxation length.
 A. Ghosh, S. Auffret, U. Ebels, and W.E. Bailey, PRL (2012)
 Y. Li and W.E. Bailey, PRB, in press (2016); cond-mat/1502.05687
 Y. Li and W.E. Bailey, PRL 116 117620 (2016)
 M. Caminale, A. Ghosh, S. Auffret, U. Ebels, K. Ollefs, F. Wilhelm, A. Rogalev, W.E. Bailey, PRB 94 014414 (2016)