Measurement of Gilbert damping parameters in nanoscale CPP-GMR spin valves

In situ device-level measurement of thermal mag-noise spectral linewidths in 60 nm diameter current-perpendicular-to-plane--giant magnetoresistive spin-valve stacks of IrMn/ref/Cu/free, with reference and free layer of similar CoFe/CoFeGe alloy, are used to simultaneously determine the intrinsic Gilbert damping for both magnetic layers. It is shown that careful alignment at a ``magic angle'' between free- and reference-layer static equilibrium magnetization can allow direct measurement of the broadband intrinsic thermal spectra in the virtual absence of spin-torque effects which otherwise grossly distort the spectral line shapes and require linewidth extrapolations to zero current (which are nonetheless also shown to agree well with the direct method). The experimental magic-angle spectra are shown to be in good qualitative and quantitative agreement with both macrospin calculations and micromagnetic eigenmode analysis. Despite similar composition and thickness, it is repeatedly found that the IrMn exchange pinned reference layer has an order of magnitude larger intrinsic Gilbert damping (α≈0.1) than that of the free layer (α≈0.01). It is argued that the large reference-layer damping results from strong, off-resonant coupling to lossy modes of an IrMn/ref couple, rather than commonly invoked two-magnon processes.