= Solution
Measure nonlocal alignment-dependent resistance in otherwise comparable devices with several injector–detector spacings $L$. Steady <spin accumulation> obeys a diffusion–relaxation equation,
$$
D\frac{d^2\mu_s}{dx^2}-\frac{\mu_s}{\tau_s}=0,\qquad
\mu_s(L)\propto e^{-L/\lambda_s},\qquad
\boxed{\lambda_s=\sqrt{D\tau_s}}.
$$
In a weakly invasive detector limit, $\Delta R_{\rm NL}=(V_P-V_{AP})/I\propto e^{-L/\lambda_s}$; a semilog plot gives slope $-1/\lambda_s$. With transparent Py/Cu interfaces, fit a spin-resistance model including contact absorption and wire cross-section rather than assuming that every amplitude reduction is a bulk Cu loss.
Keep Cu processing, cross-section and <temperature> fixed, characterize <contact resistances>, reverse injector current and subtract alignment-independent offsets. Repeat at different <temperatures> to assess interface and bulk contributions. A <Hanle spin-precession measurement> provides an independent check of spin relaxation, with diffusion times and electrode rotation included in its model.
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