= Solution
Two practical approaches retaining Co and NiFe are:
* Improve the spin-selective interfaces: control deposition and intermixing, avoid oxidation or magnetic dead layers, and reduce pinholes and spin-flip defects. The aim is large majority/minority transport asymmetry with low spin-memory loss, rather than merely the smallest total resistance.
* Optimize the Cu thickness and electrical stack: use the thinnest continuous spacer that still allows independent layer reversal and remains well below its <spin diffusion length>; reduce extraneous contact or lead resistance. This preserves a full antiparallel state and limits spin-independent dilution of the measured <magnetoresistance ratio>.
Increasing magnetic-layer thickness toward the useful spin-dependent-scattering scale can be another optimization, but growth beyond the relevant spin diffusion scale eventually gives diminishing returns. Merely increasing the bias current increases the readout voltage and possible heating; it does not, by itself, increase the intrinsic <giant magnetoresistance> ratio.
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