GMR: ρ_AP − ρ_P = 4β²ρ*λ_sf/L-dependent — parallel/antiparallel resistance difference

Baibich-Fert 1988 / Grünberg 1989 GMR (2007 Nobel): Fe/Cr multilayers show a ~50% resistance drop when external field aligns neighbouring Fe layers' magnetisations. Valet-Fert 1993 semi-classical two-current diffusion model: each spin…