The Kondo Effect (Jun Kondo, 1964): magnetic impurities in a metal scatter conduction electrons via spin-flip scattering, giving a contribution to resistivity that increases logarithmically as temperature decreases. Combined with the phonon-dominated T⁵ decrease, this produces a
resistivity minimum.
ρ(T) = ρ₀ + AT⁵ − c·B·ln(T/T_K) + ... (T ≫ T_K)
At T ≪ T_K (Kondo temperature), the impurity spin is screened by conduction electrons forming a many-body "Kondo singlet" — the resistivity saturates. The full crossover is captured by the Hamann formula. This effect explains the mysterious resistance minima observed in dilute magnetic alloys since the 1930s.