The Kondo effect in superparamagnetic 30% Co–Ni/NiO nanocomposites: detailed transport and magnetic investigations

Nanocomposites of 20% and 30% Co-doped Ni/NiO were synthesized using a microwave-assisted sol–gel auto-combustion method. The 30% Co-doped Ni/NiO nanocomposite exhibited negative magnetoresistance (M-R) at various temperatures, including 2 K, 10 K, 80 K, 100 K, and 250 K. This behavior is interpreted as indicative of a Kondo-like scattering effect. The resistivity ρ(T) upturn observed at low temperatures in the 30% Co-doped Ni/NiO nanocomposite can be accurately described by a power series equation combined with a Kondo term. Furthermore, this sample demonstrated a metal–insulator transition around the Kondo temperature T_K ≈ 29.8(5) K, with noticeable resistivity changes at magnetic fields of 0 T, 1 T, 5 T, and 8 T. Additionally, a hump in resistivity at approximately 212 K, 170 K, 243 K, 251 K, and 209 K for magnetic fields of 0 T, 0.05 T, 1 T, 5 T, and 8 T, respectively, is attributed to the charge ordering of Co³⁺ and Co⁴⁺ ions within the Ni/NiO lattice, contributing to a further metal–insulator transition.