Electronic and magnetic states in metallic compounds-I. Kondo effect and ferromagnetism in CoAl

Magnetic susceptibility and electron transport properties were investigated in CoAl alloys in the vicinity of exact stoichiometry. Resistance minima, negative magnetoresistance, and thermo-power anomalies were observed in the liquid helium region. The concentration, temperature, and field dependence of the transport properties were interpreted in terms of localized magnetic moments formed on Co atoms on Al sites (Co_x atoms) in the lattice. The effective moment on the Co_x atoms in the paramagnetic state is 5·9 μ_R. Analysis of the data in terms of the theories of the Kondo effect led to estimates of the exchange parameter and Kondo temperature of ∼ -1·3 to -2·2 eV and ∼8°K. respectively. At excess Co concentrations of more than 0·6 at.%. ferromagnetic interactions between the Co_x atoms become apparent in that the resistance minima disappear, the Weiss temperature changes from negative to positive, and remanent magnetization is observed. The nature of the ferromagnetic state is discussed along with some evidence for spin-fluctuation effects in the nearly ferromagnetic alloys.