Magnetic Properties of Strongly Interacting Systems: Part I. Theoretical Analysis for almost Localized Electrons and Exchange-Mediated Pairing. Part II. Experimental Studies on the Role of Oxygen and Strontium in Doped Lanthanum Nickelates

In the first part, magnetic properties of almost localized electrons have been analyzed theoretically. The results obtained have been used to explain the observed magnetic field dependence of the effective mass in CeB _6. Additionally, utilizing the concept of real-space pairing among strongly correlated narrow band as well as among hybridized electrons, various physical properties characterizing the superconducting state have been determined. The applicability of this theory to heavy fermion and high-T_{c } systems is discussed. In the second part, magnetic susceptibility measurements have been performed on single crystals of La_2 NiO_{4+delta} and La_{2-x}Sr _{x}NiO_4 as a function of both composition and temperature. A previously unreported discontinuity in T_{N} with delta has been observed. While a small increase in delta<=ads to a drastic lowering of T_{N}, the effect of Sr doping up to x <=q 0.20 does not influence T_ {N}. A transition to a spin-glass phase below 20 K is being reported for the first time in La _2NiO_{4+delta }, in the composition range 0.05 <=q delta <=q 0.06, and in La_{2-x}Sr _{x}NiO_4 , for 0.15 <=q x <=q 0.25.