Onset of ferromagnetism and spin fluctuations in rare earth (or actinide)-3d compounds

The R-Ni and R-Co (R = rare earth or actinide) systems form excellent tools for the study of d magnetism. Indeed the development of the nickel and cobalt magnetism in these systems is associated with the rare earth (or actinide) content. Because of the large electronegativity and volume differences between R and nickel or cobalt, a large number of well-defined compounds exist in the phase diagrams. It is therefore possible to obtain compounds close to the onset of 3d magnetism. Four different types of behaviour characteristic of the occurrence of d ferromagnetism were observed: (i) exchange enhanced Pauli paramagnetism in YNi5, CeNi5, LuCo2 and YCo2; (ii) very weak itinerant ferromagnetism in YNi3; (iii) collective electron metamagnetism in ThCo5, Ce(Co,Ni)5, Y2Ni17 and some RCo2 compounds; (iv) thermal spontaneous magnetization in Y2Ni7. These types of behaviour are all qualitatively accounted for in the classical Stoner-Wohlfarth model which takes account of the shape of the band structure resulting from the 3d-4d (or 5d or 6d) hybridization. However, a more realistic description of the observed properties shows the strong effects of spin fluctuation. (Auth.)