We present here a neutron diffraction study, both in zero field and as a function of magnetic field, of the magnetic structure of the tetragonal intermetallic EuNiGe 3 on a single crystalline sample. This material is known to undergo a cascade of transitions, first at 13.2 K towards an incommensurate modulated magnetic structure, then at 10.5 K to an antiferromagnetic structure. We show here that the low-temperature phase presents a spiral moment arrangement with wave vector k = (1 4 ,δ,0). For a magnetic field applied along the tetragonal c axis, the square root of the scattering intensity of the (1 0 1) reflection matches very well the complex metamagnetic behavior of the magnetization along c measured previously. For the magnetic field applied along the b axis, two magnetic transitions are observed below the transition to a fully polarized state.

Rdsum6.-On expose une thdorie gdndrale des fluctuations de spins et les propriiStds thermodynamiques du magndtisme dlectronique itingrant, interpolation entre les limites faible et forte du ferromagndtisme. On donne une expression unifide de la tempdrature de Curie et on discute la signification physique de la susceptibilit6 magn6tique statique de Curie-Weiss. En tant que phdnomSnes nouveaux ddcoulant de cette thdorie, on discute les moments magndtiques locaux ddpendant de la tempdrature c o m e ceux observds dans CoS2, CoSez, etc. et les propridtds magnstiques et thermiques des semiconducteurs presque magndtiques connus de Si.Abstract.-A general theory of spin fluctuations and thermodynamical properties of itinerant electron magnets is developed, interpolating between the weakly and strongly ferromagnetic limits. A unified expression is given for the Curie temperature and the physical meaning of the Curie-Weiss magnetic susceptibility is discussed. As new phenomena derived from this theory the temperature-induced local magnetic moments as observed in CoSz, CoSe2, etc. and peculiar magnetic and thermal properties of nearly ferromagnetic semiconductors such as FeSi are discussed.

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Theory of spin¯uctuations for itinerant magnetism and its application to high temperature superconductivity are reviewed. After a brief introduction to the whole subject the developments of the self-consistent renormalization theory of spin¯uctuations are summarized with particular emphasis on critical properties at the quantum phase transitions. Most of the anomalous properties in the normal state of high-T c cuprates are understood as due to the critical behaviours for the two dimensional antiferromagnetic metals. By analysing the nuclear magnetic relaxation rate and the T -linear term of resistivity, the set of parameters to specify the spin¯uctuations are determined. It is shown that by using the parameters thus obtained one can describe other quantities as well, e.g. optical conductivity. Then we proceed to the theory of superconductivity by the spin¯uctuation mechanism. After some discussion on the weak coupling treatments, the strong coupling theory is reviewed. It is shown that the set of parameters determined by the normal state properties of the high-T c cuprates just give a transition temperature of the right order of magnitude. Among the parameters, the most sensitive one for T c is the frequency spread of the spin¯uctuations. This fact enables us to present a possible uni® ed picture of the antiferromagnetic spin¯uctuation-induced superconductors, including heavy fermion superconductors and organic superconductors. This point of view may be con® rmed to a certain extent by microscopic calculations based on the¯uctuation exchange approximation for the two-dimensional Hubbard models representing not only the cuprates but also organic and trellis lattice compounds. The review is concluded with some discussions on future problems, e.g. the pseudo spin-gap in the under-doped region.

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