Spin Fluctuations in Itinerant Electron Magnetism

Abstract

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.

Competing magnetic instabilities in the weak itinerant antiferromagnetic TiAu

“…Moriya's self-consistent renormalization (SCR) theory 10,11 of spin fluctuations provides the modern theory of weak magnetism due largely to the fact that (i) the C-W behavior of χ(q = 0) is not explained by Stoner theory, and (ii) RPA theory requires modification at higher temperatures. Moriya's SCR theory not only resolved these difficulties, but it also predicted some observed quantum critical behavior of wFMs as well, for example, 1/χ Q → T 3/2 and C m /T →∝ T 1/2 for a 3D AFM system.…”

“…It was one success of Moriya's self-consistent renormalized theory of itinerant spin fluctuations that it predicts a Curie-Weiss-like susceptibility over a wide temperature range. 10,11 Neutron diffraction reveals an ordered local moment of only 0.15 µ B /Ti, identifying TiAu as a weak moment system. Ordering is at a commensurate wavevector Q AF M = (0, These properties identify TiAu as an itinerant but commensurate wAFM, whereas small itinerant moments are more often associated with incommensurate spindensity wave states.…”

Many‐electron operator approach in the solid state theory

“…Such oversimplified versions of the Hartree-Fock method, as the Stoner theory of magnetism, turned out to be not too successful. Later, some shortcomings of the latter theory were improved by semiphenomenological spin-fluctuation theories [4], which, however, in most cases do not take into account correlation effects in the ground state. To treat electron correlations, various perturbation approaches in the electron-electron interaction (e.g., diagram techniques) were used.…”

Strain Engineering of the CeNi5 System

“…The same study [5] also demonstrated for the contemporary YNi 5 system that a complex set of interactions exists between the spin and orbital moment. It has also been shown that CeNi 5 demonstrates spin fluctuations [6][7][8], which themselves are an interpolative model [9] that bridges the weakly and strongly ferromagnetic limits.…”