Ferromagnetic spin fluctuations

From the anisotropic nature of the superconductivity (d- or p-wave) and the intimate relation between the superconductivity and magnetism, the origin of the attractive force is considered to be due to the antiferro- or ferromagnetic spin fluctuations in HF superconductors. 

The spin susceptibility derived from the x and K values at Tc is enhanced by a factor of 5.4 as compared with that estimated from the band calculation due to ferromagnetic spin fluctuation,107,108 and the related cubic compound... 

If the material is ferromagnetic then the entropy, susceptibility and resistance at temperatures just above the Curie point are to be calculated in much the same way (i.e. in terms of spin fluctuations). A treatment of this problem starting from the Stoner-Wohlfarth model is due to Moriya and Kawabata (1973). 

According to the spin fluctuations theory, the Curie temperature of the band ferromagnets is inversely proportional to the density of states in the subbands near the Fermi level, NT(Ef) and Nl(E ). The increase of the lattice parameters after the hydrogen atoms incorporation to the lattice induces the decrease of the degree of wave-function hybridization between the 3r/-electrons of Fe atoms and 5r/-electrons of rare-earth metal atoms. This in turn entails the decrease of density of states in both zones NT(Ef) and Ni(EF). 

One problem with the Stoner-Wohlfarth theory is that most systems to which it has been applied exhibit a Curie-Weiss law for T > T. The Stoner-Wohlfarth theory is a molecular field theory and therefore includes neither spin correlations nor dynamics, nor is the detailed band structure taken into account explicitly. Several authors have recently turned their attention to this problem. Moriya and Takashashi (1978) used a renormalized theory of spin fluctuations (described in section 4.1.2) to obtain a Curie-Weiss law for x above T< for weak ferromagnets. They also showed that the critical indices are different, for example... 

The main purpose of the SCR theory is to show that the spin fluctuations strongly affect the equilibrium properties of weak ferromagnets, in contrast to the RPA treatment (see section 4.1.1). The function is taken to be... 

The collective excitation spectrum of strong itinerant ferromagnets is dominated by the Stoner gap Ab given by eq. (50) rather than the transverse spin fluctuations. As mentioned above the poles of ) are undamped spin... 

Strictly speaking, the compound CeSi2-x (cf. sect. 5) that has already been studied can be included in this group because Si is substituted by vacancies. It is also an example of a system that undergoes a continuous transition from ferromagnetic (CeSij 75) to IV behaviour. In this particular case, the spin fluctuations play an important role. Other systems to be included in this subsection are CePdj, when Pd is substituted by Rh (displays an IV tendency) or Ag (trivalent tendency) and CeSi2-, when Si is substituted by Ge. The specific heats of Ce(Pd, Rh)j and Ce(Pd, Ag)3 were... 

In chapter 98, Julian Sereni adds significantly to an evaluation of systematic, experimental low-temperature studies of the ambivalent behaviors of cerium (ferromagnetism, antiferromagnetism, spin glass, superconductivity, valence fluctuations, heavy Fermion, Kondo and spin fluctuations) which depend upon its environment in materials. The systematic conclusions arrived at should provide new data against which the theory can be advanced. 

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