No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Relationship between electronic conduction states and indirect magnetic exchange
Abstract
An attempt to find a quantitative relationship between magnetic transition temperature and conduction states properties in metallic materials is presented. The exchange interaction among magnetic moments is assumed to be of the Anderson's s-f type and the conduction states have s-d orbital character. The results are analyzed vs. different amount of s-d hybridization within LCAO scheme.
The Green-function technique, termed the irreducible Green functions (IGF) method, that is a certain reformulation of the equation-of motion method for double-time temperature dependent Green functions is presented. This method was developed to overcome some ambiguities in terminating the hierarchy of the equations of motion of double-time Green functions and to give a workable technique to systematic way of decoupling. The approach provides a practical method for description of the many-body quasi-particle dynamics of correlated systems on a lattice with complex spectra. Moreover, it provides a very compact and self-consistent way of taking into account the damping effects and finite lifetimes of quasi-particles due to inelastic collisions. In addition, it correctly defines the Generalized Mean Fields, that determine elastic scattering renormalizations and, in general, are not functionals of the mean particle densities only. Although some space is devoted to the formal structure of the method, the emphasis is on its utility. Applications to the lattice fermion models such as Hubbard/Anderson models and to the Heisenberg ferro- and antiferromagnet, which manifest the operational ability of the method are given. It is shown that the IGF method provides a powerful tool for the construction of essentially new dynamical solutions for strongly interacting many-particle systems with complex spectra.
Magnetization measurements have been carried out under hydrostatic pressures up to 6 kbar and constant magnetic fields (10 kOe) in CsCl-type compounds: CeMg and CeZn. a decrease in the Néel temperature, TN, with increasing pressure is observed in both compounds (dTN/dp=- 0.2 and -0.17 K kbar-1 respectively in CeMg and CeZn). The relative decrease of TN has a similar order of magnitude to that found in other cerium compounds with Kondo-type properties.
With a view to study the nature of the conduction electrons in rare earth metals and intermetallic compounds self consistent augmented-plane-wave calculations have been performed for DyZn. The results indicate that 72 per cent of conduction electrons inside the APW sphere of Dy have d character. The dependence of the nature of the conduction electrons on the exchange potential has also been studied.
The Curie temperature T0 of CeAg as a function of hydrostatic pressure exhibits a well-defined maximum at P~=0.7 GPa, giving clear evidence for Anderson lattice behavior. A rapid broadening of the resistive anomaly at T0 for P>=2.1 GPa suggests a change in the character of the magnetic ordering. At these pressures the temperature dependence of the electrical resistivity to 300 K is highly anomalous, resembling closely that of Ce compounds thought to be in or near the mixed-valence regime.
The LCAO, or Bloch, or tight binding, approximation for solids is discussed as an interpolation method, to be used in connection with more accurate calculations made by the cellular or orthogonalized plane-wave methods. It is proposed that the various integrals be obtained as disposable constants, so that the tight binding method will agree with accurate calculations at symmetry points in the Brillouin zone for which these calculations have been made, and that the LCAO method then be used for making calculations throughout the Brillouin zone. A general discussion of the method is given, including tables of matrix components of energy for simple cubic, face-centered and body-centered cubic, and diamond structures. Applications are given to the results of Fletcher and Wohlfarth on Ni, and Howarth on Cu, as illustrations of the fcc case. In discussing the bcc case, the splitting of the energy bands in chromium by an antiferromagnetic alternating potential is worked out, as well as a distribution of energy states for the case of no antiferromagnetism. For diamond, comparisons are made with the calculations of Herman, using the orthogonalized plane-wave method. The case of such crystals as InSb is discussed, and it is shown that their properties fit in with the energy band picture.
Energy bands of the simple cubic intermetallic compounds LaAg and LaCd have been calculated by using both a non-relativistic APW and relativistic KKR method in the muffin-tin-potential approximation. The density of states of LaAg is found to peak sharply just above the Fermi energy and the Fermi surface geometry shows that in this high density-of-states region the conditions are met for the band-Jahn-Teller-effect to drive the martensitic transformation observed in LaAg1_x
In
x
,x >0.05, and in LaCd. The case of YZn, which shows no martensitic transformation, is discussed briefly.
We study the effect of the conduction-electron density of states on magnetic transition temperatures in metallic systems with localized magnetic moments. We assume the indirect-exchange interaction to originate from s-f mixing of the Anderson type and we perform calculations of the coupling parameters, up to fourth-nearest neighbors, in the formalism of da Silva and Falicov. The density-of-states models are obtained from a two-component band in tight-binding scheme and we include on-site Coulomb correlation. We find that the magnetic energy has a modulated Ruderman-Kittel-Kasuya-Yoshida-like behavior as a function of the number of conduction electrons n. This is, however, dominated by two strong maxima when n is such that the Fermi level lies on a peak of the density of states. In this condition the transition temperature is enhanced by at least 1 order of magnitude with respect to the weaker background. This behavior is found for different values of the virtual excitation energy of the s-f mixing mechanism.