Magnetic susceptibility mean-field approximation

Fig. 8.31. Temperature dependence of magnetic susceptibilities for the 4TXg term at different applied fields m—mean magnetic susceptibility d—differential magnetic susceptibility (solid) a— approximate magnetic susceptibility based on the van Vleck equation.

A simple model to explain the [iSR response in CeNiSn has been offered by Kagan and Kalvius (1995). They propose that the features seen by [iSR are the manifestation of a very small difference in the ground-state energy of the spin-liquid phase and a magnetically ordered phase. This results in quasicritical behavior and is equivalent to the appearance of an effective Curie temperature, which is located formally in the negative temperature region (-7 ). Within the mean-field approximation (MFA) and for small values of T one finds for the static susceptibility ... 

Fig. 8.32. Temperature dependence of effective magnetic moments for the 4Tlg term at different applied fields m—based on the mean magnetic susceptibility d—based on the differential magnetic susceptibility (solid) a—based on the approximate magnetic susceptibility via the van Vleck equation.

Both the Anderson and the Kondo (or Coqblin-Schrieffer) model have been solved exactly for thermodynamic properties such as the 4f-electron valence, specific heat, static magnetic and charge susceptibilities, and the magnetization as a function of temperature and magnetic field B by means of the Bethe ansatz (see Schlottmann 1989, and references therein). This method also allows one to calculate the zero-temperature resistivity as a function of B. Non-equilibrium properties, such as the finite temperature resistivity, thermopower, heat conductivity or dynamic susceptibility, could be calculated in a self-consistent approximation (the non-crossing approximation), which works well and is based on an /N expansion where N is the degeneracy of the 4f level. 

---