ABSTRACT
In this chapter, we study the electron spin resonance (ESR) line width for localized moments within the framework of the Kondo lattice model. Only for a sufficiently small Kondo temperature can an ESR signal be observed for a Kondo impurity. On the other hand, for a Kondo lattice representing a heavy-fermion compound, short-range ferromagnetic (FM) correlations between the localized moments are crucial to observe a signal. The spin relaxation rate (line width) and the static magnetic susceptibility are inversely proportional to each other. The FM correlations enhance the susceptibility and hence reduce the line width. For most of the heavy-fermion systems displaying an ESR signal the FM order arises in the ab-plane from the strong lattice anisotropy.
CeB6 is a heavy-fermion compound with cubic symmetry having a Γ8 ground-state quartet. Four transitions are expected for individual Ce ions with a Γ8 ground-state multiplet, but only one has been observed. Antiferro-quadrupolar order (AFQ) arises below 4 K due to the orbital content of the Γ8-quartet. We address the effects of the interplay of AFQ and FM correlations on the ESR line width and the phase diagram. It is usually difficult to distinguish among ESR resonances due to individual localized moments and conducting heavy electron spins, especially for anisotropic Ce and Yb compounds. However, for CeB6 an itinerant picture within the AFQ phase is necessary to explain the electron spin resonances, ruling out the individual localized moments (impurities) picture.
The longitudinal magnetic susceptibility has a quasielastic central peak of line width 1/T 1 and inelastic peaks for the absorption/emission of excitation. The latter are measured by inelastic neutron scattering (INS) and provide insights into the magnetic order. We briefly summarize some of the INS results for CeB6 in the context of the picture that emerged from the ESR experiments.
