Theory of Electron Spin Resonance in Ferromagnetically Correlated Heavy Fermion Compounds

We studied 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 correlations (FM) 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 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-quartet. Four transitions are expected for individual Ce ions with a Γ8 ground-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 addressed the effects of the interplay of AFQ and FM on the ESR line width and the phase diagram. It is usually difficult to distinguish among ESR resonances due to 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. The longitudinal magnetic susceptibility has a quasi-elastic central peak of line width 1/T1 and inelastic peaks for the absorption/emission of excitations. The latter are measured via 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.