Theory of Electron Spin Resonance in Ferromagnetically Correlated Heavy Fermion Compounds

We study the electron spin resonance (ESR) line width for localized moments within the framework of the Kondo lattice model. An ESR signal for an impurity can only be observed if the Kondo temperature is sufficiently small. On the other hand, for the Kondo lattice, short-range ferromagnetic correlations (FM) between the localized spins are necessary to obtain an observable signal. The spin relaxation rate (line width) is inversely proportional to the static magnetic susceptibility. The FM enhance the susceptibility and hence reduce the line width. For most of the heavy fermion systems displaying an ESR signal the FM arise in the ab-plane from the strong lattice anisotropy. An ESR signal was observed in the cubic heavy fermion compound CeB6 which has a Γ8 ground-quartet. The orbital content of the Γ8-quartet gives rise to an antiferro-quadrupolar ordered (AFQ) phase below 4 K. Single ions with a Γ8 ground-multiplet are expected to display four transitions, however, only one has been observed. We address the effects of the interplay of AFQ and FM on the phase diagram and the ESR line width. While for anisotropic Ce and Yb compounds with ESR-signal it is difficult to distinguish if the resonance is due to localized spins or conducting heavy electron spins, an itinerant picture within the AFQ phase is necessary to explain the electron spin resonances for CeB6.  The longitudinal magnetic susceptibility has a quasi-elastic central peak of line width 1/T1 and inelastic peaks for the absorption/emission of excitation. 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.