Theory of Electron Spin Resonance in Ferromagnetically Correlated Heavy Fermion Compounds

Abstract

Abstract: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. CeB 6 is a heavy fermion compound with cubic symmetry having a Γ 8 ground-quartet. Four transitions are expected for individual Ce ions with a Γ 8 ground-multiplet, but only one has been observed. Antiferro-quadrupolar order (AFQ) arises below 4 K due to the orbital content of the Γ 8 -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 CeB 6 , 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/T 1 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 CeB 6 in the context of the picture that emerged from the ESR experiments.

Field-Angle-Resolved Magnetic Excitations as a Probe of Hidden-Order Symmetry inCeB6

“…On the other hand, the model predicts no additional resonances at the Γ point below the energy of resonance A, which means that it is unable to explain the appearance of the resonance B in ESR data at fields above 12 T [41]. Theoretical considerations about the possible origin of this high-field resonance were proposed earlier by Schlottmann [77][78][79], but the reason why it cannot be seen in INS measurements remains unclear. The dispersion of resonance B, as well as its relationship to the multipolar excitations described in our work, also remain to be clarified.…”

“…The same anisotropic field-angular dependence could be well described by a model proposed in Refs. [77][78][79] by Schlottmann.…”

Evolution of the propagation vector of antiferroquadrupolar phases inCe3Pd20Si6under magnetic field

“…Note that as soon as phase II is suppressed, giving way to the field-polarized paramagnetic phase I in the phase diagram [see Fig. 5 (g)], a new minimum in the dispersion develops near the zone center, which can be interpreted as the resonance [36]. Simultaneously, the (100) mode shifts to higher energies, as be seen in the 5.5 T data in Fig.…”

Special Issue: Electron Paramagnetic Resonance

“…Recent achievements in both theory and instrumentation have made EPR central in order to study new interdisciplinary topics. Such achievements are found in the collection of papers from the Special Issue, devoted to EPR, with contributions in technical improvement [1,2], theory in physics [3], biology [2,4,5], and biochemistry [6]. This issue is articulated around three original articles and three specific reviews.…”

“…More than 600 papers were presented and classified, showing the entire history as well as recent improvements and state of the art in the domain. Pedro Schlottmann [3] presented the theory of EPR in ferromagnetically correlated heavy fermion, and more specifically, the role of the Kondo effect (screening of magnetism of impurity by conduction electrons) on EPR signals.…”