Electron spin resonance in antiferro-quadrupolar-ordered CeB6

Abstract

CeB6 is a cubic heavy fermion compound with a Γ8 ground-quartet for which an ESR signal was observed. All other Ce or Yb compounds displaying an ESR signal have strong magnetic anisotropic and ferromagnetic correlations among the spins. The role of the ferromagnetic correlations is to narrow the resonance width rendering the signal observable. In CeB6 the orbital content of the Γ8-quartet gives rise to an antiferro-quadrupolar ordered phase below 4 K. Single ions with a Γ8 ground-multiplet are expected to display four transitions, however, only one has been observed. The following questions are addressed in this paper: (1) why is only one transition seen?, (2) why is this transition observed if the Kondo temperature is comparable to the linewidth and the resonance frequency?, and (3) are there ferromagnetic correlations between the Ce ions? The answer to these questions is associated with the antiferro-quadrupolar order. While for other 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 antiferro-quadrupolar phase is necessary for CeB6.

Angular Dependences of ESR Parameters in Antiferroquadrupolar Phase of CeB6

“…1a) [12]) and therefore ESR is entirely settled in the AFQ phase diagram area. In this case the theoretical model [5,6] should be applicable to the experimental data and it is reasonable to make their comparative analysis. The angular dependence obtained in the rotating resonator (Fig.…”

“…It is visible that the direction [100] corresponds to the maximum of g(Θ) dependence, then g(Θ) quickly drops apart from this direction. The theoretical model [5,6] contains an internal parameter φ which is the angle between two quadrupolar sublattices which provides different g(Θ) for every φ. However, according to [5,6] it is possible to localize the area of g(Θ).…”

“…The AFQ phase at zero magnetic field develops below T Q = 3.2 K. Further temperature decrease transfers this compound into AFM phase below T N = 2.4 K. Magnetic field application stabilizes the AFQ phase causing an increase of T Q as well as a suppression of AFM order below T N . The experimental observation of ESR in CeB 6 [1][2][3] induced a novel theoretical approach describing ESR in the AFQ phase [5,6]. It is based on the consideration of Γ 8 state of Ce 3+ ions forming two orbitally ordered sublattices.…”

Theory of Electron Spin Resonance in Ferromagnetically Correlated Heavy Fermion Compounds

“…The problem is conveniently studied within the framework of the Anderson lattice which leads to hybridized localized and conduction states. 40 Within the reduced Brillouin zone there are now twice as many bands,…”

“…40 For heavy fermions, the Fermi level intersects the lower hybridized band close to the band-gap, leading to one resonance with effective g-factor g ef f = (g 1 + g 2 )/2. This result depends on two angles, namely, the angle θ of the magnetic field with the crystal axis and an angle ϕ defining the AFQ long-range order.…”