Line shape of magnetic spin resonance at 60 GHz specific to the phase II ͑so-called antiferroquadrupole phase͒ of CeB 6 was studied. The applied procedure of data analysis has allowed obtaining g factor of the oscillating magnetic moments, line width, and oscillating magnetization. It is found that the approaching to the transition temperature T I-II from phase II to the paramagnetic phase I results in strong broadening of the resonance ͑the line width increases three times in the range 1.8 K Յ T Յ 3.8 K͒ whereas g factor g = 1.59 remains temperature independent. Magnetic-resonance data suggests that the magnetization of CeB 6 in the phase II consists of several contributions, one of which is responsible for the observed magnetic resonance. This term in magnetization is missing in the paramagnetic phase and corresponds to ferromagnetically interacting localized magnetic moments. The magnitude of the oscillating part of magnetization is less than total magnetization in the range T ‫ء‬ Յ T Յ T I-II and coincides with the total magnetization for T Յ T ‫ء‬ , where T ‫ء‬ ϳ 2 K. We argue that ferromagnetic correlations play a key role in the observed phenomenon in analogy with the recent experimental and theoretical results on the magnetic resonance in the dense Kondo systems. At the same time the interpretation of the magnetic-resonance data in the framework of the existing models of magnetism in CeB 6 faces substantial difficulties, which demands further development of the theory of static and dynamic magnetic properties of this heavy fermion metal.

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The existence of quadrupolar interaction driven magnetic ordering has been established for various rare earth intermetallic compounds [1][2][3][4]. In the studied cases the quadrupole magnetic moments at some critical temperature T Q become ordered whereas dipole (i.e. ordinary) magnetic moments remain disordered. With lowering temperature the magnetic dipoles may become ordered undergoing a ferromagnetic or antiferromagnetic transition inside the quadrupole phase at T D < T Q . Note that if the strength of the dipole interaction will be sufficient to change the inequality to opposite, T D > T Q , the ordering of dipoles will with necessity induce the ordering of quadrupoles and the one and only magnetic transition will occur. In the latter case, however, the quadrupole interactions may modify the characteristics of the dipolar transition [1][2][3].Cerium hexaboride is a well-known example of a compound where quadrupole magnetic interactions play an essential role [5 -10]. The dipole and quadrupole magnetic moments are associated with the Ce 3+ ions forming a simple cubic lattice [5,6]. In zero magnetic field the quadrupole ordering occurs at T Q = 3.2 K and precedes the formation of an antiferromagnetic phase (i.e. dipole ordering) at T D = 2.3 K. In the region T > T Q cerium hexaboride is a paramagnetic metal and demonstrates behaviour typical of a dense Kondo system. The application of the external magnetic field induces an enhancement of T Q and suppression of T D as shown in the magnetic phase diagram (inset in Fig. 1). This sequence of phase transitions in CeB 6 has been proved by means of neutron diffraction [5] and resonant X-ray scattering [6] studies as well as by specific heat [7], NMR [8], magnetisation [9,10] and transport measurements [11]. The microscopic nature of the quadrupole ordering still remains the subject of discussions [12]. However it is widely accepted that the crystal field splitting of the 2 F 5/2 level of Ce ion leads to the lowest in energy Γ 8 term, whose symmetry allows the existence of the quadrupole moment [12]. The neutron scattering evidence [5] of an antiferromagnetic component with a wave vector, , ] in the quadrupole phase implies two types of nonequivalent Ce ions having quadrupole moments +Q and -Q and arranged in an alternating three-dimensional structure. Thus the ordered phase of CeB 6 at T < T Q is referred as an antiferro-quadrupole phase [5 -12].The coupling between quadrupole moments and dipole moments suggests the study of the antiferro-quadrupole phase of CeB 6 by means of various magnetic techniques, not excluding a priori resonant measurements. However, for dense Kondo systems the spin fluctuations at the magnetic ions are generally believed to broaden the resonance For strongly correlated electronic systems in addition to magnetically ordered phases based on the interaction between magnetic dipoles magnetic ordering originating from the quadrupolar magnetic interactions is possible. Up to now any information about magnetic resonances specific to quadrupolar or...