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.
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...
In zero magnetic field, the famous neutron spin resonance in the f-electron superconductor CeCoIn 5 is similar to the recently discovered exciton peak in the nonsuperconducting CeB 6 . A magnetic field splits the resonance in CeCoIn 5 into two components, indicating that it is a doublet. Here we employ inelastic neutron scattering (INS) to scrutinize the field dependence of spin fluctuations in CeB 6 . The exciton shows a markedly different behavior without any field splitting. Instead, we observe a second field-induced magnon whose energy increases with field. At the ferromagnetic zone center, however, we find only a single mode with a nonmonotonic field dependence. At low fields, it is initially suppressed to zero together with the antiferromagnetic order parameter, but then reappears at higher fields inside the hidden-order phase, following the energy of an electron spin resonance (ESR). This is a unique example of a ferromagnetic resonance in a heavy-fermion metal seen by both ESR and INS consistently over a broad range of magnetic fields. PACS numbers: 71.27.+a, 76.50.+g, 78.70.Nx, 76.30.Kg INTRODUCTIONThe observation of neutron spin resonance within a broad range of materials, in particular high-T c cuprates [1], iron pnictides [2,3], and heavy-fermion superconductors [4][5][6], is recognized as an indicator of unconventional superconductivity. It was shown that sign-changing gap symmetry can lead to the existence of resonance behavior [7][8][9][10]. Of particular interest are inelastic neutron scattering (INS) results obtained on CeCoIn 5 , where a sharp resonance peak was observed within the superconducting phase [5,[11][12][13]. At first glance similar peaks were found in the antiferromagnetic (AFM) superconductor UPd 2 Al 3 [14,15], as well as in the normal state of the heavy-fermion metal YbRh 2 Si 2 [16], where superconductivity was recently discovered below ∼ 2 mK [17]. Another striking example of a resonant mode is given by the well known nonsuperconducting heavyfermion antiferromagnet CeB 6 [18,19]. The microscopic origins of such resonant magnetic excitations persisting in f-electron systems either with or without superconductivity may well differ among materials and are still hotly debated.The application of an external magnetic field may help to unmask the differences between these various excitations. For instance, among f-electron compounds, a weak quasielastic signal gives rise to a field-induced ferromagnetic (FM) excitation in CeRu 2 Si 2 [20]. In YbRh 2 Si 2 , two incommensurate excitation branches merge into a commensurate FM resonance whose energy scales linearly with magnetic field [16], whereas in UPd 2 Al 3 the energy gap initially remains almost constant inside the superconducting phase, but starts following a monotonic linear dependence at higher magnetic fields [14]. The sharp resonance in CeCoIn 5 splits into a Zeeman doublet [11] rather than a theoretically predicted triplet [21], whereas in Ce 1−x La x B 6 the magnetic field reportedly leads to a crossover from an itinerant to ...
Experimental evidence of the magnetic resonance in the antiferro-quadrupole phase of CeB6 is reported. We have shown that below orbital ordering temperature a new magnetic contribution from localized magnetic moments (LMM) emerge and gives rise to observed resonant phenomenon. This behaviour is hardly possible to expect in dense Kondo system, where LMM should vanish al low temperatures rather than emerge. From the other hand, in the quadrupole ordering concept, where magnetism of Ce magnetic ions is solely accounted, is difficult to explain splitting of magnetisation into components having different physical nature. Therefore an adequate theory explaining magnetic properties of CeB6 including magnetic resonance and orbital ordering appears on the agenda.Comment: 4 pages, Accepted paper for MISM05 proceeding
Cavity measurements of a high frequency ͑60 GHz͒ electron spin resonance ͑ESR͒ have been carried our for a single crystal of EuB 6 at temperatures 4.2-50 K in magnetic field up to 7 T, which has been aligned along ͓001͔ crystallographic direction. It is found that in the case of homogeneous magnetic field in the sample the ESR spectrum of EuB 6 is formed by a single line at all temperatures including the ferromagnetic ordering region, whereas the gradient of magnetic field in the samples induces double peak ESR structure at low temperatures. For the quantitative description of the ESR line shape we suggested an analytical approach applicable for the cavity measurements of a metal with an arbitrary value of magnetic permeability including strongly magnetic case and obtained full set of the ESR parameters, namely oscillating magnetization M 0 , g factor, and linewidth. Our analysis has explained the visible resonance line shift and revealed the coincidence of the oscillating magnetization defining the resonance amplitude with the static magnetization. The anomalous growth of the linewidth below Curie temperature T C Ϸ 15 K is observed. We argue that ESR in EuB 6 is not considerably affected by either interaction with the spin polarons or the magnetic phase separation and reflects merely the oscillation of the Eu 2+ localized magnetic moments, which can be well understood within meanfield approximation.
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