Crystal field in valence-fluctuating CeNi-based compounds

The effects of the crystal field (CF) on the paramagnetic Pr ion in a number of compounds of the type R_1−x Pr_xNi (R = Ce, La, Y), in which a transition of the cerium ions from an intermediate-valence into a Kondo state occurs as La is substituted for Ce, are investigated. The level schemes of the Pr ion in the CF are reconstructed from inelastic neutron scattering spectra and the temperature dependence of the heat capacity in different magnetic fields (B=0–8 T). The parameters of the low-symmetry CF in the compounds RNi are determined from the experimental data. It is established that in the Kondo regime the hybridization of the f electrons with conduction electrons only gives a proportional increase in all the parameters of the CF potential. At the same time, partial delocalization of the f electrons in the intermediate-valence state results in charge redistribution, which is manifested in different scales for the changes in the different CF parameters. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 947–952 (25 June 1996)     

Peculiarities of crystal field effects in CeInCu2 based heavy-fermion compounds

We have studied the evolution of the inelastic neutron magnetic scattering spectra of a compound with cubic symmetry, CeInCu₂, in the temperature range 10–130 K, and also their transformation with variation of the Kondo temperature T _K due to substitution of cerium ions in the system Ce_1−x (La,Y)_xInCu₂ at T=10 K. It turns out that the energy of the transition between the ground state and excited state of the 4 f electrons (Δ_CF) in the crystal electric field in CeInCu₂ increases with growth of the population of the ground state as the temperature is reduced, with a slight change in its intensity. Such behavior is inconsistent with the notion of classical one-ion effects of the crystal electric field. We have found that the scale of the observed variations in the excitation spectra of the 4f electrons depends on the Kondo temperature T _K and is insensitive to disorder in the rare-earth sublattice. Thus, despite the fact that T _K ≪Δ_CF, hybridization with states in the conduction band has a substantial effect on all parameters of the excitation spectrum of the ground multiplet of the 4f electrons at low temperatures. Zh. éksp. Teor. Fiz. 115, 2197–2206 (June 1999) A. A. Baikov Institute of Metallurgy     

EPR study of exchange interactions in the nonmagnetic Kondo system La1−x CexCu6

The EPR of paramagnetic impurities Gd³⁺ and Mn²⁺ was studied in nonmagnetic Kondo system La_1−x Ce_xCu₆ containing in the 1.6–200 K range. The exchange interaction parameters of gadolinium and manganese ions with conduction electrons, of cerium ions with conduction electrons and with one another, the Kondo temperature of cerium ions, and the temperature behavior of cerium-ion spin-fluctuation rate have been determined. A pseudogap in the density of states at the Fermi level has been detected in the CeCu₆ regular system, which is apparently due to s-f hybridization. This pseudogap can be destroyed by introducing an aluminum impurity, which induces strong conduction-electron scattering. It was also found that RKKY interaction among manganese ions in CeCu_6−y Mn_y is considerably stronger than it is in LaCu_6−y Mn_y, which implies enhancement of nonlocal spin susceptibility due to an f band contribution to conduction-electron states. Fiz. Tverd. Tela (St. Petersburg) 40, 593–599 (April 1998)     

Effect of composition on the anderson hybridisation in ternary cubic alloys of Ce

Summary The cubic compounds of Ce with Zn, Ag and Cd are considered to behave as Kondo lattices depending on applied pressure or alloying. In particular their magnetic behaviour is ascribed to the possible competition between the RKKY interaction and the Kondo effect. We calculate the hybridisation width Δ of the Anderson model for the two ternary cubic compounds CeCd_1−x Ag _x , CeCd_1−x Zn _x . This quantity determines, along with the positionE _f and correlationU of the Cef-state, the antiferromagnetic coupling constantJ _s-f of the Kondo Hamiltonian. We start fromab initio electronic calculations performed with the FLAPW method in the LSDA and carry out an LCAO parametrisation of the band structure for the pure binary compounds. A virtual-crystal approximation is used to obtain the LCAO parameters of the ternary alloys. We check the accuracy of this approximation on CeCd_0.5Zn_0.5 by a self-consistent FLAPW calculation on the supercell Ce₂CdZn, finding that it is satisfactory for the present purposes. We find that Δ, in the whole concentration range, is affected quantitatively much more by substitution of Cd by Ag than by Zn—isovalent to Cd—, although the latter one induces a larger volume reduction. Moreover, the Δ behaviour in CeCd_1−x Zn _x contrasts with the expected volume dependence. This shows that electronic effects may be more effective than electronic pressure alone (volume effects) in determining the Kondo properties of substitutional systems. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Calorimetric and spectroscopic study of quasi-one-dimensional Haldane magnets (Y1 ? xNdx)2BaNiO5 (x = 1, 0.75, 0.50, 0.25)

The temperature dependences of the heat capacity and optical transmission spectra of quasi-one-dimensional Haldane magnets (Y_{1 − x} Nd _x )₂BaNiO₅ (x = 1, 0.75, 0.50, 0.25) have been studied. All the compounds studied undergoes antiferromagnetic ordering. In the ordered state, the internal magnetic field produces splitting of the ground-state Kramers doublet of Nd³⁺ ion. The temperature dependences of the magnitude of splitting, as determined from spectroscopic data, were used to calculate the Schottky anomaly on the heat capacity curve. Nonequivalent centers related to neodymium ions have been found in compounds with x = 0.75, 0.50, and 0.25.     

Validity of the Anderson and Hubbard models for the description of metallic Ce and cerium heavy fermion compounds

The importance of the inclusion of inter-site f-f hybridization in electron structure calculations for metallic Ce and cerium heavy fermion compounds was studied. We demonstrate that for heavy-fermion systems such as cerium compound CeCu₂Si₂ f-f hybridization can be neglected and Anderson model application is well justified. On another hand for cerium metal f-f hybridization is strong enough to provide the contribution to hybridization function comparable to hybridization between 4f and itinerant electrons. We argue that in the case of Ce only the most general Hamiltonian combining the Hubbard and Anderson models should be used.     

Magnetism in quasibinary systems based on the valence-unstable compound CeNi

Magnetic ordering in solid solutions of Ce _x (Gd,Pr,Nd,La)_1-x Ni is studied by measuring the DC magnetization and the AC susceptibility in the temperature range of 1.8–300 K. The valence state of ceriumions in Ce _x (Gd,Pr,Nd,La)_1-x Ni quasibinary systems is studied based on X-ray absorption spectra measured at synchrotron-radiation sources in the temperature range of 5–300 K. It is shown that chemical pressure and lowering the temperature help heighten the degree of delocalization of the 4f electrons of cerium in Cex(Gd, Nd, Pr)_1-x Ni systems. It is found that the substitution of magnetic ions (Gd, Pr, and Nd) with cerium results in significantly weaker magnetic-ordering suppression than the substitution of these ions with lanthanum at equal concentrations. The obtained data reveal the strong influence of cerium electrons on localized magnetism in the studied compounds. This effect is most probably associated with the contribution of partially delocalized 4f electrons of cerium to the exchange interaction.     

Influence of the martensitic structure transformation in LaAg x In1−x on the magnetic properties of substituted Ce

The martensitic phase transformation has no observable effect on the magnetic susceptibility of Ce in LaAg _x In_1–x which agrees with the assumption of a crystal field splitting by 300 K of the² F _5/2 ground state of Ce³⁺ ions. The magnetic ordering temperature and the Kondo minimum move to higher temperatures together with the martensitic phase transformation when the Ag concentration is reduced. This behavior can be related qualitatively to the lowering of the 5d – e _g levels in the center of the Brillouin-zone with increasing In content.     

Influence of the martensitic structure transformation in LaAgxIn1?x on the magnetic properties of substituted Ce

The martensitic phase transformation has no observable effect on the magnetic susceptibility of Ce in LaAg _x In_1–x which agrees with the assumption of a crystal field splitting by 300 K of the² F _5/2 ground state of Ce³⁺ ions. The magnetic ordering temperature and the Kondo minimum move to higher temperatures together with the martensitic phase transformation when the Ag concentration is reduced. This behavior can be related qualitatively to the lowering of the 5d – e _g levels in the center of the Brillouin-zone with increasing In content.     

Microwave absorption in the Kondo lattice of CeB6 in strong magnetic fields

The characteristic features of microwave (30–120 GHz) magnetoabsorption in the magnetic Kondo lattice of CeB₆ at liquid-helium temperatures in strong magnetic fields are investigated. It is discovered that the absorption structure is the result of the superposition of the ESR of the cerium 4 f electrons, which has a pronounced doublet structure, and features whose position does not depend on the radiation frequency and which are associated with transitions in the magnetic phase diagram of CeB₆. The character of the absorption in the vicinity of the ESR can be linked to the splitting of the ² F _5/2 state in the crystal field; here the g factor is substantially renormalized from g=2.06±0.03 to g=1.83±0.03 as the temperature decreases from 4.2 K to 1.8 K, respectively. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 6, 431–435 (25 March 1996)     

Low-temperature properties of the Ce(Pd1−xNix) system

The influence of substituting Pd by Ni is described in Ce(Pd_1-xNi_x)₃ alloys with x taken up to about 0.25. Thermal and magnetization measurements point out a transition from a non-magnetic state (CePd₃) to a ferromagnetic state for x > 0.05, with a Curie temperature K. The Ce-L_2,3 absorption edges and magnetic circular dichroism (XMCD) study reveals the coexistence of strong 4f hybridization and ferromagnetic order. The Ce-L_2,3 XMCD signal measured in CePd₃ demonstrates that in the Ce-based dense Kondo materials only the 4f¹ channel gives a magnetic response.Received: 2 September 2003, Published online: 15 March 2004PACS: 75.20.Hr Local moment in compounds and alloys; Kondo effect, valence fluctuations, heavy fermions - 75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena - 71.10.Hf Non-Fermi-liquid ground states, electron phase diagrams and phase transitions in model systems     

Kondo effect and impurity-impurity interaction in (La,Ce)B6 alloys

Measurements of the magnetic susceptibility between 0.03 and 300 K and of the magnetization between 0.05 and 10K for magnetic fields up to 60kOe have been used to investigate effects from the interaction between the conduction electrons and local magnetic moments in (La_1–x Ce _x )B₆ alloys (0.0007x0.10). For Ce concentrationsx<0.006 the data show Kondo-type single impurity behavior at low temperatures with a transition from a magnetic to a non-magnetic regime of the Ce ions. In the magnetic regime the impurity susceptibility follows a Curie-Weiss law, and in the non-magnetic regime it varies withT ². An external magnetic field gradually restores the free-ion behavior of the Ce impurities. For more concentrated alloys interactions between the impurities are observed. The RKKY interaction strength derived is more than two orders of magnitude smaller than in the Kondo systemCuFe. Values of thes–f exchange integral,J, estimated from both the Kondo effect and the RKKY interactions are in good agreement. The relatively high Kondo temperature, in spite of a smallJ, can be explained by a density-of-states argument. The influence of crystal field excitation on the susceptibility is also discussed.     

Valence instability of uranium in U(Al1−x Gex)3

The electronic structure of U and Ge in the solid solutions U(Al_1−x Ge_x)₃ is investigated by measuring x-ray line shifts. It is shown that uranium has the mixed valence U³⁺ [Rn](5f ³)-U⁴⁺ [Rn](5f ²) over the entire composition range (0⩽x⩽1) and that the population of the uranium 5f shell increases by ∼0.28 5f electrons/U atom from UAl₃ (x=0) to UGe₃ (x=1). The electronic structure of Ge is close to the electronic structure of Ge metal over the entire composition range 0<x⩽1. No variation of the population of the Ge 4p shell is detected to within the experimental error (∼0.1 4p electrons/Ge atom) as the composition varies from x=0.2 to 1. It is established that the delocalization of a U 5f electron occurs as a result of its transition to the s or d band of the same uranium atom. Fiz. Tverd. Tela (St. Petersburg) 39, 1505–1508 (September 1997)     

Ce(Ⅳ)和Ce(Ⅲ)化合物系列的X射线光电子能谱研究

用XPS研究了标称四价铈盐系列:硫酸铈铵、硫酸铈、二苯基羟乙酸铈、碘酸铈、过氧化铈、二氧化铈以及三价铈盐系列:草酸亚铈、硫酸亚铈、二苯基羟乙酸亚铈、碘酸亚铈和氯化亚铈.结果表明,上述标称四价铈盐都属于混合价化合物,其Ce3d电子能谱呈现Ce⁴⁺和Ce³⁺的二组谱线结构的混合.在真空中加热后CeO₂的Ols电子能谱有两个氧峰,这说明 Ce(4f⁰)O(2p^π)←→Ce(4f¹)O(2p^n-1) 类型价态波动的可能性较大.三价铈盐的3d谱线的低结合能端约3.5—4eV处则存在摔落伴峰.     

Molecular-field theory of moment reduction in Kondo systems with crystal-field effects

Several dense Kondo compounds have a low-temperature ordered phase in which the magnetic moments are reduced with respect to the values expected for the crystal-field (CEF) ground state. In the present work the phenomenon of moment reduction is studied within a molecular-field theory combined with a variational solution of the one-impurity Anderson model with CEF effects. The calculated zero-temperature magnetization and susceptibility agree well with available exact results; the present method is easily applied to systems of any symmetry. We first study the f ¹ configuration in cubic symmetry, for small values of the ratio T _K/Δ between Kondo temperature and CEF splitting. With a Γ _∞ ground state and a field along a 〈100〉 direction, an inflection point occurs in the magnetization curve, which gives rise to a first order transition in the zero-temperature phase diagram. This feature is not found for a field along 〈110〉 or 〈111〉, for which the transition is second order. For a Γ ₇ ground state and small values of T _K/Δ, the magnetic-nonmagnetic transition is second order for all field directions. On increasing T _K/Δ an inflection point in the magnetization curve appears first for a field along 〈111〉. The theory is applied to a study of cubic CeAg, CeAl₂, CePb₃, CeIn₃, CeTe, and hexagonal CePd₂Ga₃. The bare value of the moment is found to be strongly increased by exchange coupling to excited CEF states, and the amount of Kondo reduction is found to be substantial, confirming the importance of the Kondo effect in these compounds.     

Quasiparticle band structure of CeCu2Si2 and CeAl3

The band structure for the ground state is obtained for CeCu₂Si₂ and CeAl₃ within the local-density approximation (LDA). Subsequently, the quasiparticle band structure is calculated on the basis of the Kondo Lattice Ansatz (KLA) for the Cerium 4f state using the LDA potential parameters for all other electrons. The KLA requires the specification of both the symmetry of the scattering channel, which is taken from the crystal-field (CF) ground state, and the energy dependence of the scattering phase shift, for which a Kondo resonance is assumed. Different results are obtained for two choices of the CF ground-state parameters for CeCu₂Si₂. Based on our results the low temperature specific heat is discussed.     

Interplay of negative pressure and hydrogen chemical effects in CeRhSn from first principles

Investigations within the local spin density functional theory (LSDF) of the intermetallic hydride system CeRhSnH_x were carried out for discrete model compositions in the range 0.33 ≤x_H ≤ 1.33 with the purpose of assessing the change of the cerium valence state in the neighborhood of the experimental hydride composition, CeRhSnH_0.8. In agreement with experiment, the analyses of the electronic and magnetic structures and of the chemical bonding properties point to trivalent cerium for 1 ≤x_H ≤ 1.33. In contrast, for lower hydrogen amounts the hydride system stays in an intermediate-valent state for cerium, like in CeRhSn. The influence of the insertion of hydrogen is addressed from both the volume expansion and chemical bonding effects. The latter are found to have the main influence on the change of Ce valence character. Spin polarized calculations point to a finite magnetic moment carried by the Ce 4f states; its magnitude increases with x_H in the range 1 ≤x_H ≤ 1.33.     

Electron spin resonance in the Heusler alloy YbRh2Pb

An electron spin resonance (ESR) signal was observed in a concentrated Kondo lattice, Heusler alloy YbRh₂Pb. It is attributed to the combined effect of the 4f local magnetic moments of Yb³⁺ and conduction electrons. It is shown that the significant broadening and disappearance of the ESR line at temperatures above 20 K is caused by the processes of the spin-lattice relaxation of the Yb³⁺ ions through the first excited Stark doublet with an activation energy Δ ≈ 73.5 K. A comparison of the ESR data for YbRh₂Pb and some other undoped intermetallic compounds based on ytterbium, cerium, and europium indicates that hybridized electronic states occurring as the result of hybridization between the localized 4f electrons and the collectivized conduction electrons constitute a fundamentally new source of ESR.     

Transport and magnetic properties of the compounds YbNi4In and YbNiIn4 with valence-unstable Yb

The electrical resistance, thermal emf, and magnetic susceptibility of the compounds YbNi₄In and YbNiIn₄, with valence-unstable Yb, are measured at temperatures of 4.2–300 K. The valence state of Yb is identified by measuring the x-ray L _III absorption spectra at T=300 K. YbNi₄In is shown to have a Kondo magnetic lattice and exhibit crystal-field effects. The preferred scheme is splitting of the 4f level of Yb³⁺ with doublet formation in the ground and first excited states. In the case of YbNiIn₄, a valence-unstable state of Yb is formed that makes no significant additional contributions to the transport coefficients. Fiz. Tverd. Tela (St. Petersburg) 41, 1918–1921 (November 1999)     

Magnetic ordering and the Kondo effect in the alloys,Ce2Co1−xPdxSi3

The compound Ce₂CoSi₃, crystallizing in a AlB₂-derived hexagonal structure, has been recently identified as a Kondo lattice with a non-magnetic ground state. Here, we report the influence of gradual replacement of Co by Pd on the magnetic behaviour in the pseudo-ternary solid solution, Ce₂Co_1−xPd_xSi_3, by magnetization (2–300 K), electrical resistivity (2–300 K) and heat-capacity (0.7–30 K) measurements to bring out a transformation from non-magnetic to magnetic ordering. Distinct features attributable to the existence of a competition between the Kondo effect and magnetic ordering with varying x and temperature are observed in the electrical resistivity data. The results reveal that small substitutions of Pd (x=0.2–0.3) are sufficient to induce magnetic ordering of the Ce ions at low temperatures. The strength of the Kondo interaction as indicated by the paramagnetic Curie temperature decreases monotonically with increasing Pd content. A notable finding is that there are qualitative changes in the isothermal magnetization data in the magnetically ordered state, as if there are modifications in the magnetic structure with changes in the Co/Pd composition. The importance of electronic structure relative to unit-cell volume in deciding magnetic characteristics of this class of compounds is brought out taking into account the trends in the magnetic behaviour of isostructural Ce compounds.