Kondo lattice and heavy fermions in Heusler phases: CeInAg2−xCux

Magnetic properties, electrical resistivity, specific heat and magnetic excitations have been investigated in Heusler phases CeInAg_2–xCu_x. The hybridization continuously increases from CeInAg₂ (antiferromagnetic Kondo lattice) to CeInCu₂ (heavy fermion compound). The specific heat coefficient for this last compound is found to reach 1.2 J/mole. K² at 1.4 K, the Kondo temperature is 6 K and the Wilson ratio is close to 2.     

Spin-glass, antiferromagnetism and kondo behavior in Ce2Au1−x Co x Si3 alloys

Recently, the solid solution Ce₂Au_{1− x Co} xSi₃ has been shown to exhibit many magnetic anomalies associated with the competition between magnetic ordering and the Kondo effect. Here we report high pressure electrical resistivity of Ce₂AuSi₃, ac susceptibility (X) and magnetoresistance of various alloys of this solid solution in order to gain better knowledge of the magnetism of these alloys. High pressure resistivity behavior is consistent with the proposal that Ce₂AuSi₃ lies at the left-hand side of the maximum in Doniach’s magnetic phase diagram. The ac X data reveal that there are in fact two magnetic transitions, one at 2 K and the other at 3 K for this compound, both of which are spin-glass-like. However, as the Co concentration is increased, antiferromagnetism is stabilized for intermediate compositions before attaining non-magnetism for the Co end member.     

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     

Magnetic susceptibility and electrical resistivity of CexY1−xAl2

The magnetic susceptibility and the electrical resistivity have been measured for Ce_xY_1−xAl₂ (x = 0, 0.05, 0.15, 0.25) between 4.2°K and 300°K. _x3kT vanishes at low temperature. We find a minimum in resistivity at 60°K (x = 5) and 68°K (x = 0.25).     

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.     

Mixed valence on dilute Eu in YCu2Si2

We report measurements of electrical resistivity, thermopower, thermal conductivity, magnetic susceptibility, Mößbauer andL _III x-ray absorption of intermediate valent Eu in Y_1–x Eu _x Cu₂Si₂ alloys withx=0.03,x=0.07 andx=0.10. A qualitative comparison of the physical properties of dilute Eu in YCu₂Si₂ with concentrated Eu in EuCu₂Si₂ shows only slight differences; the dominant feature is a very strong variation of the Eu valence with temperature, from about 2.8 at 4 K to about 2.45 at 800 K, in the concentrated compound as well as in the dilute alloys.     

Delocalization of the Ce 4f shell in amorphous Ce75.5Co24.5

The amorphous alloy Ce_75.5Co_24.5 prepared by melt spinning has been studied through measurements of the magnetic susceptibility, magnetization, electrical resistivity, thermoelectric power and specific heat. The results are interpreted in terms of a homogeneous intermediate valence state of the Ce ions. This is inferred from a temperature-independent magnetic susceptibility at low temperature and the absence of magnetic ordering, a large linear term in the specific heat, and aT ² dependence of the electrical resistivity at low temperature followed by a steep increase with temperature up to 50 K. At this temperature, the thermoelectric power displays a maximum. The intrinsic properties are partially obscured at low temperatures by a contribution from roughly a few percent of magnetic impurities, presumably Ce³⁺ ions. They manifest themselves by an increase of the susceptibility towards low temperatures and by a broad Schottky-like contribution to the specific heat resulting from the excitation of magnetic clusters.Dedicated to Prof. Dr. S. Methfessel on the occasion of his 60th birthday     

Giant magnetoresistance in Ce-doped manganite systems

The fascinating properties like giant magnetoresistance (GMR) effect, metal-insulator transition, charge ordering phenomenon etc. have made the divalent ion doped RMnO₃ (R = rare-earth elements) an attractive system for investigation. Resistivity of these compounds shows a peak near the ferromagnetic transition temperature (T _c ). The application of magnetic field inhibits the spin-disorder scattering and the resistivity decreases drastically. Keeping electrondoped superconductor Nd_2?x Ce _x CuO₄ in mind we have doped RMnO₃ (R = La, Pr, Nd) with tetravalent Ce ion. These compounds are very susceptible to the annealing treatment and belong to the orthorhombic perovskite phase. They show a very high value of resistivity at the peak and under the magnetic field the GMR effect is observed. For La_0.7Ce_0.3MnO₃ and Pr_0.7Ce_0.3MnO₃ the magnetoresistance ratio reaches about 54% and 82.5% respectively at 7.7 T. With the increase of the temperature the magnetic state changes from ferromagnetic to paramagnetic regime. This magnetic transition is not very sharp and the resistivity peak appears at a temperature higher than T _c .     

Evolution of 4f heavy fermion state from 5f heavy fermion state in U1−xCexPd2Al3 system

We report the effect of Ce substitution for U in the heavy-electron antiferromagnetic superconductor UPd₂Al₃. CePd₂Al₃ is an established non-superconducting heavy-electron system which undergoes antiferromagnetic ordering below 2.8 K. Thus studies on U_1−xCe_xPd₂Al₃ system provide a unique opportunity to observe the evolution from a 4f heavy-electron state to a 5f heavy-electron state. We have measured the resistivity and magnetic susceptibility from 1.5 to 300 K and the heat capacity from 2 to 20 K for several U_1−xCe_xPd₂Al₃ samples. Our studies show that the antiferromagnetic (AF) ordering temperature (T_N) of U_1−xCe_xPd₂Al₃ does not decrease monotonically from T_N = 14 K for UPd₂Al₃ to T_N = 2.8 K for CePd₂Al₃ but rather shows a local maximum of 5 K near x = 0.4.     

Effect of substitution of Ce on superconducting properties of Bi1.7Pb0.3Sr2Ca2−x Ce x Cu3O10+δ system

We have investigated the superconducting properties of the Bi_1.7 Pb_0.3Sr₂Ca_2−xCe _x Cu₃O_10+δ system with x=0.00, 0.02, 0.04, 0.08 and 0.1 by X-ray diffraction and magnetic susceptibility. The substitution of Ce for Ca has been found to drastically change the superconducting properties of the system. X-ray diffraction studies on these compounds indicate decrease in the c-parameter with increased substitution of Ce at Ca site and volume fraction of high T _c (2 : 2 : 2 : 3) phase decreases and low T _c phase increases. The magnetic susceptibility of this compound shows that the diamagnetic on set superconducting transition temperature (onset) varies from 109 K to 51 K for x=0.00, 0.02, 0.04, 0.08 and 0.1. These results suggest the possible existence of Ce in a tetravalent state rather than a trivalent state in this system; that is, Ca²⁺ → Ce⁴⁺ replacement changes the hole carrier concentration. Hole filling is the cause of lowering T _c of the system.     

A new family of ternary intermetallic superconducting/magnetic stannides

A new family of rare earth-rhodium-tin intermetallic compounds, with the representative formula (RE)Rh_xSn_y, has been synthesized in single crystal form. The compounds containing the heavier rare earths are superconducting and those with the lighter rare earths are generally magnetic. The compound ErRh_1.1Sn_3.6 exhibits reentrant superconductivity with T_c = 0.97 K and T_m = 0.57 K as determined from ac magnetic susceptibility measurements. The synthesis and X-ray characterization of the series are described and the results of electrical resistivity, upper critical magnetic field, magnetic susceptibility, specific heat and neutron scattering measurements on the Er compound are given.     

Chemical pressure effect in CeFeAs1−xPxO0.95F0.05

We investigate the chemical pressure effect due to P doping in the CeFeAs_1−xP_xO_0.95F_0.05(0≤x≤0.4) system. The compound CeFeAsO_0.95F_0.05 without P doping is on the boundary between antiferromagnet (AFM) and superconductor. The AFM order of Ce³⁺ local moments causes a significant reentrance behavior in both resistivity and magnetic susceptibility. Upon P doping, T_c increases and reaches a maximum of 21.3 K at x=0.15, and then it is suppressed to lower temperatures. Meanwhile, the AFM order of Ce³⁺ ions remains nearly the same in the whole doping range (0≤x≤0.4). Our experimental results suggest a competition between superconductivity and Kondo effect in the Ce 1111 system.     

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.     

Atomic structure and transport and magnetic properties of the Sm1−x SrxMnO3 system

This is the first study of the temperature dependences of the atomic structure by neutron diffraction, as well as of the resistivity, differential magnetic susceptibility, and magnetoresistance of the ceramic system ¹⁵⁴Sm_1−x Sr_xMnO₃ (x∼0.16–0.4). Samples (x⩾0.3) having an initially orthorhombic structure transfer upon cooling from the insulating to the metallic state and exhibit giant magnetoresistance, which at liquid-helium temperature reaches as high as 90% in magnetic fields up to 30 kOe. At lower doping levels (x⩽0.25), the compound has monoclinic structure. The resistivity of such compounds in zero magnetic field displays insulating behavior upon lowering the temperature to 77 K. Fiz. Tverd. Tela (St. Petersburg) 40, 1271–1276 (July 1998)     

Magnetic ordering in Ce2RhSi3

The results of magnetic susceptibility, heat-capacity, electrical resistivity and magnetoresistance measurements on the compound Ce₂RhSi₃ prove that this alloy orders antiferromagnetically at 6 K in support of an original report by Chevalier et al. [Solid State Commun. 49 (1984) 753] but in contrast to the conclusions from neutron diffraction results [Szytula et al., J. Magn. Magn. Mater. 118 (1993) 302].     

The effect of spin dilution on magnetism of the linear chain system β-Cu2?xZnxV2O7

We have measured the magnetic susceptibility (χ) and heat capacity (C _p) of β-Cu_2−x Zn _x V₂O₇ (x = 0, 0.05, 0.1, 0.15, 0.2, 0.3, 2) in the temperature range 2–300 K. A one-dimensional alternating exchange Heisenberg antiferromagnetism (HAF) is observed in all compositions with chains of infinite length. The intra-chain exchange remains uniform and decreases marginally with dilution of the magnetic state. A cooperative ordering is seen in the magnetic chains for all Zn concentrations (x ≤ 0.3). The temperature of occurrence of this transition decreases with increasing Zn concentration. Though the conventional spin-wave theory has been used here to describe the properties of the ordered phase, the presence of some contributions like the lattice heat capacity in C _p and the Curie-Weiss term in susceptibility introduces some uncertainties in the estimation of the proportions contributed by the spin system. Therefore, the nature of the ordered phase could not be ascertained unambiguously.     

Chemical pressure effects on the Kondo behaviour of CePt2Ge2

The compound, CePt₂Ge₂, recently reported to crystallize in a monoclinic modification of the CaBe₂Ge₂ structure, has been identified to be an antiferromagnetic Kondo lattice (T _N =2.2 K). The influence of positive and negative chemical pressure on the Kondo behavior is investigated by measurements of magnetic susceptibility and electrical resistivity () on the solid solutions, Ce_1–x La _x Pt₂Ge₂ and Ce_1–z Y _z Pt₂Ge₂. Y and La substituents cause nearly similar effects on the magnetic and electrical resistivity behaviour of CePt₂Ge₂, though the unit-cell volumes vary in opposite directions. We propose that the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction may contribute to decide the temperature at which attains a maximum at low temperatures (5 K) in CePt₂Ge₂.     

Thermal properties of dilute (La,Ce)Al2 alloys

We report on measurements of the specific heat and transport properties (resistivity, heat conductivity and thermopower) of dilute (La_1–x Ce _x )Al₂ alloys withx0.04 between 0.3 and 10 K. The specific heat experiments were extended to a magnetic field of 47 kØe; the transport measurements were performed on single crystals. For Ce concentrations less than 0.5 a/o Kondo type single impurity behavior was observed for all quantities under investigation. A mean Kondo temperature was obtained from the S=1/2 exchange model using our own data taken in the normal as well as in the superconducting state and data from the literature. Whereas neither the mean free path nor interimpurity correlations were significantly reflected in the specific heat, they obscured the transport properties above 0.5 a/o. The influence of crystal field excitation and band structure effects on the experimental results is also discussed.Work performed within the research program of the Sonderfor-schungsbereich 125—Aachen/Jülich/Köln.     

Magnetic susceptibility study of Ce in PbCeA (A=Te,Se, S)

The magnetic susceptibility of Pb_1-xCe_xA (A=S, Se and Te) crystals with Ce³⁺ concentrations 0.006≤x≤0.036 was investigated in the temperature range from 2 K to 300 K. The magnetic susceptibility data was found to be consistent with a ²F_5/2 lowest manifold for Ce³⁺ ions with a crystal-field splitting Δ=E(Γ₈)−E(Γ₇) of about 340 K, 440 K and 540 K for Pb_1-xCe_xTe, Pb_1-xCe_xSe, and Pb_1-xCe_xS, respectively. For all the three compounds the doublet Γ₇ lies below the Γ₈ quadruplet which confirms the substitution of Pb²⁺ by Ce³⁺ ions in the host crystals. The observed values for the crystal-field splitting are in good agreement with the calculated ones based on the point-charge model. Moreover, the effective Landé factors were determined by X-band (∼9.5 GHz), electron paramagnetic measurements (EPR) to be g=1.333, 1.364, and 1.402 for Ce ions in PbA, A = S, Se and Te, respectively. The small difference with the predicted Landé factor g of 10/7 for the Γ₇ (J=5/2) ground state was attributed to crystal-field admixture.     

Valence fluctuation in Ce2Co3Ge5 and crystal field effect in Pr2Co3Ge5

Polycrystalline samples of ternary rare-earth germanides R₂Co₃Ge₅ (R=La, Ce and Pr) have been prepared and investigated by means of magnetic susceptibility, isothermal magnetization, electrical resistivity and specific heat measurements. All these compounds crystallize in orthorhombic U₂Co₃Si₅ structure (space group Ibam). No evidence of magnetic or superconducting transition is observed in any of these compounds down to 2 K. The unit cell volume of Ce₂Co₃Ge₅ deviates from the expected lanthanide contraction, indicating non trivalent state of Ce ions in this compound. The reduced value of effective moment (μ_eff≈0.95 μ_B) compared to that expected for trivalent Ce ions further supports valence-fluctuating nature of Ce in Ce₂Co₃Ge₅. The observed temperature dependence of magnetic susceptibility is consistent with the ionic interconfiguration fluctuation (ICF) model. Although no sharp anomaly due to a phase transition is seen, a broad Schottky-type anomaly is observed in the magnetic part of specific heat of Pr₂Co₃Ge₅. An analysis of C_mag data suggests a singlet ground state in Pr₂Co₃Ge₅ separated from the singlet first excited state by 22 K and a doublet second excited state at 73 K.