Giant kondo resistivity in (La, Ce) B6

The electrical resistivity of the system $\text{(}\text{La}\text{, Ce)B}{}_6$ has been measured in the temperature range 0.04–300 K. The alloys show a Kondo minimum at about 20 K and a strong increase of the resistivity with decreasing temperature. The low temperature increase of the resistivity due to the Kondo effect of a sample containing 1, 2 at.% is about half as large as the room temperature resistivity of LaB₆. The Kondo temperature T_K of $\text{(}\text{La}\text{, Ce)B}{}_6$ was found to be 1.1 ± 0.2 K by comparison of the experimental results with theoretical predictions of the resistance anomaly associated with the formation of the spin-compensated state.     

Electrical resistivity and magnetoresistance of CeB6

Electrical resistivity and magnetoresistance of CeB₆ single crystal have been measured in the temperature range from 1.3 to 300 K under the magnetic field up to 85 kOe. Three characteristic phases are distinguished consistently with other measurements. The Kondo like behaviour in the resistivity observed in the high temperature phase is fitted by the conventional form for the dilute Kondo state with the Kondo temperature T_K = 5 ～ 10K and the unitarity limit resistivity $\text{?}{}_{\mathrm{u}}\text{? 110 μΩ}$ cm/Ce-atom. The negative magnetoresistance in the middle phase is stronger with increasing magnetic field and with decreasing temperature suggesting rapid destruction of the Kondo state. The magnetoresistance in the low temperature phase exhibits some anomalies suggesting sub-phases corresponding to several kinds of spin ordering.     

Kondo effect in spin glasses: Electrical resistivity

We investigate the influence of the Kondo effect on the electrical resistivity of spin glasses. Our approximation reduces in the limit of vanishing impurity concentrations to the Suhl-Nagaoka theory for the Kondo effect. The magnetic impurity interactions are taken into account in the form of time dependent two-spin correlation functions which can be measured by neutron scattering. The dynamics of the impurity spins leads to a partial destruction of the Kondo effect. For the resistivity this can be described by a temperature dependent effective spinS _eff ² (T) withS _eff ² 0 forT0 andS _eff ² S(S+1) forT, and by a reduction of the Kondo temperatureT _K. Sufficiently strong interactions lead toT _K=0. We obtain a resistivity maximum at a temperatureT _m due to the interplay of the Kondo effect and the spin dynamics;T _m depends onT _K and on the excitation spectrum, and therefore on parameters such as impurity concentration or pressure. The ratioT _m/T_f (T _f is the freezing temperature) is calculated for a single relaxation time and for a square density of relaxation modes and is compared with experimental data forAuCr,AuMn,AgMn, andCuMn. The influence of other possible modes on various spin glass properties is discussed.SFB 125 Aachen-Jülich-Köln     

Magnetic hyperfine field of arsenic in nickel by NMR/ON of71,72As

The magnetic hyperfine splitting frequencies of⁷¹AsNi and⁷²AsNi in a 0.11 Tesla external magnetic field have been determined by NMR/ON method as 66.00(6) MHz and 106.17(13) MHz respectively. Using the known magnetic moments of μ(⁷¹As)=1.6735(18) and μ(⁷²As)=−2.1566(3), the hyperfine fields were deduced asB _hf(⁷¹AsNi)=12.824(19) Tesla andB _hf(⁷²AsNi)=12.807(16) Tesla.     

Ion implantation as a tool for studying the Kondo-systemZnMn

In the case ofZnMn alloys it is shown how low temperature ion implantation can be used for the production of dilute magnetic alloys. The Kondo effect in the resistivity (T) was studied between 1.1K and 20 K. The phonon part of the resistivity was subtracted using (T)-curves from Zn-Films with similar disorder produced by self-ion implantation. A logT-dependence with a slope proportional to the Mn-concentration was found below 200 ppm. The influence of disorder on the specific Kondo slope is discussed.Dedicated to Prof. Dr. W. Buckel on the occasion of his 60th birthday.     

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.     

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.     

On the behaviour of the new Kondo lattice CeCuAl3

The temperature dependence of the electrical resistivity, thermopower, specific heat, susceptibility and magnetization of CeCuAl₃ are presented. CeCuAl₃ behaves as a Kondo lattice system with antiferromagnetic ground state properties (T _N 2.8 K). The valency of Ce in this tetragonal compound is close to 3 and the overall crystal field splitting found from our results is about 150 K. The Kondo temperatureT _K in the crystal field ground state, estimated from the magnetic susceptibility and the specific heat, is of the order of 8 K.     

Superconducting properties of ion implantedIn Mn alloys

Annealed Indium films have been implanted with 150 keV-Mn ions at low temperatures. TheT _c -depression and the low temperature part of the resistivity exhibit the Kondo effect, the Kondo temperature being strongly influenced by lattice defects. The lattice disorder has been increased by pre-implantation of In ions. The influence of lattice disorder on the magnetic properties ofIn Mn alloys is explained by applying the strong coupling theory of superconductors.It is found that the main effect results from a shift of the Fermi energy relative to the virtual bound state, due to a variation of the effective massm ₀·(1+).     

Magnetic order in a Kondo lattice: A neutron scattering study of CeCu2Ge2

Elastic and inelastic neutron scattering studies of the Kondo lattice CeCu₂Ge₂ were performed. AtT _N=4.1 K an incommensurate magnetic order develops with an ordering wave vectorq ₀=(0.28, 0.28, 0.54) and an ordered moment µ _s =0.74 µB. The crystalline electric field splits the 4f ¹-J-multiplet of the Ce ion into a ground state doublet and a quartet at 191 K. The wave function of the ground state yields an ordered moment of 1.54µB. Thus, due to the onset of the formation of a Kondo singlet the magnetic moment is considerably reduced. The magnetic relaxation rate was investigated via quasielastic neutron scattering. The temperature dependence of (T) is characteristic of heavy-fermion systems with a high temperature square root dependence and a limiting low temperature value, yielding a Kondo temperatureT _K10K. The quasielastic component of the scattered neutron intensities persists down to the lowest temperatures, well belowT _N. This quasielastic line is regarded as a characteristic feature of heavy-fermion systems and corresponds to the enhanced value of the linear term of the specific heat.     

The effect of pressure on the resistivity of a gold-chromium Kondo alloy and on the low temperature phonon resistivity of pure gold

The electrical resistivity of the Kondo alloy Au (20ppm Cr) and of pure gold has been determined in the temperature range 1.3 – 20 K at pressures up to 80 kbar. For pure gold the pressure dependence of the temperature dependent part of the lattice resistivity can be explained fairly well by the Bloch-Grüneisen theory. Expressions for the volume dependence of the ideal lattice resistivity and of the Debye-temperature for gold are derived. — The Kondo temperatureT _K of Au(Cr) is found to increase with pressure to more than twice the value atp=0 kbar.Therefrom the volume dependence of the effective exchange constantJ is calculated. The results are similar as in other Kondo alloys described previously.     

Heavy electron behavior in a new uranium-based ternary compound: UPd2Sn

Electrical resistivity, magnetic susceptibility and specific heat measurements on a new compound, UPd₂Sn, reveal that this material exhibits valence fluctuation or Kondo lattice phenomena below a characteristic temperature ∼ 10 K. In particular, the electronic specific heat coefficient appears to be strongly temperature dependent with a maximum of ∼ 270 mJ/mole U−K² at 9.7 K and an extrapolated value of ∼ 70 mJ/mole U−K² at 0 K. The compound UPd₂Sn was expected to crystallize in the same structure as the family of cubic Heusler alloys, but instead, crystallized in a more complicated structure which appears to be orthorhombic. The compound can be characterized as a nonmagnetic, nonsuperconducting heavy electron material.     

Temperature dependence of the hall effect in amorphous Ni-Si-B alloys between 1.5 and 290 K: Evidence for magnetic ordering near a resistance minimum

The Hall effect in the metallic glasses Ni₇₆Si₁₂B₁₂, Ni₇₈Si₈B₁₄ and Ni₈₀Si₁₀B₁₀ has been measured as a function of temperature (1.5 ≤ T ≤ 290 K) and magnetic field (B ^<_～ 1.2 Tesla). At room temperature the Hall resistivity ρ_H is negative and depends linearly on the magnetic field for all three compositions. With decreasing temperature Ni₈₀Si₁₀B₁₀ exhibits a negative spontaneous Hall coefficient and a transition to ferromagnetism at about 110 K. In case of Ni₇₆Si₁₂B₁₂ and Ni₇₈Si₈B₁₄ an anomalous part contributes below ～ 40 K and ρ_H changes from negative to positive sign at about 9 K. For these two alloys we find that the initial slope of the ρ_H(B) curves follows a Curie-Weiss law with Tθ ≈ 0 K and the anomalous contribution of the Hall resistivity can be described by a Brillouin function for superparamagnetism.     

Low temperature properties of a new compound based on (Yb,Zr)B<Subscript>12</Subscript> substitution

The thermodynamic, kinetic, and magnetic characteristics of (Yb, Zr)B₁₂ have been investigated in detail in the temperature range from 4 to 300 K to reveal the effect of the band structure on the properties of the YbB₁₂ Kondo insulator ground state. It is found that electron doping due to 20% substitution in the Yb sublattice significantly changes the properties of the low-temperature ground state of YbB₁₂ Kondo insulator and only slightly affects the high-temperature spin-fluctuating state forming at T > 50 K.     

Experimental study of electrical conduction in RuO2-based thick resistive films

We have found direct evidence for the dynamical instability of Pr, which could be observed in Pr_0.015 Pd by inelastic neutron scattering, and as such is first ever to be found for Pr ions. Crystalfield analysis of the measured dynamic susceptibility reveals that the spin instability is similar to that of Ce Kondo systems. We show that the previously observed resistivity increase belowT=25 K is a true Kondo effect and is not related to crystal field effects. We conclude that Pr diluted in Pd is a Kondo system withT _K =2.5 K.     

On Suhl's dispersion equations for dilute magnetic alloys in a finite magnetic field

A one dimensional, nonlinear, singular integral equation was recently shown to be equivalent to Suhl's dispersion equations for the Kondo-problem of a half-spin magnetic impurity in a finite magnetic field. We investigate this integral equation further analytically and numerically and obtain numerical solutions which we use for a calculation of transport coefficients. The normal part of the scattering potential of the magnetic impurity is included via ans-wave phase shift. The transport coefficients are universal functions of the ratiosT/T _K andB/B _K of the temperatureT and the zero magnetic field Kondo-temperatureT _K and of the magnetic inductionB and the Kondo magnetic inductionB _K. We find maxima in the electrical and thermal resistivities as functions ofT/T _K forBB _K. These are typical Kondo phenomena, and can be influenced by. Interference of and the phases of Kondo-scattering amplitudes leads to dramatic effects in the thermopower and the Hall coefficient.SFB 125The numerical part of this work was performed at the Institut für Festkörperforschung, Kernforschungsanlage Jülich, F.R. Germany     

Transport properties of CeCu6 single crystals

The electrical resistivity ? and the thermoelectric power of CeCu₆ single crystals are strongly anisotropic. The inverse of the temperature of the Kondo resistivity maximum (T_max) roughly scales the linear temperature coefficient B of ? as well as the residual value (?₀ ÷ B ÷ 1/T_max). Along the [1 0 0] direction ? follows a T² Fermi-liquid law between 30 and 90 mK. The thermoelectric power is positive over the investigated temperature range (1–300 K) and shows two contributions.     

Magnetic hyperfine fields of Nb andMo in nickel by NMR-ON of90Nb and93mMo

The magnetic hyperfine splitting frequencies of⁹⁰NbNi and^93mMoNi in an external magnetic field of 0.2 T have been determined by the NMR-ON method to be 18.52(7) and23.73(10) MHz, respectively. With the assumption of Knight shift factorK=0 and with the knowng-factors, the hyperfine fields of⁹⁰NbNi and^93mMoNi were deduced asB _HF(⁹⁰NbNi)=-4.118(16) T andB _HF(^93mMoNi)=-3.491(33) T. The rather long spin-lattice relaxation time of 32(5) min was observed for⁹⁰NbNi at an external magnetic field of 0.2T and8 mK.     

Effect of pressure on the Kondo temperatures of Au(Fe) and Au(Mn)

The electrical resistivity of four Kondo systems, Au-(5 p.p.m. Fe), Au-(39 p.p.m. Fe), Au-(17 p.p.m. Mn) and Au-(50 p.p.m. Mn) has been measured in the temperature range 1.3–20 K at pressures up to 80 kbar. The Kondo temperature T_K is found to increase initially with pressure at the rate of 1.1%/kbar for Au(Fe) and 6%/kbar for Au(Mn). The volume dependence of the effective exchange constant J_eff is derived.     

U impurities in Au

The properties of U impurities in Au have been studied by magnetic susceptibility, electrical resistivity, and electron spectroscopic measurements. The results indicate that U forms a well-defined local magnetic moment of 3.6 _B , giving rise to a Kondo resistance anomaly at low temperature. This is in agreement with spectroscopic data, which show localized impurity states well removed from the Fermi level. Comparison to calculated atomic multiplets shows the need for inclusion of crystal or ligand field effects for the multiplets. The intra-5f Coulomb correlation energy is found to be 2.6 eV.