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.     

Magnetoresistivity of the Kondo-system (La,Ce)B6

The electrical resistivity of the Kondo system (La, Ce)B₆ has been measured in longitudinal and transversal magnetic fields up to 6 Tesla in the temperature range 0.04–20K. Corresponding to the strong increase of the resistivity with decreasing temperature the alloys show a very large negative magnetoresistivity with a Kondo temperatureT _K =1.05K and a Kondo magnetic fieldB _K =1.1 Tesla. The observed anisotropy of the resistivity due to the magnetic field direction cannot be explained well by existing theories.     

稀磁合金的杂质互作用效应

本文用格林函数方法讨论Tk(Kondo温度)时杂质互作用对Kondo效应的影响。对s-d互作用哈密顿量作自洽场近似时,同时计入导致Kondo效应和产生杂质互作用的切断项,求得了杂质系统的Kondo温度和低温电阻。结果表明:在稀磁合金中,杂质间的互作用效应使Kondo温度下降,并且使T《T_k时的电阻率温度变化曲线由(1-AT²)型变为(1+BT²)型(A,B>0),从而可能在Tk温区产生电阻极大。 关键词：     

稀磁合金中“电阻极大”现象的双杂质散射理论

本文基于s-d相互作用,考虑杂质之间存在RKKY相互作用,提出了一种新的双杂质散射模型。按照这个模型,当一个杂质作自旋翻转散射时,由于杂质之间存在着RKKY关联,它的自旋作为内部自由度会受到限制。由于这种关联,抑制了杂质的自旋翻转散射,结合Kondo的logT项,能形成电阻极大。本文计算了所有可能的“DIS”图(双杂质自能图),在Kondo电阻公式中加入了A/(T₀²—T²)这样的项。其中A是一个正常数。T₀是一个临界温度。当T≤T₀时,这个公式不再有意义。这个理论和已有的分子场理论在本质上是不同的.因为它并不依赖于合金中的磁有序.因此当T≥T_c时(T_c是磁有序转变温度),这种机制仍起作用,但分子场理论则不行.这是一种顺磁效应.我们和Cd-Mn(杂质浓度从0.01到0.1at./0)的实验曲线进行了比较,发现符合得很好.最后,我们认为即使在极低浓度下这种机制也是消除Kondo logT发散的主要原因. 关键词：     

Magnetic field of bifilar lead

Results of cw⁶³Cu NMR measurements in dilute CuFe alloys containing from 0.01 up to 0.2 at.-% Fe are presented. The temperature was well above the Kondo temperature of this specimen. The broadening of the NMR line width is caused by the localized magnetic moment of the Fe impurities. The decreasing signal amplitude with increasing iron content is due to the quadrupolar interactions appearing simultaneously. The line broadening correspond strongly to that of the impurity susceptibility behaviour in CuFe. The quadrupolar disturbances are temperature independent. The influence of the simultaneously appearing magnetic and electric interactions on the spin lattice nuclear relaxation time is discussed.     

Effect of cobalt impurity ions on the magnetic and electrical properties of iron monosilicide crystals

The results of experimental investigations of Fe_{1 − x} Co _x Si crystals in the impurity limit with x = 0.001, 0.005, and 0.01 are reported. The temperature and field dependences of the magnetic susceptibility have been studied. According to the experimental data, the introduction of cobalt impurity leads to a change in the energy structure, which is most pronounced in a change in the electrical properties. The temperature, field, and concentration dependences of the resistivity have been measured. The results have been interpreted in the framework of the Kondo model.     

Superconducting and normal state properties of dilute alloys of LaSn3 containing Ce impurities

The dependence of the superconducting transition temperature T_c on Ce impurity concentration and the specific heat jump at T_c as a function of T_c are reported for the system ($\text{La}$Ce)Sn₃. The experimental results are analyzed using a theory due to Kaiser concerning the effect on superconductivity of nonmagnetic localized resonant impurity states This analysis yields values for the intraatomic Coulomb repulsion parameter U_eff and the Ce local density of states at the Fermi level N_? (E_F). The results of low temperature normal state heat capacity and magnetic susceptibility measurements which give independent estimates of N_? (E_F) are also reported. A large pressure dependence of the T_c of ($\text{La}$Ce)Sn₃ alloys was observed for pressures up to 20 kbar. This behavior is similar to that previously observed in several other superconducting matrix-Ce impurity systems in which the Ce solute 4f electron shell undergoes a continuous-pressure induced demagnetization.     

Spin density wave of itinerant heavy electrons in Ce(Ru0.85Rh0.15)2Si2

The magnetic properties of Ce(Ru_0.85Rh_0.15)₂Si₂ were studied by neutron scattering and measurements of magnetization, susceptibility, specific heat and thermal expansion as a function of temperature. We observe a crossover from a high temperature localized spin to a low temperature heavy electron state. Spin density wave (SDW) behavior appears in the heavy electron state below T_N = 5.5 K and the volume change due to spin quantum fluctuations associated with the SDW and the Kondo screening is reminiscent of moment-volume instabilities of the INVAR and anti-INVAR behavior of 3 d transition metal alloys.     

The Kondo effect of a magnetic impurity under the crystalline field

Using the Coqblin-Schrieffer exchange interaction, we investigate the Kondo effect for a magnetic impurity under the crystalline field, as occuring in La or Y alloys containing cerium impurities. The Hamann typet-matrix equation for the conduction electron scattered by a magnetic impurity is derived and solved, using the method of Zittarz and Müller-Hartmann. We find a Kondo type anomaly and a decreased Kondo temperature due to the crystalline field splittings. The resistivity, entropy and specific heat are calculated and they show some characteristic behavior due to the crystalline field splittings.     

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.     

Temperature dependence of the magnetic hyperfine field at cerium impurity in Co

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the magnetic hyperfine field (B _hf ) at Ce impurity in Co using ¹⁴⁰La→ ¹⁴⁰Ce probe. The radioactive ¹⁴⁰La produced by neutron irradiation of lanthanum metal with thermal neutrons was introduced in Co by arc melting in argon atmosphere. The present measurements cover the temperature range from 4.2– 1300 K. Two pure magnetic interactions were observed at impurity sites, corresponding to a ferromagnetic ordering of Co moments in hcp and fcc phases. The temperature dependence of B _hf for both phases, however, shows a sharp deviation from an expected standard Brillouin-like behavior for the host magnetization. The results are discussed in terms of a simple molecular-field model where the localized moment at impurity ions as well as the conduction electron contributions to the hyperfine field are taken into account.     

Local magnetism of Ce in the Kondo lattice compound CeFeGe3

Local magnetism of Ce in the Kondo lattice compound CeFeGe₃ studied by measuring local susceptibility χ_loc at ¹⁴⁰Ce probe site using TDPAC method is presented. The magnitude and temperature dependence of χ_loc reflect Kondo behaviour with Kondo temperature T_K∼150 K. This revised version was published online in August 2006 with corrections to the Cover Date.     

One- and two-channel Kondo model with logarithmic Van Hove singularity: A numerical renormalization group solution

Simple scaling consideration and NRG solution of the one- and two-channel Kondo model in the presence of a logarithmic Van Hove singularity at the Fermi level is given. The temperature dependences of local and impurity magnetic susceptibility and impurity entropy are calculated. The low-temperature behavior of the impurity susceptibility and impurity entropy turns out to be non-universal in the Kondo sense and independent of the s–d coupling J. The resonant level model solution in the strong coupling regime confirms the NRG results. In the two-channel case the local susceptibility demonstrates a non-Fermi-liquid power-law behavior.     

Kondo effect and heavy fermions in Yb compounds

The Kondo properties of Yb dilute alloys and intermetallics have been investigated using Mössbauer spectroscopy on¹⁷⁰Yb. In the dilute alloysAuYb andLaBe Yb, the Kondo logarithmic anomaly of the impurity relaxation rate has been detected, and in the concentrated Yb compounds YbBe₁₃, YbP and YbAs, and YbCuAl, the manifestations of the interplay between the Kondo effect and the magnetic ordering due to the RKKY interaction have been characterized.     

Susceptibility behaviour of Mn dissolved in liquid Cu—Al-alloys

Measurements of the magnetic susceptibility of liquid Mn_0.05—Cu—Al alloys up to 1700 K indicate Curie—Weiss like behaviour at the Cu-rich end and minima of the reciprocal Mn susceptibility values as a function of the temperature 1/_ξMn(T) at the A1-rich end of the series. The occurrence of 1/_ξMn(T) minima at temperatures, which are sensitively depending on the alloy composition, hints at a transition to localized magnetic moment behaviour even in the A1-based alloys at high temperatures. Both, the aspects of extremely increasing Kondo temperature and that of decreasing spin fluctuation times, allow to discuss the susceptibility behaviour uniformly for the whole alloy series. Thus, the results do not support a fundamental distinction (magnetic—nonmagnetic) between the alloy systems $\text{Cu}$—Mn and $\text{A1}$—Mn.     

High temperature susceptibility and fractional valence of some Ce-intermetallics

The magnetic susceptibility of eight Ce-intermetallic compounds and two concentrated alloys was measured together with that of isostructural reference compounds between 1.5 and 1200 K. The high temperature susceptibility gives a lower bound of 0.3 for the 4? occupation number, supporting L_III evidence that Ce is never tetravalent in intermetallics.     

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.     

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.     

Magnetic susceptibility and electrical resistivity of some Th2Zn17-type rare-earth zinc phases

We report measurements of the magnetic susceptibility and electrical resistivity of the iostructural compounds RE₂Zn₁₇ (RE=La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). The composition dependence of the lattice parameter and effective moment indicate that all the RE ions are trivalent except Yb which is divalent. Magnetic order is observed in compounds where RE=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho and Er. A second transition is seen for RE=Pr, Ho, Sm and Tb. Superzone boundary effects are observed in the electrical resistivity of these four alloys as well as in Er₂Zn₁₇. Resistivity measurements reveal concentrated Kondo behavior (or 4f instability) of Ce in Ce₂Zn₁₇.