Electron tunneling into ion implanted PbMn and SnMn alloys

Tunnel junctions ofPb-I-Al andSn-I-Al were implanted at low temperatures with Mn ions in the ppm region. The tunnel characteristicdI/dV was measured on the pure sample and on the alloy with different concentrations. A bound state is found in both systems at an energy 0.7 ₀. Comparison with theory confirms, that the Kondo effect is not necessary for the existence of a bound state.     

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     

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 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.     

Magnetic impurity effects inAgMn,AuFe, andCuCr studied by tunneling in superconducting proximity layers

Magnetic interactions were studied by superconducting tunneling into diluteAgMn,AuFe, andCuCr alloys in which superconductivity had been induced by a proximity effect. A detailed investigation ofAgMn, for concentrations up to 0.4 at.% Mn and at temperatures down to 0.05 K, revealed a weak impurity band within the energy gap of the density of excited states at about 0.68 of the half energy gap . The amplitude of the band, however, was twenty times smaller than expected from the theory by Müller-Hartmann and Zittartz. This makes the interpretation of the effect as being due to bulkAgMn doubtful. The localisation of the band would imply a Kondo temperature,T _K , of about 1 mK for an impurity spinS=5/2. No effect of spin-glass ordering was seen in the temperature dependence of the tunneling density of states inAgMn which contradicts a report by Schuller et al. The zero bias conductance could instead be reproduced from low temperature measurements if the temperature variation in was taken into account. InCuCr, the shape of the impurity band was found to be temperature dependent. The location of an impurity band at about 0.68 inAuFe indicates. together with previous observations, the presence of two impurity bands in this alloy. The one found here, might be due to long-range Fe pairs.     

Thermoelectric power of heavy-fermion compounds

We calculate the diffusion thermopower of the Anderson lattice as a model for heavyfermion compounds in a semi-phenomenological theory. In this theory, the thermopower is expressed by the dynamical susceptibility which describes spin fluctuations and can be measured by neutron scattering. The Kondo effect is taken into account for a singlef-electron spin which is coupled to all other spins and to the conduction electrons. This approach neglects multiple intesite-scattering of the conduction electrons. We obtain a Kondo termS ₍₁₎ ^d (T) (in which the thermopower of non-interacting spins is multiplied by a factor which describes the spin dynamics) and a resonance termS ₍₂₎ ^d (T) of opposite sign which vanishes for vanishing interactions. The superposition of both terms leads to a broad maximum of the thermopower roughly at the Kondo temperatureT _K and to an additional minimum belowT _K . ForT0 the termS ₍₁₎ ^d vanishes asT ² and the termS ₍₂₎ ^d becomes proportional toT. We also show that the Sommerfeld expansion leads to an incorrect result for the low temperature resistivity of the Anderson lattice and that the Gorter-Nordheim relation does not hold at low temperatures.     

Magnetic and non-magnetic ground states of the Kondo lattice

We use the variational method to investigate the ground state phase diagram of the Kondo lattice Hamiltonian for arbitraryJ/W, and conduction electron concentrationn _c (J is the Kondo coupling andW the bandwidth). We are particularly interested in the question under which circumstances the globally singlet (collective Kondo) Fermi liquid type ground state becomes unstable against magnetic ordering. For the collective Kondo singlet we use the lattice generalization of Yosida's wavefunction which implies the existence of a large Fermi volume, in accordance with Luttinger's theorem. Using the Gutzwiller approximation, we derive closed-form results for the ground state energy at arbitraryJ/W andn _c, and for the Kondo gap atn _c=1. We introduce simple trial states to describe ferromagnetic, antiferromagnetic, and spiral ordering in the small-J (RKKY) regime, and Nagaoka type ferromagnetism at largeJ/W. We study three particular cases: a band with a constant density of states, and the (tight binding) linear chain, and square lattice periodic Kondo models. We find that the lattice enhancement of the Kondo effect, which is described in our theory of the Fermi liquid state, pushes the RKKY-to-nonmagnetic phase boundary to much smaller values ofJ/W than it was previously thought. In our study of the square lattice case, we also find a region of itinerant, Nagaoka-type ferromagnetism at largeJ/W forn _c 1/3.     

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.     

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.     

Low temperature properties of valence fluctuating cerium impurity with spin-orbit and crystal field splittings

The Anderson Hamiltonian, with spin-orbit and cubic crystal-field splittings, is studied using Brillouin-Wigner perturbation theory. The ratio between the low temperature paramagnetic susceptibility and the electronic specific heat coefficient is reported for both intermediate valent and Kondo regimes. We find that finite spin-orbit effect is not negligible in Ce systems in the intermediate valent regime. In the nearly integral valent case, for small Kondo temperatureT _K, we also find that even for relatively large crystalfield splitting (10T _K), if the free ion ground state is of ₇ symmetry, the effect of the ₈ level is still large.     

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 expansion study of magnetic phase transitions in Kondo-lattice systems

High precision measurements of the thermal expansion coefficient, (T), of the Kondo lattice systems CeAl₂ and CeB₆ reveal the coexistence of two second-order phase transitions at the Néel temperaturesT _N =3.9 K and 2.35 K, respectively. These results are discussed with regard to the complex antiferromagnetic order known from neutron diffraction experiments on both compounds. For CeIn₃, a Kondo lattice with simpler magnetic structure, only one discontinuity in (T) occurs atT _N 10 K.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

Kondo effect,spin dynamics and magnetism in anomalous rare earth and actinide compounds

The influence of spin dynamics on the Kondo effect manifestations in the Kondo lattices is investigated within perturbation theory with respect to thes-f interaction. It may give rise to Kondo-like divergencies in the electron self-energy already in the second order, resulting in an appreciable effective mass enhancement. As for usual Kondo contributions to thermodynamic and transport properties, the effect of spin dynamics reduces roughly to the replacement ln , with the characteristic spin-fluctuation energy. The thermoelectric power of dense Kondo systems is discussed. Singular contributions to the electron self-energies in the ferro-and antiferromagnetic state are considered. Kondo-like corrections to the intersite exchange interactions, saturation magnetic moment and total energy in a magnetically ordered state are calculated. The strong-coupling regionT<T _K is investigated within the Anderson lattice model. A decrease ofT _K due to spin fluctuations is demonstrated.     

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     

Magnetic properties of CePd2Ga3

Field dependent specific heat measurements and a study of elastic neutron scattering experiments characterize CePd₂Ga₃ as a ferromagnetic Kondo compound with ordering in the basal plane belowT _C 6 K. The crystal field ground state of cerium in this hexagonal compound is the |±1/2> eigenstate. In the scope of a phenomenological model, the coupling constant and the Kondo temperature have been deduced.     

Lack of delocalized Cup states at the fermi level in the high-T c superconductor YBa2Cu3O∼7 by XANES spectroscopy

The x-ray absorption spectra of the single phase high temperature superconducting YBa₂Cu₃O₇ withT _{c, onset}=97 K have been measured between 30 K and 300 K by using synchrotron radiation.The copperK-edge x-ray absorption near edge structure (XANES) spectrum gives experimental evidence of the lack of delocalized Cup states close to the Fermi level, in fact a spectral distribution of local (at the Cu site) partial (l=1) density of states as in the insulating CuO has been found.The EXAFS (extended x-ray absorption fine structure) spectrum shows evidence of the formation of layers formed by Cu ions coordinated by oxygen square planes sharing corners. The average Cu–O distance at 30 K in the layers has been found to be 1.93 i.e. 0.02 shorter than in CuO and the average distance between Cu ions at the centers of the square planes is 3.85.The present local structure suggests that superconductivity should occur in the crystalline planes formed by CuO₄ square plane units. The lack of delocalized Cu states is in agreement with the hypothesis that conductivity is due to itinerant holesL in the oxygen derived valence band.The results of this experiment have been presented at the Symposium on the Latest Developments in Superconductivity at High Temperatures of the European Physical Society Meeting held in Pisa april 8, 1987     

Specific heat of concentrated kondo systems: (La,Ce)Al2 and CeAl2

We report on specific heat measurements of (La_1–x Ce _x )Al₂ samples, with 1.5 a/ox100 a/o, performed in magnetic fields of up to 5 T between 0.3 and 10 K. In the Ce rich alloys, and especially in CeAl₂, a-type peak of an antiferromagnetic phase transition, and at lower temperatures spin waves and very large electronic contributions are clearly visible. In higher magnetic fields, that is when antiferromagnetic order can be suppressed, the specific heat of the alloys exhibits a broadened Schottky peak. All these phenomena add up tok ln 2, i.e. to the correct entropy change per single Ce³⁺ ion in its ₇ crystal field ground state.We interpret experimental results as an interplay between cooperative magnetism and the single-ion Kondo effect which describes a gradual turning off of one magnetic moment. The broadening of the Schottky peak is directly related to the Kondo temperatureT _K , which we determine with a simple resonance level model.T _K increases by an order of magnitude whenx increases from 1.5 a/o to 100 a/o. This is interpreted as caused by a lattice contraction.A quadraticx dependence of the Néel temperature suggests that (forTT _K ) stable Ce moments can only exist through pair interactions. The very large (and almost field independent) specific heat term linear in temperature with a coefficient=135 mJ/K² mole for CeAl₂ is attributed to the Kondo effect—still present in the antiferromagnetically ordered state. Our evaluation of the experimental data is backed by a molecular field theory for a simplified antiferromagnetic structure combined with the simplest possible Kondo theory.Preliminary results have been presented at the Int. Conf. on Magnetic Alloys and Oxides, Haifa 1977 [Bredl, C.D., Steglich, F.: J. Magnetism and Magnetic Materials, in press]     

Frequency dependence of the freezing temperature in spin glasses: A comparative study of (La, Gd)B6, (Y, Gd)Al2 and (La, Gd)Al2

The frequency dependence of the freezing temperatureT _f() is determined for the dilute spin glass systems (La, Gd)B₆ and (Y, Gd)Al₂ in the frequency range 10–1,000 Hz. While for (La, Gd)B₆,T _f() is found to be weak, for (Y, Gd)Al₂ T _f() is even stronger than for the previously studied system (La, Gd)Al₂. Both, measurements of the temperature dependence of the susceptibility nearT _f and calculations of the RKKY pair interaction, suggest that this difference is correlated with a different sign of the nearest-neighbor interaction, which appears to be antiferromagnetic for (La, Gd)B₆ and ferromagnetic for (Y, Gd)Al₂ as well as (La, Gd)Al₂.     

Self-consistent perturbation theory for dynamics of valence fluctuations

Dynamical as well as equilibrium properties of model Ce systems are investigated in both the intermediate-valence and nearly integral-valence (Kondo) regimes at finite temperatures. With self-consistent account of hybridization effects between the conduction bands and the highly correlated 4f states, the 4f-electron density of states _4f () and the dynamical magnetic susceptibility () are derived. Equilibrium properties such as the static magnetic susceptibility and the averaged 4f-electron number are also computed within the same approximation scheme that neglects intersite interactions between different Ce ions. In the intermediate-valence regime the calculated line-shape of Im ()/ is close to the Lorentzian at high temperatures, but at low temperatures there appears an inelastic peak. In the Kondo regime it is shown that a sharp peak in _4f () develops at the Fermi level as the temperature decreases. The line-shape of Im ()/ is shown to be close to the Lorentzian at all temperatures. The half-width is considerably enhanced over the Korringa value expected for the local-moment system. The temperature dependence of the half-width agrees qualitatively with experimental results in Kondo compounds such as CeB₆, CeCu₂Si₂ and CeAl₃.