Magnetic to nonmagnetic transition of cerium impurities in (La,Th) alloys

The initial depression of the superconducting temperature of ($\text{La, Th}$) Ce alloys has been measured. It has been found that there is a steep maximum in the initial depression as a function of La concentration in the (La, Th) matrix, This effect is compared with the nonmonotonic behaviour of the critical temperature of $\text{La}$Ce under pressure.     

Specific heat of superconducting Th,Sc and Th,Y alloys with Ce impurities

Measurements of the specific heat jump ΔC at the superconducting critical temperature T_c on ($\text{Th, Sc}$)Ce and ($\text{Th, Y}$)Ce Ce impurity, solid solution alloy systems indicate that the former systems obey the BCS law of corresponding states (LCS) characteristic of superconductors with non-magnetic impurities while the latter systems present deviations from the LCS linear relation between reduced parameters which are attributed to the development of localized moments at the Ce ions as the Y concentration increases.     

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 susceptibility of (La,Ce) Al2 alloys

The low-field magnetic susceptibility of (La, Ce)Al₂ alloys with 1–20 at-% Ce was measured between 0.04 and 4 K. Up to 1.5 at-% Ce the impurity contribution to the susceptibility exhibits features which can be described in terms of a combined influence of the crystalline electric field and the Kondo effect. At very low temperatures the onset of interactions between the Ce impurities is indicated. The impurity coupling interactions determine the dependence on temperature and concentration of the more concentrated alloys. For the dilute alloys the impurity magnetization was determined from measurements of the susceptibility in magnetic fields up to 10 kOe. The magnetization as a function of temperature and field shows a typical anomaly which has been observed also in other Kondo systems.     

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]     

Influence of nonmagnetic impurities on the kondo effect

The influence of scattering by nonmagnetic impurities is studied in perturbation theory. While the finite lifetime of the electrons in intermediate states due to scattering by nonmagnetic impurities does not lead to a change in the logT-behaviour of the third-order self-energy, certain vertex-corrections give rise to an additional term which varies like 1/√T at low temperatures. Similar correction terms are found to occur in the higher order self-energy contributions. Although these terms diverge more strongly atT=0 than the logarithmic contributions they are quite small at finite temperatures since they depend on the lifetime τ of the electrons through a factor of (? _F τ)^?5/2 (? _F Fermi energy). The possibility of observing these interference effects experimentally is discussed.     

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.     

Superconducting—normal phase boundaries of (La,Th)Ce systems

The dependence of the superconducting transition temperature T_c on Ce impurity concentration n is reported for various ($\text{La, Th}$)Ce systems. These results document for the first time the evolution of the superconducting— normal phase boundary (T_c vs n curve) as an impurity undergoes magnetic- nonmagnetic transition which for Ce impurities in ($\text{La, Th}$)Ce systems proceeds with increasing Th concentration. Observed for only the second time, re-entrant T_c vs n curves are reported for two La-rich La, Th host compositions.     

Evidence for anisotropic kondo behavior in Ce0.8La0.2Al3

We have performed an inelastic neutron scattering study of the low energy spin dynamics of the heavy fermion compound Ce0.8La0.2Al3 as a function of temperature and external pressure up to 5 kbar. At temperatures below 3 K, the magnetic response transforms from a quasielastic form, common to many heavy fermion systems, to a single well-defined inelastic peak, which is extremely sensitive to external pressure. The scaling of the spin dynamics and the thermodynamic properties are in agreement with the predictions of the anisotropic Kondo model.     

Skew scattering of the electrons by Ce impurities in La

Low temperature measurements of the Hall effect in lanthanum containing cerium impurities show, in addition to the ordinary Hall effect, a contribution (nearly) proportional to the magnetization of the cerium magnetic moments. We ascribe this contribution to skew scattering by the cerium impurities. The asymmetry of the scattering can result from the mixing of the conduction band and impurity wave functions for impurities having orbital magnetism.     

Reverse kondo effect in the presence of crystalline electric field splitting: (La,Nd)Sn3

Resistivity measurements on ($\text{La}$,Nd)Sn₃ between 0.35 K and 8 K show for the first time the existence of a reverse Kondo-effect in an alloy containing Kramers ions with crystalline electric field (CEF) splitting. The experimental results can be fitted by an extension of the CEF-theory employing higher order terms in the spin exchange scattering.     

Size-induced transition from magnetic ordering to kondo behavior in (Ce,Al) compounds

Magnetic ordering and Kondo behavior coexist in three (Ce,Al)-based compounds: CeAl2, Ce3Al, and Ce3Al11. A common feature apparently independent of crystal structures also prevails in terms of the size-induced transition between these two magnetic phenomena. As the particle size is reduced to nanoscale, the specific heat anomaly associated with the magnetic ordering diminishes. Although the Kondo temperature also decreases, the entropy associated with Kondo anomaly exhibits a large increase. This results in an enhancement of the Kondo behavior and an increased coefficient gamma of the linear term in specific heat. For example, in 80 A CeAl2 the extrapolated r(0) reaches 9000 mJ mol Ce-1 K-2.     

Anomalous behavior of thermal expansion of α-Fe impurities in the La(Fe,Co,Si)13- based alloys modified by Mn or selected lanthanides (Ce,Pr, Ho)

The aim of the present work was to study the negative lattice expansion of the La(Fe,Si)₁₃ phase in the LaFe_11.2Co_0.7Si_1.1 alloy modified by Ce, Ho, Pr or Mn. The highest change of lattice constant was observed for sample doped with Ce, which was result of the first order phase transition, previously observed in this alloy. The gradual decrease of relative change of lattice parameter with increase of Mn content was detected. Furthermore, anomalous behavior of temperature dependence of lattice constant for α-Fe phase was also observed. The X-ray diffraction analysis showed that this phenomenom is caused by negative lattice expansion of the La(Fe,Si)₁₃ phase.     

Valence instability of Ce and La impurities in YNi2

Measurements of resistivity and lattice constants are reported for the Y_0.98RE_0.02Ni₂ series of alloys. Both the Ce and the La alloys show strong anomalies in volume and resistivity increment, especially at low temperatures. This is the first indication for a valence instability on an impurity of La.     

Clarifying the basic phase structure and magnetic behavior of directly quenched (Ce,La)2Fe14B alloys with various Ce/La ratios

To improve the performance/cost ratio of NdFeB based permanent magnets, Ce or/and La substitutions for Nd have been suggested. To better understand the effects of these substitutions, the fundamental behavior of (Ce,La)-Fe-B alloys has to be clearly understood. Here, we reported a systematic investigation on the phase structure, microstructure, and magnetic properties of melt-spun (Ce,La)₂Fe₁₄B alloys. The results confirm that partial substitution of Ce by La can effectively enhance the hard magnetism and thermal stability of (Ce_1-xLa_x)₂Fe₁₄B alloys, while over 80% La substitution leads to the decomposition of 2:14:1 phase. The lattice parameters a, c and the Curie temperature T_c of 2:14:1 phase increase linearly with the increasing La content. La substitution can effectively refine the grain, resulting in the enhancement of inter-grain exchange coupling. The (Ce_0.7La_0.3)₂Fe₁₄B alloy with a mean grain size of 25 nm exhibits high remanence, maximum energy product and intrinsic coercivity up to 0.69 T, 6.2 MGOe and 217 kA/m, respectively. The present work provides a good understanding on the melt-spun (Ce,La)-Fe-B system for further developing low cost rare earth permanent magnets.     

Anomalous specific heat in La : Ce alloys

The magnetic contribution to the specific heat in La : Ce alloys shows a broad anomaly at ≈ 1.2°K which is weakly dependent on the concentration. This anomaly, which accounts for a substantial portion of the entropy of a doublet, cannot be attributed to either usual magnetic ordering or to a dilute alloy Kondo effect, but may represent the behavior of a concentrated Kondo system.     

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.