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.     

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.     

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]     

Metamagnetism in Ce(Ga,Al)2

Effect of Al substitution on the magnetic properties of Ce(Ga_1−x Al _x )₂ (x=0, 0.1 and 0.5) system has been studied. The magnetic state of CeGa₂ is found to be FM with a T _C of 8 K, whereas the compounds with x=0.1 and 0.5 are AFM and possess T _N of about 9 K. These two compounds undergo metamagnetic transition and the critical fields are about 1.2 T and 0.5 T. respectively at 2 K. These variations are explained on the basis of helical spin structure in these compounds.     

Absorptionsspektrum des Ce3+-Ions und Kristallfeld im La(Ce)Cl

The absorption spectrum of single crystals of La_(1?x)Ce _x Cl₃(x=0,05 ... 0,005) has been observed at low temperatures in the region of the 4f→4f infrared transitions. Values for the crystal-field parameters V ₂ ⁰ , V ₄ ⁰ , V ₆ ⁰ , V ₆ ⁶ and for the spin-orbit coupling constant of the ion in the crystalline environment are determined by fitting the observed splittings of the multiplet-components\({}^2F_{\tfrac{5}{2}} \) and\({}^2F_{\tfrac{7}{2}} \) in the crystal-field. The parameters are found to be consistent with values extrapolated from rare earth-ions with neighbouring atomic numbers in the same host-lattice. The only exception is the parameter V ₆ ⁶ , which is found to be smaller in La(Ce)Cl₃ than in other rare-earth-trichlorides (V ₆ ⁶ /V ₆ ⁰ =?6,23 in La(Ce)Cl₃, V ₆ ⁶ /V ₆ ⁰ ≈?10 in other trichlorides). The correct relation V ₆ ⁶ /V ₆ ⁰ in La(Ce)Cl₃ is obtained by fitting the parameters to the observed crystal-field-splitting of the\({}^2F_{\tfrac{5}{2}} \) — component and to the observed Zeeman-splitting-factors taken from the literature. The discrepancies between the two fitting procedures are explained by an effect of nonlinear shielding of the crystal-field.     

Kondo anomalies of heat conductivity and Lorenz ratio in normal state (La,Ce) Al2

The thermal conductivity of LaAl₂ and of two dilute (La, Ce)Al₂ alloys was measured in the normal state between 0.4 and 8 K. From the lattice conductivity of LaAl₂ a high dislocation densityN _d caused by the arc melting process can be inferred. After annealingN _d is reduced by an order of magnitude. For the (La, Ce)Al₂ samples minima are observed at 5 K in theW ^e ·T vs.T curves (W ^e =electronic thermal resistivity). Below 1 K the quantityW ^e ·T is linear in (— lnT). The electronic Lorenz ratioL ^e (T)=ρ(T)/W ^e (T) ·T shows a maximum at 2 K with a value 23% aboveL ^e (0). It is for the first time that this Kondo anomaly is established in its full temperature dependence.     

Magnetic ordering in (La,Gd) Al2 alloys

The detailed dependence of the magnetic ordering temperature θ on Gd concentration n below ～ 15 at.% Gd substitution for La is reported for the system ($\text{La}$Gd) Al₂. In this concentration range, the θ vs.n curve lies markedly below the extrapolation of the linear behavior previously observed for concentrations ? 20 at.% Gd, and apparantely goes to zero at a finite concentration. The effective moment of Gd is enhanced by 1 μ_B over the free ion value.     

Reverse resistance anomaly in (La,Gd)Al2

We report resistivity measurements performed between 0.4 K and 10 K, in magnetic fields, 0≦B≦5T, on (La_1?x Gd _x )Al₂ alloys with 0.23 at%≦x≦2 at %. In zero field, the incremental resistivityρ _i due to the Gd atoms increases approximately as ln(T/1 K). The anomalous temperature dependence ofρ _i wich is the “reverse” of the Kondo effect is theoretically expected since the conduction electron-impurity exchange interaction is ferromagnetic. Using our data of the magnetoresistivity, a semiquantitative analysis of the zero field results can be made. Comparison with other physical quantities, e.g. EPR andT _c depression, taken from the literature, allows us to determine (i) the Heisenberg exchange integral which agrees with that one estimated from the spin glass properties of (La, Gd)Al₂ and (ii) the local density of states at the Gd site.     

Magnetic properties of (La,Gd)Al2 alloys

Theac anddc susceptibilities of (La_1–x Gd _x )Al₂ alloys were measured for moderate Gd concentrations (0.01x0.08) between 0.05 and 4.2 K. Sharp spin glass like maxima in ^AC are observed at temperaturesT _f which scale asT _f /x 26 K for all samples. However, scaling of ^AC is not observed indicating that the RKKY interaction at short distances which becomes more important with increasing Gd concentration, is ferromagnetic in this system. The amplitudeV ₀ of the RKKY interaction is obtained from both the approach to the saturation magnetization and ^AC forT0, yielding a mean value ₀=(1.5±0.3) 10^–26 eV cm³ which is in agreement with that observed in other systems with Gd impurities. Furthermore, the inferred value of the exchange integral is in accord with other measurements (T _c depression, reverse resistance anomaly) on (La, Gd)Al₂. A superposeddc magnetic field rounds the maximum of ^AC and shifts it to lower temperatures. ^AC is completely flat below 4.2 K in a field which saturates the isothermal remanent magnetization, showing once again the correlation between the peak in ^AC and the onset of irreversible properties.This work is part of the research program of the Sonderforschungsbereich 125 Aachen/Jülich/Köln     

Tunneling density of states of the Kondo superconductor (La1−x Ce x )Al2

Tunneling measurements on (La_1?x Ce _x )Al₂ single crystals (x=0; 0.001; 0.002) were performed at 0.1 K. Heavily dopedp-type GaAs/Zn acts as normal conducting counterelectrode where the Schottky-barrier provides the insulating layer for tunneling. Contact between counterelectrode and sample surface is not made before cooling down to 1 K. Contamined surface layers can be pierced mechanically at 0.1 K. The extremely gapless behaviour agrees with an impurity band positiony ₀<0.8 for localized exited states, although no detailed structure can be resolved.     

Luminescence characteristics and optimization of (La,Gd)Sr2(Al,B)O5:Ce phosphor for white light emitting diodes

Luminescence properties of (La,Gd)Sr₂(Al,B)O₅:Ce phosphors were investigated by changing the content of Gd and B substituent. The Gd or B substitution was confirmed to improve the emission intensity. The emission peak of LaSr₂AlO₅:Ce was shifted to the longer wavelength due to the enlargement of crystal-field splitting for Ce³⁺ activators as the result of the unit-cell volume reduction when the Gd and B atoms are substituted. In the B-substituted samples, in addition to the cell volume reduction, the change in the lattice covalency due to the large electronegativity difference between B³⁺ and Al³⁺ has additionally contributed to shift the emission peak. The crystallite sizes of phosphors were enlarged using small amount of Gd and B atoms. Also, the absorption for the 4f→5d absorption of Ce³⁺ ions in UV/visible spectrum was greatly enhanced by the substitution of both Gd and B. Therefore, it was concluded that the enhancement of the emission intensity via the substitution is due to the increase in the crystallinity and the light-absorption ability. Finally, the optimized sample La_0.89Ce_0.01Gd_0.1Sr₂Al_0.9B_0.1O₅ had about 368% improved emission intensity compared with La_0.99Ce_0.01Sr₂AlO₅.     

Die Verteilungen des elektrischen und magnetischen Mikrofeldes im relativistischen Plasma

The probability distributions of the electric and magnetic microfield at a fixed space point in a relativistic, non-degenerate, homogeneous, and isotropic plasma in thermodynamic equilibrium are determined using the Markov-method and neglecting the particle interaction. They are found to be Holtsmarkians like the electricnon-relativistic microfield distribution, but the characteristic field strengths are increasing functions of temperature which are calculated.     

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.     

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(ν) 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(ν) is found to be weak, for (Y, Gd)Al₂ T _f(ν) 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.