Magnetic and thermal properties of Ce2Pd2Sn

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Abstract

X-ray diffraction, susceptibility and specific heat studies on ternary Ce2Pd2Sn intermetallic compound and on its isostructural non-magnetic compound La2Pd2Sn are reported. On Ce2Pd2Sn in absence of magnetic field, we confirm the existence of two magnetic transitions at TN=4.8 K (antiferromagnetic transition) and TC=2.2 K (ferromagnetic transition) and the specific heat results, measured down to 0.5 K, allow to recognize the transition at 2.2 K as a first-order transition. A description based on molecular field calculations for the S=1/2 resonant level model leads to a characteristic Kondo TK of about 3 K.

Introduction

The physics of Rare Earth intermetallic compounds with the magnetic atom located in a non-centro symmetric position has renewed its interest after the recent discovery of the coexistence of magnetism and superconductivity in CePt3Si [1]. In this type of structures, the anisotropic environment of the magnetic ion provides the conditions for competing magnetic structures that can be favoured by small changes in stoichiometry. Such is the case of the Ce2T2X (T=Ni, Pd and X=In, Sn, Pb) family of compounds [2], among which Ce2T2+xSn1−x presents an extended range of solid solution ranging between 0.04<x<0.2 [3].

The crystal structure of Ce2Pd2+xSn1−x is a derivative of the tetragonal U3Si2 type, characterized by columns of trigonal and tetragonal prisms surrounding Pd1 and Pd2 (or Sn) atoms, respectively. Consequently, Ce atoms have five nearest neighbours in the basal plane and two along the “c” axis. The competition between these two magnetic Ce–Ce interactions induces a magnetocrystalline anisotropy leading to peculiar magnetic structures [3]. The local environment of Ce atoms can be described as being surrounded by three trigonal prisms, resembling those of the ferromagnetic-CePd compound and two tetragonal prisms resembling antiferromagnetic-CePdSn compound.

Two magnetic transitions were observed in Ce2Pd2.04Sn0.96 by χ susceptibility measurements. An AF transition was recognized by a cusp at TN=4.8 K and a ferromagnetic one at TC=3 K determined by a moderate increase of the susceptibility [4]. From neutron diffraction studies, those authors determined that, at intermediate temperature (i.e., TN>T>TC), the magnetic structure is incommensurate with Ce3+ magnetic moments sinusoidally modulated along the tetragonal axis and a propagation vector q=(qx, 0, 0) varying between 0.112>qx>0.075 [4]. Since at TC q=(qx, 0, 0) locks into (0, 0, 0), a discontinuity in the thermal evolution of the propagation vector can be expected.

Despite the AF type cusp of the upper transition (TN=4.8 K), some other magnetic features suggest a complex interplay of magnetic interactions. Although the high temperature (T>100 K) extrapolation of the inverse magnetic susceptibility indicates a negative Curie–Weiss temperature, a negative curvature below 100 K ends at around 4.8 K [3]. The electrical resistivity ρ shows a maximum at this temperature after undergoing a minimum at ∼20 K. Both features, the increase of the electronic scattering down to ∼7 K and the coherence effect below that temperature are typical of Kondo (usually AF) systems. However, if the temperature of the ρ(T) maximum is computed as the Kondo temperature (i.e., TK∼7 K), it would be unlikely to have a ferromagnetic-ground state as it seems to occur in this compound.

With these questions in mind, we have performed low temperature study of Ce2Pd2Sn1.1, Ce2Pd2Sn and Ce2Pd2Sn0.9 samples, in order to determine the TC transition, the temperature dependence of the specific heat between TC and TN, and the TK value from the amplitude of the specific heat jump at 4.8 K.

Section snippets

Synthesis and X-ray powder diffraction

The polycrystalline Ce and La samples were prepared by conventional tri-arc melting of an appropriate amounts of pure Ce(4N), La(4N), Pd(4N) and Sn(4N), under a high purity argon atmosphere on a water-cooled copper hearth. The buttons were remelted several times to ensure good homogeneity. The weight losses after arc melting were lower than 0.2 wt% of total mass (about 1 g for both samples); thus, the alloy compositions were assumed to be the nominal ones. The samples were annealed 3 weeks at 750 

Magnetization measurements

The susceptibility measurements were carried out using a superconducting quantum interference device (SQUID) magnetometer. Fig. 1 shows the magnetic susceptibility for Ce2Pd2Sn as a function of temperature in the 1.8–7 K range; we observe two distinct anomalies and an hysteresis behaviour:

  • (i)

    a sharp maximum around TN=4.9 K suggesting the occurrence of an AF phase transition, which will be confirmed from the magnetization in magnetic field measurements;

  • (ii)

    at lower temperatures the susceptibility

Specific heat measurements

The magnetic 4f-derived contribution Cmag(T) to the total specific heat of Ce2Pd2Sn, obtained by subtracting the phonon contribution (La2Pd2Sn), is shown in Fig. 3. The thermal variation is characterized by a large jump at 4.8 K, followed by a tail at higher temperature. In the ordered phase a small peak at 2.1 K, with signs of a first-order characteristic transition, marks the stabilization of a ferromagnetic phase. Below that temperature, there is a significant decrease of Cmag(T) which

Evaluation of Kondo temperature

The low temperature specific heat in the magnetic Kondo compounds, with a doublet crystal-field ground state, are well described within an approach based on molecular field calculations for S=1/2 resonant level model [8], [9]. We have shown [9] that within this framework, a magnetic state is possible only if |Jm|/TK>π/2 (Jm being the molecular field parameter), with reduced magnetic ground state moments (spontaneous magnetization in the case of a ferromagnetic order or staggered magnetization

Conclusions

We have reported that the Ce2Pd2Sn compound exhibits two magnetic transitions, the upper-ordering temperature, TN=4.8 K, shows characteristic features of an antiferromagnetic transition but preceded at higher temperature by magnetic correlations. The lower transition, TC=2.2 K, indicates a change in magnetic ground state toward a ferromagnetic type arrangement of the moments.

The S=1/2 resonant level model leads to trace the temperature-dependent specific heat with only two adjustable parameters: T

Acknowledgment

The authors like to thank J.P. Lambour for his help in the specific heat measurements.

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