Magnetic and superconducting properties of A7B3 compounds (A=Th or La and B=Ni, Co, Fe or Pd, Rh, Ru)

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Abstract

High-temperature magnetization, low temperature specific heat, and ac-susceptibility measurements on Th7X3 (X=Ni, Co, and Fe) and La7Z3 (Z=Pd, Rh, and Ru) superconductors are presented. The magnetic behaviour of the Th7X3 group has the characteristics of Pauli paramagnets. The La7Z3 group also shows Pauli paramagnetism for Z=Pd and Ru, and signs of spin fluctuations for La7Rh3. Specific heat (CP) measurements indicate a conventional BCS behaviour for La7Pd3 and La7Rh3, like that of the previously measured Th7X3 superconductors. In the case of La7Ru3, the reduced specific heat jump at the transition, the anomalous temperature dependence of CP under magnetic field and a large upper critical field slope are discussed.

Introduction

The coexistence of magnetism and superconductivity is a subject with still many open questions. Although it is well known that magnetic impurities suppress superconductivity, via a “pair breaking” effect [1], there are many examples of intermetallic compounds where the superconducting (SC) state persists, or is even enhanced, in the presence of a magnetic ligand [2]. The concept of “coexistence” is quite broad because of the many possibilities of sharing conduction electrons between the magnetic and SC components. An intrinsic difference between localized moments (related to f-electrons) and those arising from band magnetism has to be taken into account. The first do not contribute to the conduction band, while the second can be strongly affected by the electronic structure modifications driven by the “chemical potential” compensation at the compound formation. There are some nearly-magnetic systems where SC electrons condensate from a band which is also directly involved in the magnetic response, like in the unconventional superconductor UPt3 [3]. In other compounds, superconductivity may even suppress a magnetic phase as for example in CeCu2Si2 under pressure [4]. There are other cases where the magnetic and SC components simply belong to different atomic sub-lattices. Such seems to be the case of the Th7X3 family of compounds [5], [6], where the SC transition TS increases (from 1.37 K of pure Th to about 2 K) and behaves as in conventional BCS superconductors [7] despite the presence of 30% of magnetic components: X=Ni, Co and Fe.

In order to gain insight into the role of the electronic structure in the SC state properties, we have also studied the magnetic behaviour of the Th7X3 family and compared it with the La7Z3 compounds formed by SC La and 4d-transition elements. The selected Z components are Pd, Rh, and Ru, whose electronic structure correspond one to one with the 3d-ligands of Th. With the exception of La7Ru3, these compounds form with a Th7Fe3 type hexagonal structure [8], which consists in three sub-lattices of Th atoms (1 ThI, 3 ThII, and 3 ThIII). La7Ru3 forms with the related Sr7Pt3 orthorhombic type structure [9] (the latter structure consists in four sub-lattices of Sr atoms: 1 SrI, 2 SrII, 2 SrIII, and 2 SrIV), though another not identified phase was reported [10].

Section snippets

Experimental details and results

Samples were prepared by arc melting the pure constituent elements in a water cooled copper crucible under Ar atmosphere. The buttons were flipped over and remelted for several times in order to assure homogeneity. The mass losses were found to be negligible. The La samples were wrapped in Ta foil and encapsulated in quartz tubes under vacuum, while the Th samples were wrapped in Ta, Zr, and Ta foils successively and encapsulated in quartz tubes under He atmosphere [5], [6]. The samples were

Discussion

The high-temperature magnetic measurements on Th7Ni3 and Th7Co3 confirm the absence of long-range magnetic order in these compounds. As proposed in Ref. [7], a demagnetization of the 3d-elements is expected from the charge transfer of Th conduction electrons to the 3d-orbitals owing to the 3d-levels lie below the Fermi level in the electronic structure of the compounds. Then, the Th 7s6d-band should become the main responsible for the electronic properties of the system. The behaviour shown by

Conclusions

These two selected groups of compounds give the chance to compare how superconductivity develops in similar crystal symmetry, but slightly different electronic configurations. Some differences and similarities are clearly observed in the normal state through the temperature dependencies of their respective magnetizations. The charge transference is particularly evident in the Th compounds with a large concentration of magnetic component. On the other hand in La7Z3 compounds, the electronic

Acknowledgements

We want to thank Dr. J.G. Huber for providing the Th7X3 samples, Dr. R. Zysler and Dr. C. Geibel for fruitful discussions, and Dr. J. Luzuriaga for carefully reading the manuscript.

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