(Der Text ist in englischer Sprache verfasst!)

CeNi₁₂B₆

Cerium based intermetallic compounds are of special interest with respect to their variety of ground states resulting from the competition between interactions favoring the formation of symmetry breaking long range magnetic order driven by RKKY-type Ce-Ce intersite correlations versus interactions such as Kondo screening or frustration effects favoring a symmetry conserving ground state such as the paramagnetic Kondo lattice state. The relevance of RKKY-type exchange coupling is generally expected to be lower in systems with large Ce-Ce interatomic distances. In this context CeNi₁₂B₆ is of special interest as it exhibits Ce-Ce interatomic distances as large as 6.07Ǻ.

A very interesting feature of the CeNi₁₂B₆ structure displayed in Fig. 1 is the existence of slightly puckered 3₃4₂ Ni-nets oriented in the ab-plane. Such d-metal sheets may give rise to highly anisotropic features of the Fermi surface. The remaining nickel atoms are inter- bounded to form pairs of empty [Ni4] tetrahedra coupled by common edges which are highlighted in green colour in Fig.1. We prepared large single crystals via the floating zone method and investigate the ground state properties of the interesting system with possibly highly anisotropic electronic structure and anisotropic magnetic exchange correlations by means of specific heat and magnetisation studies as well as single crystal neutron diffraction.

Contact: H. Michor, E. Bauer, O. Solohub

Rare earth rich 4f-3d system

The R₃M family of rare-earth – d-transition-metal compounds with the highest rare-earth content amoung all binary systems. They crystallize in a low symmetry orthorhombic structure of the Fe₃C-type (space group Pnma), which forms only with d-metals having an almost filled d-band (Co, Ni, Rh). The rare-earth ions occupy two non-equivalent crystallographic positions of 4c and 8d. The M atoms are located within trigonal prisms formed by the R ions.

An increase of the rare earth content in R-M binary compounds leads to a hybridization of the rare-earth 5d electrons with the unfilled 3d-band that results in the absence of a magnetic moment on the Co and Ni atoms in R₃M. The observed narrowing of the 3d-band in R₃M in comparison with pure 3d-metals was attributed to reduction in the overlapping of 3d-orbitals owing to the large M-M distance (0.4 nm) in Fe₃C-type lattice. Therefore, one may expect a strong correlation between f- and d-electrons and a localisation of spin fluctuations in real space in R₃T. Specific heat measurements of some R₃M compounds revealed a strong dependence of the T-linear specific heat coefficient γ on the type of R-ion and its concentration. In particular, the γ-value of Y₃Co was found to be significantly lower (15 mJ/molK²) than that of isostructural Gd3Co (122 mJ/molK²) and (Gd_0.2Y_0.8)₃Co (350 mJ/molK²). Such a behaviour can be attributed to the presence of a huge contribution of spin fluctuations in the d-electron subsystem induced by the f-d exchange interaction. Moreover, the significant upturn C_p/T for x=0.9 may be associated with the critical behaviour in the vicinity of a quantum phase transition. The above findings motivate further investigations on (Gd_xY_1-x)₃M with M = Co, Ni, Rh, .. and other R₃M and which have not yet been studied.

This project is a cooperation with Prof. N. Baranov at Russian Academy of Science and Ural State University, Ekaterinburg.

Contact: H. Michor

The family of RT₉X₄ compounds

The family of tetragonal (variant of the cubic NaZn₁₃-type) compounds, RT₉X₄, has been very fertile in revealing a wide spectrum of important features including heavy fermion non-Fermi liquid behavior, model type Kondo lattice behavior, itinerant electron metamagnetism with indications for metamagnetic quantum criticality and others which are the subject of on-going research.

• CeNi₉Ge₄ shows unusual non-Fermi liquid behaviour of the specific heat over at least two decades of temperature with a record value of the electronic specific heat coefficient γ at low temperature:γ ~ 6 J/molK² at 80mK in this system without any sign of magnetic order. The ground state properties are currently studied by neutron and muon spectroscopy and single crystal investigations.
• CeNi₉Si₄ is an exceptional system which exhibits a model type Kondo lattice behavior almost perfectly well described in terms of the degenerate (J = 5/2) Coqblin-Schrieffer model. It is hitherto the first Kondo lattice system where the thermoelectric power S(T) shows indeed close agreement with earlier theoretical results of the degenerate Anderson lattice without intersite interactions.
• The observation of weak itinerant ferromagnetism in LaCo_13-xSi_x with Tc approaching zero temperature approximately at the stoichiometric composition 1-9-4 suggests that LaCo₉Si₄ may be at or nearby a ferromagnetic QCP. A detailed investigation of this compound comprising magnetic, specific heat, and NMR measurements as well as ab initio electronic structure calculations, in fact revealed strongly exchange enhanced Pauli paramagnetism at magnetic fields below 3T and an itinerant metamagnetic phase transition with indications for metamagnetic quantum criticality at a critical field which is about 3.5T for H parallel and 6T for H perpendicular to the tetragonal c-axis. These critical fields for the appearance of metamagnetism are the lowest values ever found for rare earth intermetallic compounds.

Contact: H. Michor