Pauline HARAUX: New 3d-4f Intermetallic Systems at Intermediate Valence

Abstract:
The main part of this work completes and clarifies previous studies on the YbMn6Ge6-xSnx compounds and their Fe derivatives. A second exploratory part focuses on potentially promising new compounds in which an anomalous rare earth is associated with a magnetic 3d sublattice.
The YbMn6Ge6-xSnx series shows signatures of quantum criticality at the Yb magnetic instability near x ~ 5.23. The first specific heat measurements performed in this work show the heavy fermion character of the alloys (γ > 100 mJ.K-2.mol-1). These measurements also corroborate the existence of a quantum critical point around x ~ 5.23, given the increase in the Sommerfeld coefficient γ (thus the effective mass) in the corresponding alloys, in agreement with theoretical predictions. In addition, XANES and XMCD measurements under pressure, using a low-gradient transmitter medium (He), show the beginning of a peak in the pressure dependence of the signals, similarly to what is observed in the composition dependence. A thermoelectric study has confirmed the importance of this technique for the study of heavy fermion systems, with thermoelectric response strongly dependent on the state of Yb. The Fe for Mn substitution in YbMn6-yFeyGe0.85Sn5.15 alloys (0.00 ≤ y ≤ 1.00) enhances 3d antiferromagnetism. This yields the reduction of the exchange field generated at the Yb site and induces the collapse of the magnetic character of Yb. This kind of substitution constitutes a new way to cross the Yb magnetic instability.
In the last part, we have investigated new compounds involving an anomalous rare earth associated with a magnetic 3d sublattice. The numerous synthesis of YbMn6-yCoyGe6-xSnx and CeMnGe1-xSix compounds allow to specify the stability domains and the structural transitions in these series. However, the quality of the samples needs to be improved. The YbMn6-yCoyGe6-xSnx series might be very promising since it could associate two critical points within the same systems.

Keywords:
intermetallics, ytterbium, cerium, intermediate valence, magnetism, XANES, XMCD, heat capacity.

Composition of the jury:
> Reporters:
- Claire COLIN, Associate professor, Institut Néel, Université de Grenoble
- Olivier TOUGAIT, Professor, UCCS, Université de Lille
> Examiners:
- Emilie GAUDRY, Professor, IJL, Université de Lorraine
- Pierric LEMOINE, Researcher, ISCR, Université de Rennes
> Direction of thesis:
- Thomas Mazet, Thesis Director, Professor, IJL, Université de Lorraine
- Lucas Eichenberger, Thesis Co-Director, Associate Professor, IJL, Université de Lorraine