- Fig. 1
- Example of a cage compound : a filled skuterrudite RPt4Ge12. The preserved mobility of the R magnetic atom can yield very specific magnetic phenomena.
In cage compounds, atoms enclosed in oversized cavities (cages) retain a relatively large latitude of displacement (Fig. 1). These systems have recently attracted much attention, driven by favorable properties as regards thermoelectric applications. Our studies are of a fundamental purpose and focus on systems where the cage guest is a magnetic ion from the rare earth series.
Investigations of magnetic cage systems, mostly filled skutterudites, have evidenced phenomena that relate them to "heavy fermion" physics, metal-insulator transition, unconventional superconductivity, non-magnetic ordered phases etc. In the literature that reports these investigations, the interpretations usually neglect the possible involvement of the guest ion displacement. However, the physical context realized in the cages is unique, coupling the magnetic degrees of freedom, spin and orbit, with those of the displacement of the magnetic atom. Among the physical consequences of this coupling, one may imagine :
- a change in the pair interactions between rare earth ions, their strength depending on their relative positions. With offset rare earth ions, the system can lower its interaction energy. Such a mechanism is determinental in the antiferromagnetic range of the rare earth hexaboride , where the minimization of the exchange energy yields the formation of atomic displacement waves (Fig. 2) ;
- Fig. 2
- Thermal dependence of X-ray structure factors reflecting the formation of atomic displacements wave in antiferromagnetic GdB6.
- a single ion effect, due to the crystal field : the crystalline electric field acting on the rare earth at the cage center is very different from that exerted at an offset position. The high symmetry at the center determines orbitally degenerate crystal field levels whereas, at the margins of the cage, a low symmetry crystal field lifts the orbital degeneracy : the crystal field level scheme is highly dependent on the guest position (Fig. 3). At low temperature, an out of center position can become energetically favorable, leading to a specific Jahn-Teller effect. This centrifugal Jahn-Teller effect could explain part of the CeB6 enigma .
Following our investigations of the rare earth hexaborides, we turned to other cage systems, in particular, filled skuterrudites from the RPt4Ge12 series  (Fig. 1).
At Néel Institute :
- Synthesis and characterization : Synthesis of filled skuterrudites, DTA, X-ray and SEM characterizations
- Fig. 3
- Illustration of the centrifugal Jahn-Teller effect : the degeneracy of the 4f ground state at the cage center (left) is lifted for an offset magnetic ion (right).
- thermodynamical measurements : specific heat (PPMS), to identify phase transitions, determine the magnetic entropy etc.
- Magnetic Measurements : magnetization, susceptibility measurements (SQUID and extraction methods magnetometry)
- Measurements of the thermal expansion and magnetostriction : investigation of the coupling between the magnetic cage and the lattice.
Large scale facilities (ESRF, ILL, ...) :
- Neutron spectroscopy : Investigation of the magnetic rattling modes and of the crystal field schemes
- Powder neutron diffraction : structural investigation and search for magnetic orders
- X-ray diffraction on single crystal : investigation of the distribution of guest ions in their cages, both in the paramagnetic and the ordered states.
Members of the team
Rose-Marie Galéra, Christine Opagiste, Mehdi Amara.
 Macroscopic and microscopic investigation of the antiferromagnetic phase of TbB6, M. Amara, R.-M. Galéra, I. Aviani, and F. Givord, Phys. Rev. B, 82 (2010) 224411 [HAL,PRB]
 CeB6 Macroscopically Revisited, M. Amara and R.-M. Galéra, Phys. Rev. Lett., 108 (2012) 026402 [arXiv,PRL]
 First neutron studies of the magnetism and rattling modes in CePt4Ge12, R.-M. Galéra, C. Opagiste, M. Amara, M. Zbiri And S. Rols, Journal of Physics : Conference Series 592 (2015) 012011 [HAL,JPhysConfSeries]
Optique et Matériaux - OPTIMA
Rassembler une chaine de compétence complète qui va de la synthèse et l’élaboration de matériaux nouveaux à l’étude des propriétés optiques non linéaires et plasmoniques
Micro et NanoMagnétisme - MNM
Complementary expertise in fabrication, characterisation, and simulations for studies in nanomagnetism with applications in spin electronics and micro-systems
Systèmes Hybrides de basse dimensionnalité - HYBRID
Propriétés électroniques, optiques, vibrationnelles, mécaniques, et leur couplage à l’échelle quantique, de nouveaux systèmes hybrides (nanotubes, graphène, matériaux bi-dimensionnels, fonctionnalisés) que l’équipe développe.