Preparation of single crystals of CeAl2 by the Czochralski method

Lattice dynamics in CePd2Al2 and LaPd2Al2

The interaction between phonons and 4f electrons, which is forming a new quantum state (quasi-bound state) beyond Born-Oppenheimer approximation, is very prominent and lattice dynamics plays here a key role. There is only a small number of compounds in which the experimental observation suggest such a scenario. One of these compounds is CePd₂Al₂. Here the study of phonon dispersion curves of (Ce,La)Pd₂Al₂ at 1.5, 7.5, 80 and 300 K is presented. The inelastic X-ray scattering technique was used for mapping the phonon modes at X and Z points as well as in Λ and Δ directions, where the symmetry analysis of phonon modes was performed. The measured spectra are compared with the theoretical calculation, showing very good agreement. The measurements were performed in several Brillouin zones allowing the reconstruction of phonon dispersion curves. The results are discussed with respect to the magneto-elastic interaction and are compared with other cerium compounds. The phonon mode symmetry A_1g was found to be unaffected by the interaction, which is in contrast to previous assumptions.

Size effects in the magnetic behaviour of TbAl(2) milled alloys

The study of the magnetic properties depending upon mechanical milling of the ferromagnetic polycrystalline TbAl(2) material is reported. Rietveld analysis of the x-ray diffraction data reveals a decrease in the grain size down to 14 nm and a -0.15% variation in the lattice parameter, after 300 h of milling time. Irreversibility in the zero field cooled-field cooled (ZFC-FC) dc susceptibility and clear peaks in the ac susceptibility between 5 and 300 K show that the long-range ferromagnetic structure is inhibited in favour of a disordered spin arrangement below 45 K. This glassy behaviour is also deduced from the variation of the irreversibility transition with the field (H(2/3)) and frequency. The magnetization process of the bulk TbAl(2) is governed by domain-wall thermal activation processes. In contrast, in the milled samples, cluster-glass properties arise as a result of cooperative interactions due to the substitutional disorder. The interactions are also influenced by the nanograin structure of the milled alloys, showing a variation in coercivity with the grain size, below the crossover between the multi- and single-domain behaviours.