ABSTRACT
This paper presents a study of the lattice dynamics in BaFe2Se3. We combined first-principle calculations, infrared measurements and a thorough symmetry analysis. Our study confirms thatPnmacannot be the space group of BaFe2Se3, even at room temperature. The phonons assignment requiresPmto be the BaFe2Se3space group, not only in the magnetic phase, but also in the paramagnetic phase at room temperature. This is due to a strong coupling between a short-range spin-order along the ladders, and the lattice degrees of freedom associated with the Fe-Fe bond length. This coupling induces a change in the bond-length pattern from an alternated trapezoidal one (as inPnma) to an alternated small/large rectangular one. Out of the two patterns, only the latter is fully compatible with the observed block-type magnetic structure. Finally, we propose a complete symmetry analysis of the BaFe2Se3phase diagram in the 0-600 K range.
ABSTRACT
We present here a quantitative analysis of the ground state and magnetic properties of Ce3Pt23Si11, based on a crystalline electric field description within the mean-field approximation. In this face-centered cubic compound, the point group symmetry at the Ce site is orthorhombic. One main difficulty in this low symmetry case is that the CEF potential for Ce3+ ions is determined by five independent parameters, while only two magnetic excitations are observed by inelastic neutron scattering. Moreover the anisotropy of the magnetic susceptibility of the Ce ion, that permits an independent determination of the second-order CEF parameters is hidden by the cubic symmetry of the compound. A specific procedure is developed for this purpose that combines genetic algorithms and more conventional optimization methods. A set of CEF parameters is found that best reproduces the different experimental observations in both the paramagnetic and ferromagnetic phases of Ce3Pt23Si11. The analysis accounts for two seemingly contradictory properties: a strong local anisotropy that aligns the moment along a fourfold axis and a rather weak anisotropy of the bulk magnetization with an easy threefold magnetization axis. Ce3Pt23Si11 is shown to be a model system where single site anisotropies compete within a crystal structure of overall high symmetry.
ABSTRACT
In solid state science, multifunctional materials and especially multiferroics have attracted a great deal of attention, as they open the possibility for next generation spintronic and data storage devices. Interestingly, while many of them host coexisting 3d and 4f elements, the role of the coupling between these two magnetic entities has remained elusive. By means of single crystal neutron diffraction and inelastic neutron scattering experiments we shed light on this issue in the particular case of the multiferroic oxide DyMn2O5. This compound undergoes a first order magnetic transition from a high temperature incommensurate phase to a low temperature commensurate one. Our investigation reveals that although these two phases have very different magnetic structures, the spin excitations are quite similar indicating a fragile low temperature ground state with respect to the high temperature one. Such a rare scenario is argued to be a manifestation of the competition between the exchange interaction and 4f magnetic anisotropy present in the system. It is concluded that the magnetic structure, hence the ferroelectricity, can be finely tuned depending on the anisotropy of the rare earth.
