Transition énergétique

Orthoferrites are a class of magnetic materials with a magnetic ordering temperature above 600 K, predominant G-type antiferromagnetic ordering of the Fe-spin system and, depending on the rare-earth ion, a spin reorientation of the Fe spin taking place at lower temperatures. DyFeO3 is of particular interest since the spin reorientation is classified as a Morin transition with the transition temperature depending strongly on the Dy-Fe interaction. Here, we report a detailed study of the magnetic and structural properties of microcrystalline DyFeO3 powder and bulk single crystal using neutron diffraction and magnetometry between 1.5 and 450 K. We find that, while the magnetic properties of the single crystal are largely as expected, the powder shows strongly modified magnetic properties, including a modified spin reorientation and a smaller Dy-Fe interaction energy of the order of 10 μeV. Subtle structural differences between powder and single crystal show that they belong to distinct magnetic space groups. In addition, the Dy ordering at 2 K in the powder is incommensurate, with a modulation vector of 0.0173(5) c∗, corresponding to a periodicity of ∼58 unit cells.

SwissFEL team has demonstrated the generation of widely tunable two-color x-ray free-electron laser (FEL) pulses with unprecedented photon energy ratio between the two colors of about three (350 and 915 eV), in addition to a tunable time separation between the two pulses from negative time delays to up to 500 fs. These new capabilities open new opportunities to study ultrafast x-ray-induced energy transfer and relaxation processes in physics, chemistry, and biology.

The gas-phase reaction dynamics and kinetics in a laser induced plasma are very much dependent on the interactions of the evaporated target material and the background gas. For metal (M) and metal–oxygen (MO) species ablated in an Ar and O₂ background, the expansion dynamics in O₂ are similar to the expansion dynamics in Ar for M⁺ ions with an MO⁺ dissociation energy smaller than O₂. This is different for metal ions with an MO⁺ dissociation energy larger than for O₂. This study shows that the plume expansion in O₂ differentiates itself from the expansion in Ar due to the formation of MO⁺ species. It also shows that at a high oxygen background pressure, the preferred kinetic energy range to form MO species as a result of chemical reactions in an expanding plasma, is up to 5 eV.

The interplay between a topological electronic structure and magnetism may result in intricate physics. In this work, we describe a case of rather peculiar coexistence or competition of several magnetic phases below seemingly single antiferromagnetic transition in LnSbTe (Ln = Ho and Tb) topological semimetals, the magnetic members of the ZrSiS/PbFCl structure type (space group P4/nmm). Neutron diffraction experiments reveal a complex multi-step order below T_N = 3.8 K (Ln = Ho) and T_N = 6.4 K (Ln = Tb). Magnetic phases can be described using four propagation vectors k₁ = (¹/₂ 0 0) and k₂ = (¹/₂ 0 ¹/₄) at a base temperature of 1.7 K, which transform into incommensurate vectors k₁′ = (¹/₂ – δ 0 0) and k₃ = (¹/₂ – δ 0 ¹/₂) at elevated temperatures in both compounds. Together with the refined models of magnetic structures, we present the group theoretical analysis of magnetic symmetry of the proposed solutions. These results prompt further investigations of the relation between the electronic structure of those semimetals and the determined antiferromagnetic ordering existing therein.

This work aimed to produce intermetallic samples of platinoid metals (active metal matrix) and lanthanides (co-metal) and via the method of Coupled Reduction, i.e. a thermal treatment of the combination of the lanthanide oxide and noble metal at high temperature, as high as 1100 °C, under a constant flow of H₂. We have demonstrated by means of several techniques, such as Scanning Electron Microscope, Energy Dispersive X-Ray Spectroscopy, Alpha Spectrometry and Radiographic Imaging, that this method, at defined experimental conditions (temperature, pressure and concentration) yields a metallic lanthanide thin film when using platinum as active metal matrix. Conversely, the formation of a bulk intermetallic compound was obtained when using Pd as matrix. Those systems will have applications in different nuclear physic and radiochemistry studies, such as irradiation targets for production of superheavy elements and for nuclear data determination.

Yanting Qian has received the MSc/PhD competition award in the FISA2022-EURADWASTE'22 conference. She works on the retention of redox-sensitive Tc on Fe-bearing clay minerals.