Andrew Boothroyd

Abstract:

We use spherical neutron polarimetry to determine the ground state magnetic structure of ${\mathrm{Mn}}_3\mathrm{Sn}$ . We find that ${\mathrm{Mn}}_3\mathrm{Sn}$ adopts an inverse triangular structure with spins parallel to $〈100〉$ (type III) rather than spins parallel to $〈110〉$ (type IV). Density functional theory calculations reveal no energy difference between these two structures, suggesting that the selection is caused by subtle effects such as sixth-order anisotropy. Partial control of the magnetic domain population through a moderate magnetic field is key to distinguishing between the two models. We find that three of the six domains are approximately equally populated, while the others have negligible population. Upon entering the low temperature incommensurate phase, the domain structure is lost. The domains in this phase are decoupled from the magnetic field and can therefore not be controlled by any known method.

Abstract:

Tin is known for its asymmetric crystal structure and numerous solid phase transitions, with molecular dynamics studies suggesting the beta to gamma phase transition exhibits a strong orientation dependence. In this study, shock compression experiments are conducted on tin single crystals and polycrystals to probe the effects of the crystal orientation on this phase transition through Hugoniot measurements, with peak pressures between 9 and 13 GPa. A strong order-of-magnitude orientation dependence of the elastic limit is found; however, the transition and post-transition behavior show at best only qualitative differences to the velocimetry profiles, with no quantitative variation. A dependence of the transition on the peak pressure is also observed. Explanations of these results based on potential transformation pathways identified through prior static high pressure work are discussed.

Abstract:

Geometrical frustration in the face-centered-cubic (fcc) lattice presents a fundamental challenge in determining antiferromagnetic order, as the ground state is highly sensitive to subtle differences in competing magnetic interactions and structural symmetry. Here, we explore the magnetostructural interplay in two halide double perovskites, Cs2NaFeCl6 and Cs2AgFeCl6. Although both materials have a cubic structure at room temperature, neutron diffraction shows that they adopt different antiferromagnetic structures upon cooling. Cs2NaFeCl6 experiences a transition to an AFM-III order below 2.6 K, governed by J 1 and J 2 (first and second nearest-neighbor) magnetic exchange interactions. Cs2AgFeCl6, however, adopts an AFM-I order below 17 K, accompanied by a significant tetragonal distortion confirmed from both neutron diffraction and polarized Raman spectroscopy. Thermal expansion measurements reveal anomalous lattice expansion at the magnetic transitions in both compounds but are substantially stronger in Cs2AgFeCl6. Combining these findings with density functional theory (DFT) studies, we conclude that the strength of magnetoelastic coupling dictates the magnetic ground state. A strong J 1 in Cs2AgFeCl6 induces a large tetragonal lattice distortion, relieving magnetic frustration and stabilizing the AFM-I phase. In contrast, weaker magnetoelastic coupling in Cs2NaFeCl6 causes minimal distortion, favoring the AFM-III phase via the J 1–J 2 mechanism. Our findings show that magnetic interactions can be a primary driving force for structural phase transitions in these materials, while the strong structural distortion could determine the selection of magnetic ground-state ordering.