Andrew Boothroyd

Abstract:

Spall occurs when materials are subjected to shock impacts; under this loading, the material properties can be modified through microstructural changes and phase transitions. The effect of these changes on subsequent spall has been underexplored. The anisotropy of tin’s ambient crystal structure and the accessibility of the β → γ solid-solid phase transition under shock loading means that tin offers a rich domain in which to study spall failure. Through testing single-crystal and polycrystal samples shocked above and below this transition, the effects of these variables on the deformation behaviour of tin can be determined. Although no orientation dependent spall behaviour is observed, unusual strainrate-dependent behaviour is observed, indicating likely mechanisms for the high-rate behaviour of tin.

Abstract:

Magnets with isotropic easy-plane symmetry host Goldstone modes that can be leveraged for efficient spin transport. Here, we present a time-resolved optical polarimetry technique that allows us to detect and characterize such low-frequency modes, and use it to observe the Goldstone mode in the multi-Q broken helix phase of EuIn2As2. The strength of our technique comes from the ability to distinguish between nematic and magnetization dynamics in order to yield information about the mode structure, in addition to its frequency. We find that the nearly uniform spin precession characteristic of a Goldstone mode is realized only when a small magnetic field is used to unpin the broken helix from local strain generated during crystal growth. In this regime, the mode frequency scales linearly with the applied field due to the ground state C2z symmetry of the broken helix. Our work shows how optical polarimetry can be used to study the Goldstone modes of complex magnets.

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.