Andrew Boothroyd

Abstract:

We present an extensive x-ray and neutron scattering study of the structure and magnetic excitations of ${\mathrm{Nd}}_2{\mathrm{PdSi}}_3$ , a sister compound of ${\mathrm{Gd}}_2{\mathrm{PdSi}}_3$ that was recently found to host a skyrmion lattice phase despite its centrosymmetric crystal structure. Dispersive magnetic excitations were measured throughout the Brillouin zone and modeled to determine the magnetic interactions between Nd ions. Our analysis reveals that the magnetic interactions in this system extend over large distances and are significantly affected by a crystallographic superstructure formed by ordering of the Pd and Si atoms. The results suggest that the mechanism for the skyrmion phase formation in this family of materials, specifically ${\mathrm{Gd}}_2{\mathrm{PdSi}}_3$ , is through the long-range Ruderman-Kittel-Kasuya-Yosida interactions rather than short-range triangular-lattice frustration.

Abstract:

We report on the synthesis and characterization of $\mathrm{M}{\mathrm{n}}_3\mathrm{Ga}$ thin films with controlled phase and orientation using DC magnetron sputtering. High-quality Mn-deficient hexagonal $\mathrm{M}{\mathrm{n}}_3\mathrm{Ga}$ films with C-plane and M-plane orientations were achieved on various single-crystal substrates, and their structural properties were systematically investigated by x-ray diffraction. Magnetization, Hall effect, and neutron diffraction measurements confirm the formation of antiferromagnetic order below $T_{\mathrm{N}}=460\text{–}470$ K and reveal its characteristic magnetic behavior. A sizable anomalous Hall effect is observed coexistent with weak net magnetization, consistent with the 120° noncollinear spin structure. The ability to tune both the crystal phase (hexagonal vs tetragonal) and orientation through composition and thermal treatment provides a platform for exploring the intrinsic magnetic and topological properties of $\mathrm{M}{\mathrm{n}}_3\mathrm{Ga}$ , with promising implications for antiferromagnetic spintronic applications.

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.