Andrew Boothroyd

Abstract:

According to previous theoretical work, the binary oxide CuO can become a room temperature multiferroic via tuning of the superexchange interactions by application of pressure. Thus far, however, there has been no experimental evidence for the predicted room-temperature multiferroicity. Here, we show by neutron diffraction that the multiferroic phase in CuO reaches 295 K with the application of 18.5 GPa pressure. We also develop a spin Hamiltonian based on density functional theory and employing superexchange theory for the magnetic interactions, which can reproduce the experimental results. The present study provides a stimulus to develop room-temperature multiferroic materials by alternative methods based on existing low temperature compounds, such as epitaxial strain, for tunable multifunctional devices and memory applications.

Abstract:

Co3Sn2S2 has recently been predicted to be a Weyl semimetal in which magnetic order is key to its behavior as a topological material. Here, we report unpolarized neutron diffraction and spherical neutron polarimetry measurements, supported by magnetization and transport data, which probe the magnetic order in Co3Sn2S2 below TC=177 K. The results are fully consistent with ferromagnetic order in which the spins on the Co atoms point along the crystal c axis, although we cannot rule out some canting of the spins. We find no evidence for a type of long-ranged (k=0) in-plane 120° antiferromagnetic order which had previously been considered as a secondary phase present at temperatures between ∼90 K and TC. A discontinuous change in bulk properties and neutron polarization observed at T=125 K when samples are cooled in a field and measured on warming is found to be due to a sudden reduction in ferromagnetic domain size. Our results lend support to the theoretical predictions that Co3Sn2S2 is a magnetic Weyl semimetal.

Abstract:

In this work, we explore experimentally and theoretically the spectrum of magnetic excitations of the Fe3+ and Yb3+ ions in ytterbium iron garnet (Yb3Fe5O12). We present a complete description of the crystal-field splitting of the 4f states of Yb3+, including the effect of the exchange field generated by the magnetically ordered Fe subsystem. We also consider a further effect of the Fe-Yb exchange interaction, which is to hybridize the Yb crystal field excitations with the Fe spin-wave modes at positions in the Brillouin zone where the two types of excitations cross. We present detailed measurements of these hybridized excitations, and we propose a framework that can be used in the quantitative analysis of the coupled spectra in terms of the anisotropic 4f-3d exchange interaction.

Abstract:

CeRh 3 Si 2 has been reported to exhibit metamagnetic transitions below 5 K, a giant crystal field splitting, and anisotropic magnetic properties from single crystal magnetization and heat capacity measurements. Here we report results of neutron and x-ray scattering studies of the magnetic structure and crystal-field excitations to further understand the magnetism of this compound. Inelastic neutron scattering and resonant inelastic x-ray scattering reveal a J z = 1 / 2 ground state for Ce when considering the crystallographic a direction as quantization axis, thus explaining the anisotropy of the static susceptibility. Furthermore, we find a total splitting of 78 meV for the J = 5 / 2 multiplet. The neutron diffraction study in zero field reveals that, on cooling from the paramagnetic state, the system first orders at T N 1 = 4.7 K in a longitudinal spin density wave with ordered Ce moments along the b axis (i.e., the [0 1 0] crystal direction) and an incommensurate propagation vector k = ( 0 , 0.43 , 0 ). Below the lower-temperature transition T N 2 = 4.48 K , the propagation vector locks to the commensurate value k = ( 0 , 0.5 , 0 ) , with a so-called lock-in transition. Our neutron diffraction study in applied magnetic field H ∥ b axis shows a change in the commensurate propagation vector and development of a ferromagnetic component at H = 3 kOe , followed by a series of transitions before the fully field-induced ferromagnetic phase is reached at H = 7 kOe . This explains the nature of the steps previously reported in field-dependent magnetization measurements. A very similar behavior is also observed for the H ∥ [0 1 1] crystal direction.