Frustration and Topology in Magnetic Materials

Magnetic materials have found robust technological applications both in memory storage and medical imaging. The quickly expanding family of spintronics looks to improve and expand upon these applications by examining the intersection of magnetism with other solid-state physics fields such as the properties of exfoliable 2D van der Waals materials and the symmetry protected edge states seen in topological materials. Here I will describe two stories from my graduate research at Ohio State that explore the interplay between magnetism and each of these two solid state pillars. The first story will explore the synthesis and characterization of novel 2D materials of stoichiometry Mn2Tr2Ch5 (Tr = Ga/I, Ch = S/Se) in which the van der Waals layer consists of two slabs of Mn(S/Se)6 octahedra capped by (Ga/In)(S/Se)4 tetrahedra. Mn2Ga2S5 and Mn2In2Se5 are grown atomically pure for the first time and are shown to be indirect band gap semiconductors with highly frustrated magnetism resulting in spin glassy behavior. This story will further explore the subsequent modification of these materials through polytypic and polymorphic structural transitions seen in a family of (Mn1-xFex)2Ga2S5 alloys and details the resulting changes in magnetic properties. The second half of my talk will detail our theoretical search and experimental realization of Weyl magnons in candidate material MnTe2. A high throughput approach was used to identify materials that must host topological magnons in a generic spin model by utilizing just the magnetic space group and the Wyckoff sites. From this search we identified MnTe2 as a suitable candidate material of which large single crystals have been grown and magneto-Raman and time of flight inelastic neutron scattering measurements were used to experimentally confirm the presence of Weyl magnon crossings in this material.