CM Seminar - From Quantum Magnets to Magnetic Thermoelectrics: Short-Range Spin Correlations and the Secrets They Keep

Short-range magnetic correlations have been increasingly recognized in recent years for their importance in contexts as widely varied as geometrically frustrated magnetism and functional magnetocaloric materials. Neutron scattering provides experimental access to these short-range correlations through magnetic diffuse scattering, but characterizing magnetic short-range order with quantitative accuracy has remained a difficult task. Here, I introduce magnetic pair distribution function (mPDF) analysis as a powerful tool for the detailed elucidation of shortrange magnetic correlations by Fourier transforming the diffuse magnetic scattering into real
space, and I present the application of mPDF to two material systems for which the details of the short-range magnetic correlations are crucial for their unusual properties. The first is TmMgGaO4, an Ising triangular lattice antiferromagnet with rich low-temperature behavior, including a predicted topological Kosterlitz-Thouless transition. Our mPDF results reveal intricate spin correlations consistent with this theoretical prediction, providing strong experimental support for the proposed Kosterlitz-Thouless phase. The second material is MnTe, an antiferromagnetic semiconductor with excellent thermoelectric properties at elevated temperatures. The presence of short-range antiferromagnetic correlations above the Neel temperature contributes to the enhanced thermoelectric response. Using mPDF, we reveal the real-space nature of these magnetic correlations and gain deeper insight into the origin of the unusually large thermopower of MnTe.

Bio: Ben Frandsen is an assistant professor in the Department of Physics and Astronomy at Brigham Young University in Utah. He earned his PhD in condensed matter physics at Columbia University in New York in 2016, working with Professors Tomo Uemura and Simon Billinge on muon spin relaxation and x-ray and pair distribution function studies of strongly correlated electron systems. His graduate research took him to beautiful Vancouver many times to perform muon experiments at TRIUMF. Following his graduate work, he joined the group of Bob Birgeneau at UC Berkeley in California as a postdoctoral researcher, working primarily on experimental studies of iron-based superconductors. He joined the faculty at BYU in August 2018. His research at BYU is focused on investigating the local atomic and magnetic structure of complex materials using beams of x rays, neutrons, and muons. He was selected as a recipient of the U.S. Department of Energy Early Career Award in 2020 to study local atomic and magnetic structure in energy-relevant materials.