Welcome to the Frandsen Group!

About the Group

We are an experimental condensed matter physics group focused on investigating the structure and magnetism of fascinating--and often technologically promising--materials, such as superconductors, strongly correlated electron systems, multiferroics, magnetocalorics, molten salts for nuclear reactors, and more. We use beams of neutrons, x-rays, and muons produced at large-scale accelerator facilities to probe the atomic and magnetic correlations in these materials, together with advanced computational modeling to gain quantitative insight into the spatial arrangement of atoms and spins in a given material. Specific techniques include atomic and magnetic pair distribution function (PDF) analysis of neutron/x-ray total scattering data and muon spin relaxation/rotation (μSR). Interested and motivated undergraduate and prospective graduate students are encouraged to reach out to learn more about our research and find opportunities to participate.

Research Projects

Thermoelectrics, Magnetocalorics, and Multiferroics--Oh My!

This project focuses on the connection between the local atomic and magnetic structure and the energy-relevant properties of magnetocaloric, thermoelectric, and multiferroic materials. Magnetocaloric materials exhibit large temperature changes with the application and removal of a magnetic field, offering promising applications in solid-state refrigeration and waste heat harvesting. Thermoelectric materials experience an electrical voltage when subjected to a temperature gradient or vice versa, also providing novel routes for energy-efficient cooling and waste heat harvesting. Multiferroic materials show cross-order coupling between electric polarization and magnetic order, potentially enabling unique functionalities for energy transformation, information science, and signal processing. We are using combined atomic and magnetic pair distribution function analysis, together with muon spin spectroscopy, to establish the local atomic and magnetic structure of representative compounds for these material classes and better understand the origin of their outstanding properties. In the process, we are developing new experimental and computational methods for magnetic pair distribution function analysis, which will be widely applicable to many other materials, as well. Funding: US Department of Energy, Early Career program.

Superconductors, Geometrically Frustrated Magnets, Magnetic Nanoparticles, and More

We maintain broad interest and involvement in structural studies of numerous material systems where knowledge of the local atomic and magnetic structure can add value. We have ongoing projects on iron-based superconductors, geometrically frustrated triangular lattice antiferromagnets, magnetic nanoparticles, Mott insulator systems, high-entropy alloys and oxides, and more. We are always open to collaborations on interesting material systems.

Molten Salts For Improved Nuclear Reactors

Molten salt reactors (MSRs) are a promising nuclear reactor design concept in which molten ionic salts function as the coolant and/or fuel source in the reactor. MSRs have many potential advantages over standard designs in commercial use today, including greatly enhanced safety/security and the ability to produce critical medical radioisotopes in addition to vast amounts of carbon-free electricity. To make MSRs a reality, it is necessary to understand and predict the behavior of the salts in operating conditions. Gaining a detailed knowledge of the local structure of the molten salts on the atomic scale is an essential step in this direction, since the local interactions between constituent atoms determine the macroscopic properties. In this project, we use cutting-edge neutron and x-ray total scattering and computational modeling techniques to establish the structure of relevant molten salts. We work closely with collaborators in BYU Chemical Engineering. Funding: US Department of Energy, Nuclear Energy University Program (pending).

Selected Publications

Real-space visualization of short-range antiferromagnetic correlations in a magnetically enhanced thermoelectric

By Raju Baral, Jacob A. Christensen, Parker K. Hamilton, Karine Chesnel, and Benjamin A. Frandsen (et al.)

Abstract:

Thermoelectric materials hold tremendous promise for energy applications, but developing economically and environmentally viable high-performance thermoelectrics remains challenging. Recently, the paramagnon drag effect was discovered, in which local thermal fluctuations of the magnetization known as paramagnons drag charge carriers and impart to them a magnetic contribution to thermopower. This is significant because it opens a new avenue for optimizing thermoelectric properties in magnetic semiconductors. In this work, neutron scattering is used to visualize the nanoscale magnetism responsible for paramagnon drag in the antiferromagnetic semiconductor MnTe. First-principles calculations quantitatively reproduce the short-range magnetic correlations observed above the ordering temperature. These results shed light on how magnetism enhances thermoelectricity and provide a template for studies of other magnetic semiconductors as potential high-performance thermoelectrics.

Spin dynamics of the spin-chain antiferromagnet RbFeS₂

By Benjamin A. Frandsen (et al.)

Abstract:

We report transport and inelastic neutron scattering studies on electronic properties and spin dynamics of the quasi-one-dimensional spin-chain antiferromagnet RbFeS₂. An antiferromagnetic phase transition at TN≈195 K and dispersive spin waves with a spin gap of 5 meV are observed. By modeling the spin excitation spectra using linear spin wave theory, intra and interchain exchange interactions are found to be SJ1=100(5) meV and SJ3=0.9(3) meV, respectively, together with a small single-ion anisotropy of SD_zz=0.04(1) meV. Comparison with previous results for other materials in the same class of Fe³⁺ spin-chain systems reveals that although the magnetic order sizes show significant variation from 1.8 to 3.0μ_B within the family of materials, the exchange interactions SJ are nevertheless quite similar, analogous to the iron pnictide superconductors where both localized and delocalized electrons contribute to the spin dynamics.

Local structure determination using total scattering data

By Benjamin A. Frandsen (et al.)

Abstract:

This article describes the application of total scattering and atomic pair distribution function (PDF) studies to the study of local and intermediate range structure in complex materials. We give a brief introduction to the methods, and then survey a sampling of different applications of them in various inorganic chemistry applications, including energy materials, nanoparticles, layered materials, metal organic frameworks and host-guest systems, polycrystalline thin films, atomic clusters, hybrid-perovskites. We also discuss studies of local magnetism and amorphous materials.

Uniform structural phase transition in V₂O₃ without short-range distortions of the local structure

By Ethan R. A. Fletcher and Benjamin A. Frandsen (et al.)

Abstract:

The local structure of V₂O₃, an archetypal strongly correlated electron system that displays a metal-insulator transition around 160 K, has been investigated via pair distribution function (PDF) analysis of neutron and x-ray total scattering data. The rhombohedral-to-monoclinic structural phase transition manifests as an abrupt change on all length scales in the observed PDF. No monoclinic distortions of the local structure are found above the transition, although coexisting regions of phase-separated rhombohedral and monoclinic symmetry are observed between 150 and 160 K. This lack of structural fluctuations above the transition contrasts with the known presence of magnetic fluctuations in the high-temperature state, suggesting that the lattice degree of freedom plays a secondary role behind the spin degree of freedom in the transition mechanism.

Local structure of Mott insulating iron oxychalcogenides La₂O₂Fe₂OM₂ (M = S, Se)

By Benjamin A. Frandsen (et al.)

Abstract:

We describe the local structural properties of the iron oxychalcogenides, La₂O₂Fe₂OM₂ (M=S,Se), by using pair distribution function analysis applied to total scattering data. Our results from neutron powder diffraction show that M = S and Se possess similar nuclear structures at low and room temperatures. The local crystal structures were studied by investigating deviations in atomic positions and the extent of the formation of orthorhombicity. Analysis of the total scattering data suggests that buckling of the Fe2O plane occurs below 100 K. The buckling may occur concomitantly with a change in octahedral height. Furthermore, within a typical range of 1–2 nm, we observed a short-range orthorhombiclike structure suggestive of nematic fluctuations in both of these materials.

Superparamagnetic dynamics and blocking transition in Fe₃O₄ nanoparticles probed by vibrating sample magnetometry and muon spin relaxation

By Benjamin A. Frandsen, Charlotte Read, Jade Stevens, Colby Walker, Mason Christiansen, Roger G. Harrison, and Karine Chesnel

Abstract:

The magnetic properties of Fe₃O₄ nanoparticle assemblies have been investigated in detail through a combination of vibrating sample magnetometry (VSM) and muon spin relaxation (μSR) techniques. Two samples with average particle sizes of 5 and 20 nm, respectively, were studied. For both samples, the VSM and μSR results exhibit clear signatures of superparamagnetism at high temperature and magnetic blocking at low temperature. The μSR data demonstrate that the transition from the superparamagnetic to the blocked state occurs gradually throughout the sample volume over an extended temperature range due to the finite particle size distribution of each nanoparticle batch. The transition occurs between approximately 3 and 45 K for the 5-nm nanoparticles and 150 and 300 K for the 20-nm nanoparticles. The VSM and μSR data are further analyzed to yield estimates of microscopic magnetic parameters including the nanoparticle spin-flip activation energy EA, magnetic anisotropy K, and intrinsic nanoparticle spin reversal attempt time τ₀. These results highlight the complementary information about magnetic nanoparticles that can be obtained by bulk magnetic probes such as magnetometry and local magnetic probes such as μSR.