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.

Promoting Many-Body Quantum Entanglement in Geometrically Frustrated Magnets with Disorder

Quantum information technologies rely on quantum entanglement, or the intrinsic linking of one quantum object to another. An important research objective is to gain a fundamental understanding of many-body quantum entanglement involving large numbers of quantum objects. Certain magnetic materials known as geometrically frustrated magnets provide a valuable platform for this topic of study because they may exhibit many-body-entanglement at low temperature. This project advances the search for promising quantum-entangled frustrated magnets through a systematic investigation of the role of atomic-scale disorder in promoting or hindering many-body entanglement. The results illuminate strategies for utilizing disorder to promote quantum-entangled ground states and contribute to a deeper understanding of many-body quantum entanglement in general. Funding: US National Science Foundation LEAPS Program.

Novel magnets, Magnetic Nanoparticles, Metal-Insulator Transitions, High-Entropy Materials, 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 novel magnets such as altermagnets and low-dimensional magnets, magnetic nanoparticles, Mott insulator systems and materials with metal-insulator transitions, high-entropy alloys and oxides, and more. We are always open to collaborations on interesting material systems.

Selected Publications

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.

Ab initio modeling and experimental investigation of Fe₂P by DFT and spin spectroscopies

By Benjamin A. Frandsen and Ethan J. Gibson (et al.)

Abstract:

Fe₂P alloys have been identified as promising candidates for magnetic refrigeration at room-temperature and for custom magnetostatic applications. The intent of this study is to accurately characterize the magnetic ground state of the parent compound, Fe₂P, with two spectroscopic techniques, μSR and NMR, in order to provide solid bases for further experimental analysis of Fe₂P-type transition metal based alloys. We perform zero applied field measurements using both techniques below the ferromagnetic transition TC=220 K. The experimental results are reproduced and interpreted using first principles simulations, validating this approach for quantitative estimates in alloys of interest for technological applications.