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

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By B A Frandsen (et al.)
Abstract: We report the successful synthesis and characterization of a new type I–II–V bulk form diluted magnetic semiconductor (DMS) Li(Zn,Mn,Cu)As, in which charge and spin doping are decoupled via (Cu,Zn) and (Mn,Zn) substitution at the same Zn sites. Ferromagnetic transition temperature up to  ∼33 K has been observed with a coercive field  ∼40 Oe for the 12.5% doping level. μ SR measurements confirmed that the magnetic volume fraction reaches nearly 100% at 2 K, and the mechanism responsible for the ferromagnetic interaction in this system is the same as other bulk form DMSs.
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Abstract: We present a temperature-dependent atomic and magnetic pair distribution function (PDF) analysis of neutron total scattering measurements of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. The known antiferromagnetic ground-state structure fits the low-temperature data closely with refined parameters that agree with conventional techniques, confirming the reliability of the newly developed magnetic PDF method. The measurements performed in the paramagnetic phase reveal significant short-range magnetic correlations on a 1nm length scale that differ substantially from the low-temperature long-range spin arrangement. Ab initio calculations using a self-interaction-corrected local spin density approximation of density functional theory predict magnetic interactions dominated by Anderson superexchange and reproduce the measured short-range magnetic correlations to a high degree of accuracy. Further calculations simulating an additional contribution from a direct exchange interaction show much worse agreement with the data. The Anderson superexchange model for MnO is thus verified by experimentation and confirmed by ab initiotheory.
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By B. A. Frandsen (et al.)
Abstract: We present muon spin rotation 
(
μ
SR
)
 and susceptibility measurements on single crystals of isoelectronically doped 
URu
2

x
T
x
Si
2
 (T = Fe, Os) for doping levels up to 50%. Zero field (ZF) 
μ
SR
measurements show long-lived oscillations demonstrating that an antiferromagnetic state exists down to low doping levels for both Os and Fe dopants. The measurements further show an increase in the internal field with doping for both Fe and Os. Comparison of the local moment-hybridization crossover temperature from susceptibility measurements and our magnetic transition temperature shows that changes in hybridization, rather than solely chemical pressure, are important in driving the evolution of magnetic order with doping.
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By B A Frandsen (et al.)
Abstract: We report the synthesis and characterization of a bulk form diluted magnetic semiconductor, (La1−x Ca x )(Zn1−y Mn y )AsO, with a layered crystal structure isostructural to that of the 1 1 1 1 type Fe-based high-temperature superconductor LaFeAsO and the antiferromagnetic LaMnAsO. With Ca and Mn codoping into LaZnAsO, the ferromagnetic ordering occurs below the Curie temperature   ~30 K. Taking advantage of the decoupled charge and spin doping, we investigate the influence of carrier concentration on the ferromagnetic ordering state. For a fixed Mn concentration of 10%,  increases from 24 K to 30 K when the Ca concentration increases from 5% to 10%. Further increase of Ca concentration reduces both the coercive field and saturation moment. Muon spin relaxation measurements confirm the ferromagnetically ordered state, and clearly demonstrate that (La1−x Ca x )(Zn1−y Mn y )AsO shares a common mechanism for the ferromagnetic exchange interaction with (Ga,Mn)As. Neutron scattering measurements show no structural transition in (La0.90Ca0.10)(Zn0.90Mn0.10)AsO below 300 K.
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By B. Frandsen (et al.)
Abstract: The origin of magnetism in metals has been traditionally discussed in two diametrically opposite limits: itinerant and local moments. Surprisingly, there are very few known examples of materials that are close to the itinerant limit, and their properties are not universally understood. In the case of the two such examples discovered several decades ago, the itinerant ferromagnets ZrZn2 and Sc3In, the understanding of their magnetic ground states draws on the existence of 3d electrons subject to strong spin fluctuations. Similarly, in Cr, an elemental itinerant antiferromagnet with a spin density wave ground state, its 3d electron character has been deemed crucial to it being magnetic. Here, we report evidence for an itinerant antiferromagnetic metal with no magnetic constituents: TiAu. Antiferromagnetic order occurs below a Néel temperature of 36 K, about an order of magnitude smaller than in Cr, rendering the spin fluctuations in TiAu more important at low temperatures. This itinerant antiferromagnet challenges the currently limited understanding of weak itinerant antiferromagnetism, while providing insights into the effects of spin fluctuations in itinerant–electron systems.
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Abstract: An experimental determination of the magnetic pair distribution function
(mPDF) defined in an earlier paper [Frandsen et al. (2014). Acta Cryst. A70, 3–
11] is presented for the first time. The mPDF was determined from neutron
powder diffraction data from a reactor and a neutron time-of-flight total
scattering source on a powder sample of the antiferromagnetic oxide MnO. A
description of the data treatment that allowed the measured mPDF to be
extracted and then modelled is provided and utilized to investigate the lowtemperature
structure of MnO. Atomic and magnetic co-refinements support
the scenario of a locally monoclinic ground-state atomic structure, despite the
average structure being rhombohedral, with the mPDF analysis successfully
recovering the known antiferromagnetic spin configuration. The total scattering
data suggest a preference for the spin axis to lie along the pseudocubic [101]
direction. Finally, r-dependent PDF refinements indicate that the local
monoclinic structure tends toward the average rhombohedral R3m symmetry
over a length scale of approximately 100 A ° .