Engineering topics

Christianson, Andrew D.

Publications and source records attributed to Christianson, Andrew D..

9 records

DOE OSTI2024

Anisotropic magnetism of polymorphic ErAl 3

ErAl 3 can form in either a trigonal (α) or cubic (β) polymorph and this paper investigates the physical properties of these polymorphs through characterizations of single crystals grown in an aluminum flux. Here, we demonstrate that polymorph selection can be achieved based on the nominal composition of the crystal growth. Magnetic measurements confirm that both β-ErAl 3 and α-ErAl 3 order antiferromagnetically at low temperatures. β-ErAl 3 undergoes antiferromagnetic ordering at a Néel temperature T N = 5.1 K, and the transition is suppressed continually with applied field. α-ErAl 3 displays more complex behavior, with successive magnetic transitions at T N = 5.7 K and T 2 = 4.6 K for zero field, where heat capacity and dilatometry measurements evidence that these transitions are second and first order, respectively. Under magnetic field, strong anisotropy is revealed in α-ErAl 3 , with several steplike metamagnetic transitions observed below T 2 for H ∥ c. These transitions produce sequential magnetization plateaus near one-half of the apparent saturation magnetization. The electrical resistivity of α-ErAl 3 is strongly coupled to its magnetism. At T = 2 K, we observe a positive magnetoresistance reaching 60%, with distinct anomalies at the metamagnetic transitions. The results are summarized in H-T phase diagrams that demonstrate complex magnetic behavior for α-ErAl 3 , suggesting an important role of competing interactions in this metallic system that possesses characteristics of Ising physics.

36 MATERIALS SCIENCE↗

DOE OSTI2024

MnRhBi 3 : A Cleavable Antiferromagnetic Metal

Cleavable metallic antiferromagnets may be of use for low-dissipation spintronic devices; however, few are currently known. Here, in this study, we present orthorhombic MnRhBi 3 as one such compound and present a thorough study of its physical properties. Exfoliation is demonstrated experimentally, and the cleavage energy and electronic structure are examined by density functional theory calculations. It is concluded that MnRhBi 3 is a van der Waals-layered material that cleaves easily between neighboring Bi layers and that the Bi atoms have lone pairs extending into the van der Waals gaps. A series of four phase transitions are observed below room temperature, and neutron diffraction shows that at least two of the transitions involve the formation of antiferromagnetic order. Anomalous thermal expansion points to a crystallographic phase transition and/or strong magnetoelastic coupling. This work reveals a complex phase evolution in MnRhBi 3 and establishes this cleavable antiferromagnetic metal as an interesting material for studying the interplay of structure, magnetism, and transport in the bulk and ultrathin limits, as well as the role of lone pair electrons in interface chemistry and proximity effects in van der Waals heterostructures.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Spin dynamics of the centrosymmetric skyrmion material GdRu 2 Si 2

Magnetic skyrmion crystals are traditionally associated with non-centrosymmetric crystal structures; however, it has been demonstrated that skyrmion crystals can be stabilized by competing interactions in centrosymmetric crystals. To understand and optimize the physical responses associated with topologically nontrivial skyrmion textures, it is important to quantify their magnetic interactions by comparing theoretical predictions with spectroscopy data. Here, we present neutron diffraction and spectroscopy data on the centrosymmetric skyrmion material GdRu 2 Si 2 and show that the key spectroscopic features can be explained by magnetic interactions calculated using density functional theory. We further show that the recently proposed 2-q “topological spin stripe” structure yields better agreement with our data than a 1-q helical structure and identify how the magnetic structure evolves with temperature.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Nanometric modulations of the magnetic structure of the element Nd

The rare earth neodymium arguably exhibits the most complex magnetic ordering and series of magnetic phase transitions of the elements. Here we report the results of small-angle neutron scattering (SANS) measurements as a function of temperature and applied magnetic field to study magnetic correlations on nanometer length scales in Nd. We support these results with additional neutron diffraction, thermodynamic and synchrotron x-ray measurements. The SANS measurements reveal the presence of modulation vectors characterizing the ordered spin configuration which exhibit changes in magnitude and direction that are phase dependent. Between 5.9 and 7.6 K an additional modulation vector is observed with magnitude Q = 0.12 Å -1 . This scattering appears to originate from order of the Nd layers which contain a center of inversion. Tracking this modulation vector as a function of magnetic field indicates a phase boundary at ≈1 T. The modulation vectors observed in this study establish the presence of nanometer length scale spin textures which are likely stabilized by frustrated Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Cubic double perovskites host noncoplanar spin textures

Magnetic materials with noncoplanar magnetic structures can show unusual physical properties driven by nontrivial topology. Topologically-active states are often multi-q structures, which are challenging to stabilize in models and to identify in materials. Here, we use inelastic neutron-scattering experiments to show that the insulating double perovskites Ba 2 YRuO 6 and Ba 2 LuRuO 6 host a noncoplanar 3-q structure on the face-centered cubic lattice. Quantitative analysis of our neutron-scattering data reveals that these 3-q states are stabilized by biquadratic interactions. Our study identifies double perovskites as a highly promising class of materials to realize topological magnetism, elucidates the stabilization mechanism of the 3-q state in these materials, and establishes neutron spectroscopy on powder samples as a valuable technique to distinguish multi-q from single-q states, facilitating the discovery of topologically-nontrivial magnetic materials.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Quantum magnetism in the frustrated square lattice oxyhalides YbBi 2 IO 4 and YbBi 2 ClO 4

Square-lattice systems offer a direct route for realizing two-dimensional (2D) quantum magnetism with frustration induced by competing interactions. In this work, the square lattice materials YbBi 2 IO 4 and YbBi 2 ClO 4 were investigated using a combination of magnetization and specific-heat measurements on polycrystalline samples. Specific-heat measurements provide evidence for long-range magnetic order below $T$ N = 0.21 K (0.25 K) for YbBi 2 IO 4 (YbBi 2 ClO 4 ). On the other hand, a rather broad maximum is found in the temperature-dependent magnetic susceptibility, located at $T$ max = 0.33 K (0.38 K) in YbBi 2 IO 4 (YbBi 2 ClO 4 ), consistent with the quasi-2D magnetism expected for the large separation between the magnetic layers. Estimation of the magnetic entropy supports the expected Kramers' doublet ground state for Yb 3+ and the observed paramagnetic behavior is consistent with a well-isolated doublet. Roughly two-thirds of the entropy is consumed above $T$ N , due to a combination of the quasi-2D behavior and magnetic frustration. The impact of frustration is examined from the viewpoint of a simplified $J$ 1 -$J$ 2 square lattice model, which is frustrated for antiferromagnetic interactions. Specifically, a high-temperature series expansion analysis of the temperature-dependent specific-heat and magnetization data yields $J$ 2 /$J$ 1 = 0.30 (=0.23) for YbBi 2 IO 4 (YbBi 2 ClO 4 ). In conclusion, this simplified analysis suggests strong frustration that should promote significant quantum fluctuations in these compounds, and thus motivates future work on the static and dynamic magnetic properties of these materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Spinon continuum in the Heisenberg quantum chain compound Sr 2 ⁢V 3 ⁢O 9

Magnetic excitations in the spin chain candidate Sr 2 ⁢V 3 ⁢O 9 have been investigated by inelastic neutron scattering on a single crystal sample. A spinon continuum with a bandwidth of ~22 meV is observed along the chain formed by alternating magnetic V 4+ and nonmagnetic V 5+ ions, which reveals the importance of the orbital degree of freedom in determining the chain axis as identified by prior electronic structure calculations. Incipient magnetic Bragg peaks due to weak ferromagnetic interchain couplings emerge when approaching the magnetic transition at T N ~ 5.3 K, while the excitations remain gapless within the instrumental resolution. Comparisons to the Bethe ansatz, density matrix renormalization group calculations, and effective field theories confirm Sr 2 ⁢V 3 ⁢O 9 as a host of weakly coupled S = 1/2 chains dominated by antiferromagnetic intrachain interactions of ~7.1(1) meV.

1-dimensional spin chains↗

DOE OSTI2023

Field-tuned quantum renormalization of spin dynamics in the honeycomb lattice Heisenberg antiferromagnet YbCl 3

The basis for our understanding of quantum magnetism has been the study of elegantly simple model systems. However, even for the antiferromagnetic honeycomb lattice with isotropic spin interactions–one of the simplest model systems–a detailed understanding of quantum effects is still lacking. Here, using inelastic neutron scattering measurements of the honeycomb lattice material YbCl 3 , we elucidate how quantum effects renormalize the single-magnon and multimagnon excitations and how this renormalization can be tuned and ultimately driven to the classical limit by applying a magnetic field. Additionally, our work reveals that the quantum effects tuned by the magnetic field not only renormalize the magnetic excitations but also induce a distinctive sharp feature inside the multimagnon continuum. From a more general perspective, this result demonstrates that structures within magnetic continua can occur over a wide experimental parameter space and can be used as a reliable means of identifying quantum phenomena.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Anisotropic magnetism of the Shastry-Sutherland lattice material BaNd 2 PtO 5

For this work, single crystals were grown and characterized to investigate the physical properties and magnetic ground state of BaNd 2 PtO 5 , a candidate to host physics of the Shastry-Sutherland model. Analysis of single crystal x-ray diffraction data yields an updated crystal structure, similar to the prior report but now in space group P4/mbm. Magnetization and specific heat measurements reveal an antiferromagnetic transition at T N =1.9K and a large magnetic anisotropy with in-plane magnetization much larger than out-of-plane magnetization. Single crystal neutron diffraction at zero field reveals a propagation vector of ($\frac{1}{2}$$\frac{1}{2}$$\frac{1}{2}$) for the magnetic ground state as compared to the ($\frac{1}{2}$$\frac{1}{2}$0) wave vector observed in the related body-centered material BaNd 2 ZnO 5 . The ground state is found to be a fully compensated antiferromagnet with diffraction data well fitted within the magnetic space group P S –1 (BNS setting #2.7). The ordered moment is mostly in the basal plane with nearest neighbors forming ferromagnetic dimers, however it also has a finite out-of-plane component unlike the related easy-plane materials BaNd 2 ZnO 5 and BaNd 2 ZnS 5 . Field-induced transitions are observed below T N when the field is applied within the basal plane, and in-plane anisotropy of the associated critical fields is observed. The magnetization is strongly impacted by misalignment of the field away from high symmetry directions. These results suggest complex magnetic structures may arise in the field-induced states, and they highlight the need for extreme care when studying this and related Shastry-Sutherland materials.

36 MATERIALS SCIENCE↗