Engineering topics

Frontzek, Matthias D.

Publications and source records attributed to Frontzek, Matthias D..

10 records

DOE OSTI2024

Quantum critical behavior of the hyperkagome magnet Mn 3 CoSi

β -Mn-type family alloys Mn 3 T X ( T = Co , Rh, and Ir; X = Si and Ge) have a three-dimensional antiferromagnetic (AF) corner-shared triangular network, i.e., the hyperkagome lattice. The antiferromagnet Mn 3 RhSi shows magnetic short-range order over a wide temperature range of approximately 500 K above the Néel temperature T N of 190 K. In this family of compounds, as the lattice parameter decreases, the long-range magnetic ordering temperature decreases. Mn 3 CoSi has the smallest lattice parameter and the lowest T N in the family. The quantum critical point (QCP) from AF to the quantum paramagnetic state is expected near a cubic lattice parameter of 6.15 Å . Although the Néel temperature of Mn 3 CoSi is only 140 K, the emergence of the quantum critical behavior in Mn 3 CoSi is discussed. We study how the magnetic short-range order appears in Mn 3 CoSi by using neutron scattering, μ SR , and bulk characterization such as specific heat capacity. According to the results, the neutron scattering intensity of the magnetic short-range order in Mn 3 CoSi does not change much at low temperatures from that of Mn 3 RhSi , although the μ SR short-range order temperature of Mn 3 CoSi is largely suppressed to 240 K from that of Mn 3 RhSi . Correspondingly, the volume fraction of the magnetic short-range order regions, as shown by the initial asymmetry drop ratio of μ SR above T N , also becomes small. Instead, the electronic-specific heat coefficient γ of Mn 3 CoSi is the largest in this Mn 3 T Si system, possibly due to the low-energy spin fluctuation near the quantum critical point. Published by the American Physical Society 2024

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

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↗

DOE OSTI2023

Multi-$\mathcal{k}$ magnetic structure and large anomalous Hall effect in candidate magnetic Weyl semimetal NdAlGe

Here, the magnetic structure, magnetoresistance (MR), and Hall effect of the noncentrosymmetric magnetic semimetal NdAlGe are investigated, revealing an unusual magnetic state and anomalous transport properties that are associated with the electronic structure of this compound. The magnetization and MR measurements are both highly anisotropic and indicate an Ising-like magnetic system. The magnetic structure is complex in that it involves two magnetic ordering vectors, including an incommensurate spin density wave and commensurate ferrimagnetic state in zero field. We have discovered a large anomalous Hall conductivity that reaches ≈430 Ω –1 cm –1 , implying that it originates from an intrinsic Berry curvature effect stemming from Weyl nodes found in the electronic structure. These electronic structure calculations indicate the presence of nested Fermi surface pockets with nesting wave vectors like the measured magnetic ordering wave vector and the presence of Weyl nodes in proximity to the Fermi surface. We associate the incommensurate magnetic structure with the large anomalous Hall response to be the result of the combination of Fermi surface nesting and the Berry curvature associated with Weyl nodes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

ANDiE the Autonomous Neutron Diffraction Explorer

Here, we developed the Autonomous Neutron Diffraction Explorer (ANDiE) to autonomously perform neutron diffraction measurements to discover the magnetic ordering behavior in a material. Neutron diffraction is one of the few techniques that can directly probe the magnetic ordering of the atoms in a material. As such beamtime at neutron diffraction facilities is in high demand.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2023

Search for octupolar order in NpO 2 by neutron powder diffraction

The magnetic structure of neptunium dioxide (NpO 2 ) remains a mystery despite decades of research. We revisited the search for magnetic ordering in NpO 2 by performing a powder neutron diffraction experiment at low temperatures and relatively large values of Q. Diffraction data were collected for 300 MK, 15 K, and 35 K to understand the phase transition near 25 K. However, no significant changes in the neutron diffraction pattern were identified. Using the octupolar magnetic form factor j 6 , the maximum theoretical expected value of the magnetic moment (3.27 μ B ), the calculated magnetic Bragg peak intensities would not be observable in neutron powder diffraction data. This value is much larger than the previously estimated magnetic moment of ~0.1 μ B . In conclusion, supported by experimental results, these calculations suggest that unpolarized neutron powder diffraction is unsuitable for measuring octupolar ordering in NpO 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Relationship between magnetoresistance behavior and magnetic states in intercalated compounds Fe x TiS 2

Here, temperature and field-dependent neutron powder diffraction (NPD) measurements have been performed to reveal the nature of the unusual evolution of the magnetoresistance behavior with increasing Fe content in the intercalated compounds Fe x TiS 2 (with x=0.25,0.33,0.50,0.55) synthesized by solid-phase reaction method with prolonged homogenization heat treatment. As derived from neutron diffraction measurements, both the Fe 0.25 TiS 2 and the Fe 0.50 TiS 2 compound exhibit an antiferromagnetic (AFM) order below their respective Néel temperatures T N ≈52 K and T N ≈140K, which results in the presence of a large magnetoresistance accompanying the field-induced phase transition from AFM to the ferromagnetic (FM) state. At low temperatures, this AFM-FM transition is irreversible, confirmed by the irreversibility of changes in the NPD patterns and the presence of remnant magnetoresistance. In contrast, Fe 0.33 TiS 2 shows short-range magnetic order at T f ≈44 K due to a triangular network of intercalated Fe atoms and frustrations of exchange interactions with a field-induced FM alignment of Fe magnetic moments in an applied magnetic field as revealed by NPD measurements. The field-induced transformations of the cluster glass magnetic state in this compound lead to a significant decrease in electrical resistivity. According to NPD data, a reduced impact of an external magnetic field on the electrical resistivity of the compound Fe 0.55 TiS 2 can be ascribed to the presence of ferromagnetic or ferrimagnetic order in compounds with the Fe concentrations above x=0.50. The results obtained indicate that the distribution of the Fe atoms, along with their concentration in Fe x TiS 2 layered compounds, plays a decisive role in the formation of the magnetic state and the behavior of the magnetoresistance.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Selective thermal neutron transmutation doping with Gd masks in GaN semiconductors

For the first time, selective neutron transmutation doping was successfully performed on GaN through Gd masks, showing the feasibility of patternable doping through neutron irradiations. In collaborating with Oak Ridge National Laboratory, GaN with Gd masks was irradiated by directional neutrons. By using the Gd properties with high neutron absorption cross-sections, it was confirmed that the area covered by Gd masks on GaN was completely shielded from neutrons, and the uncovered area was doped with the generated Ge and 14 C. Property differences were clearly identified in various ways when comparing the area exposed by neutrons and the area unexposed by neutrons. The discoloration by the ejected protons and elastic scattering of neutrons appeared only in the exposed area while the unexposed area remained the same as before irradiation. The aspect ratio of the patterned profile of GaN was estimated by comparing patterns on the front and backside. From energy dispersive X-ray spectroscopy spectra and secondary ion mass spectrometry measurement, the concentration difference between Ge and 14 C as well as the exact concentration of produced 14 C was also checked. Further, Schottky barrier diodes on the selectively doped GaN were fabricated and investigated to study the electrical properties.

42 ENGINEERING↗

DOE OSTI2022

Quantitative texture analysis at the WAND 2 and HIDRA diffractometers

Data collection and analysis strategies have been developed for efficient and reliable crystallographic texture measurements at two recently upgraded neutron diffractometers: the Wide Angle Neutron Diffractometer Squared (WAND 2 ) and the High Intensity Diffractometer for Residual Stress Analysis (HIDRA) at the High Flux Isotope Reactor located at Oak Ridge National Laboratory. These methods are demonstrated using measurements on a variety of textured samples, including multi-phase steel composites and polycrystalline calcite (CaCO 3 ). Reference measurements were also made at VULCAN, the engineering diffractometer located at the Spallation Neutron Source. The texture data obtained on the different instruments are in agreement, and WAND 2 is more time efficient than HIDRA. Two analysis methods were investigated, single-peak fitting to obtain individual pole figures for inversion and Rietveld texture analysis using MAUD. Here, the impact of the differences between the various textures obtained was evaluated through the calculation of diffraction elastic constants, which is one application of the texture data collected. Both instruments were found to provide texture data that are suitable for complementing other analyses, such as residual stress mapping.

47 OTHER INSTRUMENTATION↗

DOE OSTI2022

Magnetic Interactions of the Centrosymmetric Skyrmion Material Gd 2 PdSi 3

The experimental realization of magnetic skyrmion crystals in centrosymmetric materials has been driven by theoretical understanding of how a delicate balance of anisotropy and frustration can stabilize topological spin structures in applied magnetic fields. Recently, the centrosymmetric material Gd 2 PdSi 3 was shown to host a field-induced skyrmion crystal, but the skyrmion stabilization mechanism remains unclear. Here, we employ neutron-scattering measurements on an isotopically enriched polycrystalline Gd 2 PdSi 3 sample to quantify the interactions that drive skyrmion formation. Our analysis reveals spatially extended interactions in triangular planes, and large ferromagnetic interplanar magnetic interactions that are modulated by the Pd/Si superstructure. The skyrmion crystal emerges from a zero-field helical magnetic order with magnetic moments perpendicular to the magnetic propagation vector, indicating that the magnetic dipolar interaction plays a significant role. To conclude, our experimental results establish an interaction space that can promote skyrmion formation, facilitating identification and design of centrosymmetric skyrmion materials.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Mechanical Activation and Cation Site Disorder in MgAl 2 O 4

The synthesis and crystallographic site occupancy were investigated for MgAl 2 O 4 with and without mechanical activation of the precursor powders. Heating to 1200 °C or higher resulted in the formation of a single spinel phase regardless of whether the powders were mechanically activated or not. Neutron diffraction analysis was used to determine cation site occupancy and revealed that mechanical activation resulted in a lower degree of cation site inversion compared to the nonactivated materials, which indicated that the powders were closer to thermodynamic equilibrium. This is the first study to characterize the effects of mechanical activation on crystallographic site occupancy in magnesium aluminate spinel using neutron diffraction.

36 MATERIALS SCIENCE↗