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At least 1,315 records · Page 73
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Integrated simulations of premagnetized and self-magnetizing dynamic screw pinch-driven MagLIF

Magnetically driven implosions such as in magnetized liner inertial fusion (MagLIF) on the Z accelerator suffer from magneto-Rayleigh–Taylor instabilities (MRTI) that dynamically redistribute liner mass during implosion, limiting fusion fuel compression and confinement, which ultimately degrades performance. Driving the implosion with an initially helical drive field that dynamically shifts the direction of the magnetic field surrounding the liner (i.e., a dynamic screw pinch, DSP) is a method proposed to mitigate MRTI in-flight and improve target performance. In DSPs, the axial drive magnetic field component implodes the liner and diffuses through the shocked, melted liner material into the fuel throughout the implosion. Liners can be designed to enable enough axial magnetic flux to diffuse through the liner material to effectively magnetize the fuel region without the need of an initial axial magnetic field (i.e., from external field coils). We present results from three-dimensional radiation-magnetohydrodynamic simulations of MagLIF implosions employing drive magnetic fields composed of axial and azimuthal components (a helical drive field). These simulated DSP-driven MagLIF targets demonstrate improved fuel conditions and thermonuclear yield compared to a traditional MagLIF target implosion. Synthetic x-ray radiography of the imploding liner material and x-ray emission images of the fuel region at the time of peak neutron yield rate indicate superior implosion morphology for DSP-MagLIF implosions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Role of Si substitution in modulating the coercivity and magnetic anisotropy in Fe3CoB2 nanocrystallites

In this work, we report the impact of Si substitution on the structural and magnetic properties of Fe3CoB2 nanocrystallites as a rare-earth-free hard-magnet candidate. X-ray diffraction confirmed the formation of the tetragonal phase with an average crystallite size of ∼40 nm, while the transmission electron microscopy image revealed nanosized crystallites embedded in an amorphous matrix. Room-temperature Mössbauer spectra exhibited two sextets and two doublets, indicating magnetically ordered Fe sites in the tetragonal phase along with high-spin paramagnetic FeIII species in disordered regions. ZFC–FC magnetization curves displayed field-dependent blocking behavior, consistent with paramagnetic Fe species. Singular point detection measurements revealed a high anisotropy field of ∼10.7 kOe in the pristine Fe3CoB2, and systematically decreased with Si substitution. Magnetization measurements revealed a monotonic decrease in saturation magnetization with Si content, while coercivity was observed to increase, likely due to microstructural refinement. These results establish Fe3CoB2 as a rare-earth-free material with strong magnetic anisotropy, ideal for hard magnets.

Abbas, Hur [Department of Physics, University of T↗
DOE OSTI2021

Neutron diffraction study of magnetism in van der Waals layered MnBi 2n Te 3n+1

Two-dimensional van der Waals MnBi 2 n Te 3 n +1 ( n = 1, 2, 3, 4) compounds have been recently found to be intrinsic magnetic topological insulators rendering quantum anomalous Hall effect and diverse topological states. Here, we summarize and compare the crystal and magnetic structures of this family, and discuss the effects of chemical composition on their magnetism. We found that a considerable fraction of Bi occupies at the Mn sites in MnBi 2 n Te 3 n +1 ( n = 1, 2, 3, 4) while there is no detectable Mn at the non-magnetic atomic sites within the resolution of neutron diffraction experiments. The occupancy of Mn monotonically decreases with the increase of n . The polarized neutron diffraction on the representative MnBi 4 Te 7 reveals that its magnetization density is exclusively accumulated at the Mn site, in good agreement with the results from the unpolarized neutron diffraction. The defects of Bi at the Mn site naturally explain the continuously reduced saturated magnetic moments from n = 1 to n = 4. The experimentally estimated critical exponents of all the compounds generally suggest a three-dimensional character of magnetism. Our work provides material-specified structural parameters that may be useful for band structure calculations to understand the observed topological surface states and for designing quantum magnetic materials through chemical doping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Multiferroicity and phase diagram of ferro-rotational magnet RbFe(SO 4 ) 2

Ferro-rotational magnet RbFe(SO 4 ) 2 has attracted attention for its stable ferro-rotational phase and electric-field-controlled magnetic chirality. This work presents the multiferroic properties and H–T phase diagram of RbFe(SO 4 ) 2 , which have been underexplored. Our measurements of magnetic susceptibility, ferroelectric polarization, and dielectric constant under various magnetic fields reveal four distinct phases: (I) a ferroelectric and helical magnetic phase below 4 K and 6 T, (II) a paraelectric and collinear magnetic phase below 4 K and above 6 T, (III) a paraelectric and non-collinear magnetic phase below 4 K and above 9 T, and (IV) a paraelectric and paramagnetic above 4 K. This study clarifies the multiferroic behavior and H–T phase diagram of RbFe(SO 4 ) 2 , providing valuable insights into ferro-rotational magnets.

ferro-rotation↗
DOE OSTI2020

Designing stellarators using perpendicular permanent magnets

We have developed a fast method to design perpendicular permanent magnets for simplifying stellarator coils based on existing codes. Coil complexity is one of the main challenges for stellarators. To date, only electromagnetic coils have been used to generate 3D fields for stellarators. Permanent magnets provide an alternative way to produce the desired magnetic field for optimized stellarators. In this paper, we revisit the concept of representing surface current using magnetic dipoles and carry out numerical validations. A surface magnetization is proven to be equivalent to the surface current that can be linearly solved by existing coil design codes. An incremental multi-layer method has been developed to obtain a practical solution that is attainable with present permanent magnets. With this method, we can reproduce a half-Tesla NCSX configuration using specially designed neodymium magnets together with simple planar coils. It shows that stellarator coils could be substantially simplified by adopting permanent magnets.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Critical structural parameter determining magnetic phases in the F⁡e 2 ⁢M⁢o 3 ⁢O 8 altermagnet system

A systematic structural investigation of F⁡e 2 ⁢M⁢o 3⁢ O 8 as a function of pressure, temperature, and magnetic field reveals that the 𝑃⁢6 3 ⁢𝑚⁢𝑐 space group remains robust over a wide range of conditions. No changes in the long-range crystal structure are observed for pressures up to 10 GPa, temperatures between 11 K and 300 K, and magnetic fields up to 9 T. The magnetostructural response, quantified by Δ⁢𝑐/𝑐, is determined for magnetic fields applied transverse to the crystallographic 𝑐 axis, demonstrating strong magnetoelastic coupling. The well-known magnetic-field-induced transition is confirmed to be first order and isostructural, occurring between two distinct altermagnetic states. Importantly, this transition can also be accessed using magnetic fields applied within the 𝑎𝑏 plane. When expressed in terms of the 𝑐/𝑎 ratio, the structural evolution under pressure (0–10 GPa) maps continuously onto that observed across the full Zn substitution range in (F⁡e 1−𝑦⁢ Z⁢n 𝑦 )2 M⁢o 3 ⁢O 8 ⁢(0 ≤ 𝑦 ≤ 1), indicating a common underlying structural control parameter. Furthermore, these results demonstrate that the 𝑐-axis lattice parameter is the primary structural degree of freedom governing magnetic behavior under external tuning parameters. More broadly, they indicate that magnetic order in this family of complex oxides (𝐴 2 ⁢M⁢o 3 ⁢O 8 , 𝐴 = Co,Mn,Ni) can be systematically tuned via pressure. This tunability highlights these materials as promising platforms for controllable magnetic switching, particularly in thin films integrated with piezoelectric substrates.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Magnetic structure and exchange interactions in the layered semiconductor CrPS 4

Compounds with two-dimensional (2D) layers of magnetic ions weakly connected by van der Waals bonding offer routes to enhance quantum behavior, stimulating both fundamental and applied interest. CrPS 4 is one such magnetic van der Waals material, however, it has undergone only limited investigation. Here we present a comprehensive series of neutron scattering measurements to determine the magnetic structure and exchange interactions. The observed magnetic excitations allow a high degree of constraint on the model parameters not normally associated with measurements on a powder sample. The results demonstrate the 2D nature of the magnetic interactions, while also revealing the importance of interactions along 1D chains within the layers. The subtle role of competing interactions is observed, which manifest in a nontrivial magnetic transition and a tunable magnetic structure in a small applied magnetic field through a spin-flop transition. Our results on the bulk compound provide insights that can be applied to an understanding of the behavior of reduced layer CrPS 4 .

36 MATERIALS SCIENCE↗
DOE OSTI2020

Detection of uncompensated magnetization at the interface of an epitaxial antiferromagnetic insulator

We have probed directly the temperature and magnetic field dependence of pinned uncompensated magnetization at the interface of antiferromagnetic FeF 2 with Cu, using FeF 2 -Cu-Co spin valves. Electrons polarized by the Co layer are scattered by the pinned uncompensated moments at the FeF 2 -Cu interface giving rise to giant magnetoresistance. We determined the direction and magnitude of the pinned uncompensated magnetization at different magnetic fields and temperatures using the angular dependencies of resistance. The strong FeF 2 anisotropy pins the uncompensated magnetization along the easy axis independent of the cooling field orientation. Most interestingly, magnetic fields as high as 90 kOe cannot break the pinning at the FeF 2 -Cu interface. This proves that the pinned interfacial magnetization is strongly coupled to the antiferromagnetic order inside the bulk FeF 2 layer. Studies as a function of FeF2 crystalline orientation show that uncompensated spins are only detected in a spin valve with (110) crystal orientation, but not in valves containing FeF 2 (100) and FeF 2 (001). This observation is in agreement with symmetry-related considerations which predict the equilibrium boundary magnetization for the FeF 2 (110) layer.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Phase diagram of CeSb 2 from magnetostriction and magnetization measurements: Evidence for ferrimagnetic and antiferromagnetic states

Cerium diantimonide (CeSb 2 ) is one of a family of rare-earth-based magnetic materials that exhibit metamagnetism, enabling control of the magnetic ground state through an applied magnetic field. At low temperatures, CeSb 2 hosts a rich phase diagram with multiple magnetically ordered phases for many of which the order parameter is only poorly understood. In this work, we report a study of its metamagnetic properties by scanning tunneling microscopy (STM) and magnetization measurements. We use STM measurements to characterize the sample magnetostriction with subpicometer resolution from magnetic field and temperature sweeps. This allows us to directly assess the bulk phase diagram as a function of field and temperature and relate spectroscopic features from tunneling spectroscopy to bulk phases. Our magnetostriction and magnetization measurements indicate that the low-temperature ground state at zero field is ferrimagnetic. Quasiparticle interference mapping shows evidence for a reconstruction of the electronic structure close to the Fermi energy upon entering the magnetically ordered phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Interplay between magnetism and superconductivity in UTe 2

Time-reversal symmetry breaking (TRSB) in UTe 2 was inferred from observations of a spontaneous Kerr response in the superconducting state after cooling in zero magnetic field, while a finite c-axis magnetic field training was further used to determine the nature of the nonunitary composite order parameter of this material. Here, we present an extensive study of the magnetic-field-trained Kerr effect, which unveils a unique critical state of pinned “ferromagnetic vortices.” We show that a remanent Kerr signal appears following the removal of a training magnetic field, which reflects the response to the TRSB order parameter and the external magnetic field through the paramagnetic susceptibility. Furthermore, this unambiguously demonstrates the importance of the magnetic fluctuations and their intimate relation to the composite order parameter. Focusing the beam on the center of the sample, we are able to accurately determine the maximum field that is screened by the critical state and the respective critical current. Measurements in the presence of magnetic field show the tendency of the superconductor to produce shielding currents that oppose the increase in vortex-induced magnetization due to the diverging paramagnetic susceptibility.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Weak itinerant magnetic phases of La 2 Ni 7

La 2 Ni 7 is an intermetallic compound that is thought to have itinerant magnetism with small moment (~0.15 µ B /Ni) ordering below 65 K. A recent study of single crystal samples by Ribeiro et. al. [Phys. Rev. B 105, 014412 (2022)] determined detailed anisotropic H-T phase diagrams and revealed three zero-field magnetic phase transitions at T 1 ~61.0 K, T 2 ~56.5 K, and T 3 ~42 K. In that study only the highest temperature phase is shown to have a clear ferromagnetic component. Here we present a single crystal neutron diffraction study determining the propagation vector and magnetic moment direction of the three magnetically ordered phases, two incommensurate and one commensurate, as a function of temperature. The higher temperature phases have similar, incommensurate propagation vectors, but with different ordered moment directions. At lower temperatures the magnetic order becomes commensurate with magnetic moments along the c direction as part of a first-order magnetic phase transition. In this study, we find that the low-temperature commensurate magnetic order is consistent with a proposal from earlier DFT calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Field-Induced Magnetic States in the Metallic Rare-Earth Layered Triangular Antiferromagnet TbAuAl 4 Ge 2

Magnetic frustration in metallic rare- earth lanthanides (Ln) with 4f electrons is crucial for producing interesting magnetic phases with high magnetic anisotropy where intertwined charge and spin degrees of freedom lead to novel phenomena. Here we report on the magnetic, thermodynamic, and electrical transport properties of TbAuAl 4 Ge 2 . Tb ions form two-dimensional triangular lattice layers which stack along the crystalline c axis. The magnetic phase diagram reveals multiple nearly degenerate ordered states upon applying field along the magnetically easy ab-plane before saturation. The magnetoresistance in this configuration exhibits intricate field dependence that closely follows that of the magnetization while the specific heat reveals a region of highly enhanced entropy, suggesting the possibility of a nontrivial spin textured phase. For fields applied along the c axis (hard axis), we find linear magnetoresistance over a wide range of fields. Further, we compare the magnetic properties and magnetoresistance with an isostructural GdAuAl 4 Ge 2 single crystal. These results identify TbAuAl 4 Ge 2 as an environment for complex quantum spin states and pave the way for further investigations of the broader LnAuAl 4 Ge 2 family of materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Effects of Co substitution on the structural and magnetic properties of Sr ( Ni 1 - x Co x ) 2 P 2

Although SrNi 2 P 2 adopts the common ThCr 2 Si 2 structure for T ≥ 325 K, being in an uncollapsed tetragonal state rather than a collapsed tetragonal version, it is a special case for the ThCr 2 Si 2 class: on cooling below 325 K it adopts a one-third collapsed orthorhombic phase where one out of every three P rows bonds across the Sr layers. On the other hand, SrCo 2 P 2 only exhibits the uncollapsed ThCr 2 Si 2 structure from room temperature down to 1.8 K. Regardless of their low-temperature structures, neither SrNi 2 P 2 nor SrCo 2 P 2 manifests magnetic transitions down to 50 mK and 2 K, respectively. In this work we report the effects of Co substitution in Sr(Ni 1 - x Co x ) 2 P 2 , which allows for tuning the transition between the one-third collapsed and the uncollapsed structure. We find a rapid decrease of the one-third collapsed structural transition temperature with increasing Co fraction, until reaching full suppression for x ≥ 0.1. Substitution levels in the range 0.11 ≤ x ≤ 0.58 show no signs of any transition down to 1.8 K in the magnetization or resistance measurements in the range 1.8 ≤ T ≤ 300 K. However, different magnetically ordered states emerge for x ≥ 0.65, and disappear for x ≥ 0.99, recovering the known paramagnetic properties of the parent compound SrCo 2 P 2 . Further, these results are summarized in a phase diagram, built upon the characterization by energy dispersive x-ray spectroscopy, x-ray diffraction, temperature-dependent resistance, and field- and temperature-dependent magnetization measurements done on single crystals with different Co fraction. Both the magnetic and structural properties are compared to other systems with ThCr 2 Si 2 structure that exhibit magnetic ordering and collapsed tetragonal transitions. The magnetic ordering and moment formation are well described by Takahashi's spin fluctuation theory of itinerant electron magnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Local atomic and magnetic structure of multiferroic (Sr,Ba)⁢(Mn,Ti)⁢O 3

Here, we present a detailed study of the local atomic and magnetic structure of the type-I multiferroic perovskite system (Sr,Ba)(Mn,Ti)O 3 using x-ray and neutron pair distribution function (PDF) analysis, polarized neutron scattering, and muon spin relaxation (μSR) techniques. The atomic PDF analysis reveals widespread nanoscale tetragonal distortions of the crystal structure even in the paraelectric phase with average cubic symmetry, corresponding to incipient ferroelectricity in the local structure. Magnetic PDF analysis, polarized neutron scattering, and μSR likewise confirm the presence of short-range antiferromagnetic correlations in the paramagnetic state, which grow in magnitude as the temperature approaches the magnetic transition. We show that these short-range magnetic correlations coincide with a reduction of the tetragonal (i.e., ferroelectric) distortion in the average structure, suggesting that short-range magnetism can play an important role in magnetoelectric and/or magnetostructural phenomena even without genuine long-range magnetic order. The reduction of the tetragonal distortion scales linearly with the local magnetic order parameter, pointing to spontaneous linear magnetoelectric coupling in this system. These findings provide greater insight into the multiferroic properties of (Sr,Ba)(Mn,Ti)O 3 and demonstrate the importance of investigating the local atomic and magnetic structure to gain a deeper understanding of the intertwined degrees of freedom in multiferroics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Pressure control of magnetic order and excitations in the pyrochlore antiferromagnet MgCr 2 ⁢O 4

MgCr 2 O 4 is one of the best-known realizations of the pyrochlore-lattice Heisenberg antiferromagnet. The strong antiferromagnetic exchange interactions are perturbed by small further-neighbor exchanges such that this compound may in principle realize a spiral spin liquid (SSL) phase in the zero-temperature limit. However, a spin Jahn-Teller transition below T N ≈ 13 K yields a complicated long-range magnetic order with multiple coexisting propagation vectors. We present neutron scattering and thermomagnetic measurements of MgCr 2 O 4 samples under applied hydrostatic pressure up to P = 1.7 GPa demonstrating the existence of multiple close-lying nearly degenerate magnetic ground states. We show that the application of hydrostatic pressure increases the ordering temperature by around 0.8 K per GPa and increases the bandwidth of the magnetic excitations by around 0.5 meV per GPa. We also evidence a strong tendency for the preferential occupation of a subset of magnetic domains under pressure. In particular, we show that the $k$ = (0,0,1) magnetic phase, which is almost negligible at ambient pressure, dramatically increases in spectral weight under pressure. This modifies the spectrum of magnetic excitations, which we interpret unambiguously as spin waves from multiple magnetic domains. Moreover, we report that the application of pressure reveals a feature in the magnetic susceptibility above the magnetostructural transition. We interpret this as the onset of a short-range ordered phase associated with $k$ = (0,0,1), previously not observed in magnetometry measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Chiral Magnetism and High-Temperature Skyrmions in B20-Ordered Co-Si

Magnets with chiral crystal structures and helical spin structures have recently attracted much attention as potential spin-electronics materials, but their relatively low magnetic-ordering temperatures are a disadvantage. While cobalt has long been recognized as an element that promotes high-temperature magnetic ordering, most Co-rich alloys are achiral and exhibit collinear rather than helimagnetic order. Crystallographically, the B20-ordered compound CoSi is an exception due to its chiral structure, but it does not exhibit any kind of magnetic order. Here, we use nonequilibrium processing to produce B20-ordered Co 1+ x Si 1₋ x with a maximum Co solubility of x = 0.043. Above a critical excess-Co content x c = 0.028 ), the alloys are magnetically ordered, and for x = 0.043, a critical temperature $T_c$ = 328 K is obtained, the highest among all B20-type magnets. The crystal structure of the alloy supports spin spirals caused by Dzyaloshinskii-Moriya interactions, and from magnetic measurements we estimate that the spirals have a periodicity of about 17 nm. In this work, our density-functional calculations explain the combination of high magnetic-ordering temperature and short periodicity in terms of a quantum phase transition where excess-cobalt spins are coupled through the host matrix.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Complex magnetic properties in the mixed 4 f - 5 d double perovskite iridates X 2 ZnIrO 6 ( X = Nd , Sm , Eu , Gd )

Here in this paper, we report on the synthesis and magnetic properties of a series of double perovskites Ln 2 ZnIrO 6 with Ln = Nd, Sm, Eu, Gd. These compounds present examples of the rare case of double perovskites (general formula A 2 BB'O6) with a magnetic 4ƒ-ion on the A site in combination with the strongly spin-orbit coupled 5d transition-metal ion Ir 4+ on the B sublattice. We discuss the impact of different rare earths on the macroscopic magnetic properties. Gd 2 ZnIrO 6 and Eu 2 ZnIrO 6 show ferrimagnetic or canted antiferromagnetic order below T N = 23 and 12 K, respectively. Sm 2 ZnIrO 6 orders antiferromagnetically at T N = 13 K. Nd 2 ZnIrO 6 exhibits more complex magnetic properties with a strong field dependence ranging from a two-step spin reorientation at μ 0 H = 0.01 T to an antiferromagnetic ground state at intermediate external fields to a spin-flop phase for 0H ≥ 4 T. This unique behavior suggests an interesting interplay between Nd 3+ and Ir 4+ . To further shed light on the relevant magnetic interaction, the zero-field magnetic ground state of Nd 2 ZnIrO 6 is examined via neutron powder diffraction. In general, the magnetic properties of Ln 2 ZnIrO 6 hint towards a substantial contribution of the rare-earth ions and a significant correlation between 5d and 4ƒ magnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Frustrated magnetism in the layered triangular lattice materials K 2 Co(SeO 3 ) 2 and Rb 2 Co(SeO 3 ) 2

A study of the layered triangular lattice materials K 2 Co(SeO 3 ) 2 and Rb 2 Co(SeO 3 ) 2 is reported here. These isostructural compounds crystallize in the trigonal space group R-3m (No. 166). The magnetic Co 2+ ions form a spin-1/2 triangular lattice and display strongly frustrated magnetism. We find no evidence in the susceptibility or heat capacity for in-plane ordering of the spins; rather the in-plane inverse susceptibility is typical of that seen for frustrated magnets. At around 10 K, a ferromagnetic component appears in the magnetic susceptibility perpendicular to the triangular planes. The heat capacity at low applied fields does not show sharp features, but in larger applied fields a sharp ferromagneticlike transition appears. Field- and direction-dependent magnetization measurements at 2 K indicate that both materials have a magnetic easy axis perpendicular to the triangular planes. The magnetic properties are sensitive to the field applied perpendicular to the triangular planes, which induces a metamagnetization transition at 0.75 T. Spin polarization develops along with the increasing out-of-plane field, resulting in a saturated ferromagnetic state at high fields. In contrast, the magnetic properties are resistant to the in-plane field. Our results suggest that these materials are embodiments of Ising spins on triangular lattices with easy-axis anisotropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗