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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Magnon-mediated qubit coupling determined via dissipation measurements

Controlled interaction between localized and delocalized solid-state spin systems offers a compelling platform for on-chip quantum information processing with quantum spintronics. Hybrid quantum systems (HQSs) of localized nitrogen-vacancy (NV) centers in diamond and delocalized magnon modes in ferrimagnets—systems with naturally commensurate energies—have recently attracted significant attention, especially for interconnecting isolated spin qubits at length-scales far beyond those set by the dipolar coupling. However, despite extensive theoretical efforts, there is a lack of experimental characterization of the magnon-mediated interaction between NV centers, which is necessary to develop such hybrid quantum architectures. Here, we experimentally determine the magnon-mediated NV–NV coupling from the magnon-induced self-energy of NV centers. Our results are quantitatively consistent with a model in which the NV center is coupled to magnons by dipolar interactions. This work provides a versatile tool to characterize HQSs in the absence of strong coupling, informing future efforts to engineer entangled solid-state systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Emergent magnetism and exchange bias effect in iron oxide nanocubes with tunable phase and size

Abstract We report a systematic investigation of the magnetic properties including the exchange bias (EB) effect in an iron oxide nanocube system with tunable phase and average size (10, 15, 24, 34, and 43 nm). X-ray diffraction and Raman spectroscopy reveal the presence of Fe 3 O 4 , FeO, and α -Fe 2 O 3 phases in the nanocubes, in which the volume fraction of each phase varies depending upon particle size. While the Fe 3 O 4 phase is dominant in all and tends to grow with increasing particle size, the FeO phase appears to coexist with the Fe 3 O 4 phase in 10, 15, and 24 nm nanocubes but disappears in 34 and 43 nm nanocubes. The nanocubes exposed to air resulted in an α -Fe 2 O 3 oxidized surface layer whose thickness scaled with particle size resulting in a shell made of α -Fe 2 O 3 phase and a core containing Fe 3 O 4 or a mixture of both Fe 3 O 4 and FeO phases. Magnetometry indicates that the nanocubes undergo Morin (of the α -Fe 2 O 3 phase) and Verwey (of the Fe 3 O 4 phase) transitions at ∼250 K and ∼120 K, respectively. For smaller nanocubes (10, 15, and 24 nm), the EB effect is observed below 200 K, of which the 15 nm nanocubes showed the most prominent EB with optimal antiferromagnetic (AFM) FeO phase. No EB is reported for larger nanocubes (34 and 43 nm). The observed EB effect is ascribed to the strong interfacial coupling between the ferrimagnetic (FiM) Fe 3 O 4 phase and AFM FeO phase, while its absence is related to the disappearance of the FeO phase. The Fe 3 O 4 / α -Fe 2 O 3 (FiM/AFM) interfaces are found to have negligible influence on the EB. Our findings shed light on the complexity of the EB effect in mixed-phase iron oxide nanosystems and pave the way to design exchange-coupled nanomaterials with desirable magnetic properties for biomedical and spintronic applications.

Physics↗
DOE OSTI2024

Magnetic tunability in tetragonal Mn–Rh–Ir–Sn inverse Heusler compounds

Abstract Gaining control over magnetic structure has been an ongoing challenge in materials that form complex, nanoscale, and non-collinear magnetic configurations. Recently, it was predicted that tuning the ratio of the Dzyaloshinskii–Moriya interaction to the uniaxial magnetic anisotropy in tetragonal inverse Heuslers through changes in composition could allow a range of interesting magnetic states to be accessed, from simple ferrimagnetism, to helical and antiskyrmionic phases. Here, we show tunability of the magnetic phase behavior in the Mn–Rh–Sn system through Ir substitution on the Rh substructure. Iridium substitution correlates to an increase in the strength of ferromagnetic exchange couplings, at the expense of antiferromagnetic exchange couplings. However, we do not observe the complex non-collinear magnetic phases proposed previously, likely due to the extremely narrow composition window where these phases are predicted to form in a bulk sample. This work highlights the sensitivity of complex magnetic structures to stoichiometry, which makes them difficult to discover empirically.

Physics↗
DOE OSTI2022

Experimental realization of linearly polarized x-ray detected ferromagnetic resonance

We present the first theoretical and experimental evidence of time-resolved dynamic x-ray magnetic linear dichroism (XMLD) measurements of GHz magnetic precessions driven by ferromagnetic resonance in both metallic and insulating thin films. Our findings show a dynamic XMLD in both ferromagnetic Ni 80 Fe 20 and ferrimagnetic Ni 0.65 Zn 0.35 Al 0.8 Fe 1.2 O 4 for different measurement geometries and linear polarizations. A detailed analysis of the observed signals reveals the importance of separating different harmonic components in the dynamic signal in order to identify the XMLD response without the influence of competing contributions. In particular, RF magnetic resonance elicits a large dynamic XMLD response at the fundamental frequency under experimental geometries with oblique x-ray polarization. The geometric range and experimental sensitivity can be improved by isolating the 2ω Fourier component of the dynamic response. These results illustrate the potential of dynamic XMLD and represent a milestone accomplishment toward the study of GHz spin dynamics in systems beyond ferromagnetic order.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Experimental and theoretical investigation of FeCrVAl and related compounds

Abstract We have carried out a combined theoretical and experimental investigation of FeCrVAl, and the effect of Mn and Co doping on its structural, magnetic, and electronic band properties. Our first principles calculations indicate that FeCrVAl, FeCr 0.5 Mn 0.5 VAl, and FeCr 0.5 Co 0.5 VAl exhibit nearly perfect spin polarization, which may be further enhanced by mechanical strain. At the same time, FeCrV 0.5 Mn 0.5 Al and FeCrV 0.5 Co 0.5 Al exhibit a relatively small value of spin polarization, making them less attractive for practical applications. Using arc melting and high vacuum annealing, we synthesized three compounds FeCrVAl, FeCr 0.5 Mn 0.5 VAl, and FeCr 0.5 Co 0.5 VAl, which are predicted to exhibit high spin polarization. The room temperature x-ray diffraction patterns of all samples are fitted with full B2 type disorder with a small amount of FeO 2 secondary phase. All samples show very small saturation magnetizations at room temperature. The thermomagnetic curves M(T) of FeCrVAl and FeCr 0.5 Co 0.5 VAl are similar to that of a paramagnetic material, whereas that of FeCr 0.5 Mn 0.5 VAl indicates ferrimagnetic behavior with the Curie temperature of 135 K. Our findings may be of interest for researchers working on Heusler compounds for spin-based electronic applications.

Physics↗
DOE OSTI2021

Magnetism and spin dynamics in room-temperature van der Waals magnet Fe 5 GeTe 2

Two-dimensional van der Waals (vdWs) materials have gathered a lot of attention recently. However, the majority of these materials have Curie temperatures that are well below room temperature, making it challenging to incorporate them into device applications. In this work, we synthesized a room-temperature vdW magnetic crystal Fe 5 GeTe 2 with a Curie temperature T c = 332 K, and studied its magnetic properties by vibrating sample magnetometry (VSM) and broadband ferromagnetic resonance (FMR) spectroscopy. The experiments were performed with external magnetic fields applied along the c -axis (H∥ c ) and the ab -plane (H∥ ab ), with temperatures ranging from 300 to 10 K. We have found a sizable Landé g -factor difference between the H∥ c and H∥ ab cases. In both cases, the Landé g -factor values deviated from g = 2. This indicates contribution of orbital angular momentum to the magnetic moment. The FMR measurements reveal that Fe 5 GeTe 2 has a damping constant comparable to Permalloy. With reducing temperature, the linewidth was broadened. Together with the VSM data, our measurements indicate that Fe 5 GeTe 2 transitions from ferromagnetic to ferrimagnetic at lower temperatures. Our experiments highlight key information regarding the magnetic state and spin scattering processes in Fe 5 GeTe 2 , which promote the understanding of magnetism in Fe 5 GeTe 2 , leading to implementations of Fe 5 GeTe 2 based room-temperature spintronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Predicted strong coupling of solid-state spins via a single magnon mode

Abstract We propose an approach to realize a hybrid quantum system composed of a diamond nitrogen-vacancy (NV) center spin coupled to a magnon mode of the low-damping, low-moment organic ferrimagnet vanadium tetracyanoethylene. We derive an analytical expression for the spin-magnon cooperativity as a function of NV position under a micron-scale perpendicularly magnetized disk, and show that, surprisingly, the cooperativity will be higher using this magnetic material than in more conventional materials with larger magnetic moments, due to in part to the reduced demagnetization field. For reasonable experimental parameters, we predict that the spin-magnon-mode coupling strength is g ∼ 2π×10 kHz. For isotopically pure 12 C diamond we predict strong coupling of an NV spin to the unoccupied magnon mode, with cooperativity C λ = 15 for a wide range of NV spin locations within the diamond, well within the spatial precision of NV center implantation. Thus our proposal describes a practical pathway for single-spin-state-to-single-magnon-occupancy transduction and for entangling NV centers over micron length scales.

Candido, Denis R. (ORCID:0000000267083280)↗
DOE OSTI2025

Interfacial alloying contributions to the magnetic proximity effect in Pt/dysprosium iron garnet

The magnetic proximity effect (MPE), a magnetic response from a heavy metal layer induced by an adjacent magnetic layer, plays an important role in the behavior of heavy-metal/ferro- or ferrimagnet thin film heterostructures. However, in the case of heavy-metal/rare earth iron garnet bilayers the presence and magnitude of the MPE has been debated. We previously reported an MPE in a Pt/dysprosium iron garnet (DyIG) heterostructure where the two layers were grown without breaking vacuum, finding that the magnetic moment present in the Pt layer scales with the magnitude of the DyIG moment and varies nonmonotonically through the DyIG compensation temperature. However, the origin of the magnetic signal from the Pt layer was not identified. In contrast, Pt/DyIG grown with a vacuum break did not exhibit an MPE. Here we investigate the origin of the magnetic moment in the Pt layer using x-ray absorption spectroscopy, x-ray circular dichroism, composition mapping and simulations of ion bombardment. In conclusion, the results are consistent with the presence of an intermixed Fe-Dy-Pt interfacial alloy layer formed during the growth of the Pt on the DyIG and resulting in a measurable MPE.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Insulating ground state and 2−𝑘 magnetic structure of candidate Weyl hydrogen-atom K2⁢Mn3⁢(AsO4)3

The ideal Weyl "hydrogen-atom" semimetal exhibits only a single pair of Weyl nodes and no other trivial states at the Fermi energy. Such a material would be a panacea in the study of Weyl quasi-particles, allowing direct unambiguous observation of their topological properties. The alluaudite-like K2⁢Mn3⁢(AsO4)3 compound was recently proposed as such a material. Here, we use comprehensive experimental work and first-principle calculations to assess this prediction. We find K2⁢Mn3⁢(AsO4)3 crystallizes in the 𝐶⁢2/𝑐 symmetry with a quasi-one-dimensional Mn sublattice, growing as small needle-like crystals. Bulk property measurements reveal magnetic transitions at ≈8 and ≈4 K, which neutron scattering experiments show correspond to two distinct magnetic orders, first a partially ordered ferrimagnetic 𝐤𝟏=(0,0,0) structure at 8 K and a second transition of 𝐤𝟐=(1,0,0) at 4 K to a fully ordered state. Below the second transition, both ordering vectors are necessary to describe the complex magnetic structure with modulated spin magnitudes. Both of the best-fit magnetic structures in this work are found to break the symmetry necessary for the generation of Weyl nodes, though one of the magnetic structures allowed by 𝐤𝟏 does preserve this symmetry. However, the crystals are optically transparent and ellipsometry measurements reveal a large band gap, undermining expectations of semimetallic behavior. Density functional theory calculations predict an insulating antiferromagnetic ground state, in contrast to previous reports, and suggest potential frustration on the magnetic sublattice. Given the wide tunability of the alluaudite structure, we consider ways to push the system closer to a semimetallic state.

Taddei, Keith M [Argonne National Laboratory]↗
DOE OSTI2025

Prototypes of Nonrelativistic Spin Splitting and Polarization in Symmetry Broken Antiferromagnets

Antiferromagnets that break both space-time reversal and translation-spin-rotation symmetries were recently predicted [L.-D. Yuan, Z. Wang, J.-W. Luo, E. I. Rashba, and A. Zunger, Phys. Rev. B 102, 014422 (2020)] to possess splitting between the otherwise spin-degenerate energy bands even without the relativistic spin-orbit coupling (SOC). Here, we point out that such nonrelativistic spin splitting (NRSS)—in particular, “spin splitting type 4” (SST-4) symmetry-broken antiferromagnets—can be divided into subgroups having distinct patterns of spin splitting and spin textures, depending on additional auxiliary symmetries of spin interconversion and polarity. These SST-4 subgroups include the 𝛼-type (no spin-interconverting symmetry) having spin splitting at the Brillouin zone center, as well as the 𝛽 subgroup in which a rotation symmetry is applied and determines the alternating spin texture and the 𝛾 subgroup having exclusively reflection spin-interconverting symmetry. Unlike ferrimagnets, the 𝛼-type compounds are shown to have tiny net magnetization at finite temperature and thus avoid the adverse effect of the stray field. The 𝛼 and 𝛽 subgroups can be either polar or nonpolar, whereas the 𝛾 subgroup is polar only, providing a basis for possible switching by external fields. The combination of NRSS-enabling and auxiliary symmetries is used here as a filter for identifying previously synthesized compounds as specific prototypes. Their characteristic splitting and spin polarization are calculated by density functional theory to the benefit of potential future experimental testing. Interesting results are as follows: (i) SOC-independent NRSS can exceed the magnitude of the SOC-induced Rashba and Dresselhaus spin splitting in semiconductors. (ii) Examples of predicted 𝛼-type insulating compounds include BiCrO 3 (nonpolar) and Mn 2 ⁢ScSbO 6 (polar), the latter having spin splitting of 158 meV and 160 meV in the valence and conduction bands, respectively. (iii) The 𝛽-type (Cu 2 ⁢Y 2 ⁢O 5 and FeF 2 ) and 𝛾-type compounds (Mn 4 ⁢Nb 2 ⁢O 9 and FeScO 3 ) are distinguished both by their auxiliary symmetries and polarity. The spin textures of 𝛾-type compounds are mirror reflected with spin degeneracy of the wave vectors on that mirror. These observations will likely broaden the experimental playing field of NRSS physics significantly.

antiferromagnets↗
DOE OSTI2022

Controlling Antiferromagnetic Magnon Polarization by Interfacial Exchange Interaction

We demonstrate highly efficient control of antiferromagnetic (AFM) magnon spins by the interfacial exchange interaction in heterostructures of ferrimagnetic yttrium iron garnet (YIG) and AFM Cr 2 O 3 . At low temperatures, Cr 2 O 3 is antiferromagnetically ordered. The interfacial exchange interaction exerted by YIG lifts the degeneracy between the AFM magnon modes in Cr 2 O 3 , resulting in a net spin polarization and a spin current dominated by left-handed magnons, even at zero magnetic field, which is detected by the spin Seebeck effect with a 5-nm-thick Pt film. In the AFM magnon-dominated region, even if the magnetic field is not sufficiently strong to induce the spin-flop transition in Cr 2 O 3 , the total spin Seebeck signal polarity flips when the YIG magnetization switches. It clearly indicates that the Cr 2 O 3 magnon polarization is controlled by YIG magnetization through the interfacial exchange interaction. The demonstration of the efficient control of AFM magnon polarization opens a pathway to manipulate AFM magnon quantum states using FM-AFM heterostructures.

42 ENGINEERING↗
DOE OSTI2020

Ab initio description of the Bi 2 Sr 2 CaCu 2 O 8+δ electronic structure

Bi-based cuprate superconductors are important materials for both fundamental research and applications. As in other cuprates, the superconducting phase in the Bi compounds lies close to an antiferromagnetic phase. Here, our density functional theory calculations based on the strongly-constrained-and-appropriately-normed exchange correlation functional in Bi 2 Sr 2 CaCu 2 O 8+δ reveal the persistence of magnetic moments on the copper ions for oxygen concentrations ranging from the pristine phase to the optimally hole-doped compound. We also find the existence of ferrimagnetic solutions in the heavily doped compounds, which are expected to suppress superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Itinerant ferromagnetism in van der Waals Fe 5 – x Ge Te 2 crystals above room temperature

Two-dimensional (2D) van der Waals (vdW) magnets have recently attracted increasing attention, as they provide a novel system for exploring 2D magnetism. However, intrinsic ferromagnetism in 2D systems has almost exclusively been observed at low temperatures, limiting their technological relevance. Fe N Ge Te 2 ( N = 3 , 4, and 5) systems are currently becoming the most attractive 2D vdW materials due to their relatively high Curie temperatures and large saturation magnetization. However, the nature of their complex yet intriguing magnetic behaviors is still unclear, in part due to the multiple inequivalent iron sites and iron vacancies. In this study, we show evolution of magnetic ordering transitions in Fe 5 – x Ge Te 2 with high Curie temperature and a strong saturation magnetization using photoemission electron microscopy and transport measurements. At 275 K, the ferromagnet transitions to a ferrimagnet, and below 110 K transitions to a state with glassy clusters. These are evidenced from temperature-dependent magnetic stripe domain evolution and anisotropic magnetoresistance measurements. Our findings show a clear magnetic ground state of Fe 5 – x Ge Te 2 at room temperature which signals that Fe 5 – x Ge Te 2 system is a very promising candidate for spintronic devices and provides a material design pathway to further increase the Curie temperature and saturation moments in vdW ferromagnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Crystal orientation dependence of interfacial magnetic anisotropy at heavy-metal/magnetic-garnet interfaces

Ferrimagnetic insulators capped with a heavy metal are becoming an increasingly interesting materials system in spintronics due to their unique ability for electrical manipulation and detection of magnetic states and spin textures via spin-orbit torques. The ability to engineer magnetic anisotropy is a powerful tool for tuning the recently discovered phenomena in these bilayers such as electrical switching or the stabilization of topological magnetic textures. Here, we observe large shifts in the magnetic anisotropy in Tm 3 Fe 5 O 13 and Y 3 Fe 5 O 12 thin films due to heavy-metal capping layers, which strongly depends on the orientation of the substrate and therefore the orientation of the epitaxial films. This work suggests large Rashba spin-orbit coupling at the metal/ferrimagneticinsulator interface, which can be engineered in spintronic devices that utilize spin-orbit torques for electrical control of the magnetization in magnetic insulators.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Effects of filling, strain, and electric field on the Néel vector in antiferromagnetic CrSb

CrSb is a layered antiferromagnet (AFM) with perpendicular magnetic anisotropy, a high Néel temperature, and large spin-orbit coupling (SOC), which makes it interesting for AFM spintronic applications. To elucidate the various mechanisms of Néel vector control, the effects of strain, band filling, and electric field on the magnetic anisotropy energy (MAE) of bulk and thin-film CrSb are determined and analyzed using density functional theory. The MAE of the bulk crystal is large (1.2 meV per unit cell). Due to the significant ionic nature of the Cr-Sb bond, finite slabs are strongly affected by end termination. Truncation of the bulk crystal to a thin film with one surface terminated with Cr and the other surface terminated with Sb breaks inversion symmetry, creates a large charge dipole and average electric field across the film, and breaks spin degeneracy, such that the thin film becomes a ferrimagnet. The MAE is reduced such that its sign can be switched with realistic strain, and the large SOC gives rise to an intrinsic voltage controlled magnetic anisotropy. A slab terminated on both faces with Cr remains a compensated AFM, but with the compensation occurring nonlocally between mirror-symmetric Cr pairs. In-plane alignment of the moments is preferred, the magnitude of the MAE remains large, similar to that of the bulk, and it is relatively insensitive to filling.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Interplay of flat electronic bands with Holstein phonons

Existing quantum Monte Carlo studies have investigated the properties of fermions on a Lieb (CuO 2 ) lattice interacting with on-site or near-neighbor electron-electron coupling. Attention has focused on the interplay of such interactions with the macroscopic degeneracy of local zero-energy modes, from which Bloch states can be formed to produce a flat band in which energy is independent of momentum. The resulting high density of states, in combination with the Stoner criterion, suggests that there should be pronounced instabilities to ordered phases. Indeed, a theorem by Lieb rigorously establishes the existence of ferrimagnetic order. Here we study the charge density wave phases induced by electron-phonon coupling on the Lieb lattice, as opposed to previous work on electron-electron interactions. Our key result is the demonstration of charge density wave phases at one third and two thirds fillings, characterized by long-range density-density correlations between doubly occupied sites on the minority or majority sublattices, and an accompanying gap. Our team also computes the transition temperature to the ordered phase as a function of the electron-phonon coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Robust long-range magnetic correlation across antiphase domain boundaries in Sr 2 CrReO 6

Here, we report a resonant elastic x-ray scattering study of a thin-film sample of Sr 2 CrReO 6 , which has one of the highest ferrimagnetic transition temperatures among ordered double perovskites.We found resonantly enhanced magnetic Bragg peaks at Q = (odd, odd, odd) at both the rhenium L 2 and L 3 edges, which coincide with the structural Bragg peaks of Sr 2 CrReO 6 . By analyzing the widths of these Bragg peaks, we extracted very different structural and magnetic correlation lengths. The former is about 15 nm, while the latter is constrained by the instrumental resolution to be at least 90 nm. We argue that a finite structural correlation length is consistent with the existence of antiphase nanodomains in our sample. On the other hand, a much larger magnetic correlation length indicates that the magnetic correlation extends far beyond the boundaries of individual domains and is consistent with strong antiferromagnetic coupling between different antiphase domains. Last, from the azimuthal dependence of the magnetic intensity, we show that the magnetic moments lie perpendicular to the c axis, which explains the earlier bulk magnetization data.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Unifying femtosecond and picosecond single-pulse magnetic switching in Gd-Fe-Co

Many questions are still open regarding the physical mechanisms behind the magnetic switching in Gd-Fe-Co alloys by single optical pulses. Phenomenological models suggest a femtosecond scale exchange relaxation between sublattice magnetization as the driving mechanism for switching. The recent observation of thermally induced switching in Gd-Fe-Co by using both several picosecond optical laser pulse as well as electric current pulses has questioned this previous understanding. This has raised the question of whether or not the same switching mechanics are acting at the femtosecond and picosecond scales. In this work, we aim at filling this gap in the understanding of the switching mechanisms behind thermal single-pulse switching. To that end, we have studied experimentally thermal single-pulse switching in Gd-Fe-Co alloys, for a wide range of system parameters, such as composition, laser power, and pulse duration. We provide a quantitative description of the switching dynamics using atomistic spin dynamics methods with excellent agreement between the model and our experiments across a wide range of parameters and timescales, ranging from femtoseconds to picoseconds. Furthermore, we find distinct element-specific damping parameters as a key ingredient for switching with long picosecond pulses and argue that switching with pulse durations as long as 15 ps is possible due to a low damping constant of Gd. Our findings can be easily extended to speed up dynamics in other contexts where ferrimagnetic Gd-Fe-Co alloys have been already demonstrated to show fast and energy-efficient processes, e.g., domain-wall motion in a track and spin-orbit torque switching in spintronics devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗