Local Magnetic Moments Research Articles

Strong Correlations and Disorder Driven Metal to Insulator Transition in High Entropy (MoVNbW)S2 Monolayer

Abstract Combining quantum confinement and high entropy in the recently synthesized two-dimensional (2D) high entropy transition metal (TM) dichalcogenides offers a platform for unique and unexplored electronic properties. Using density functional theory and tight-binding Hamiltonians, we have explored the role of disorder and strong electronic correlations on the metal-to-insulator transitions of (MoVNbW)S2 monolayer. Our GGA+U results identify that the 1H phase is a half-metal that undergoes a metal-to-insulator transition in the spin (↑) channel with a Mott gap of 0.18 eV. The strongly correlated V-3d states drive the metal-to-insulator transition by suppressing the long-range TM−TM hopping. In addition, the 1H phase exhibits strong short-range ferromagnetism via the
superexchange interactions of TM−V pairs, with localized magnetic moments on V atoms in the range of 1.3 ∼ 1.6 μB . In contrast, geometric distortion in the 1T phase drives the metal-to-insulator transition by altering the charge density, opening a band gap of 0.37 eV. The 1T phase exhibits localized magnetic moments on V atoms (1.7 ∼ 1.9 μB ) with superexchange interactions occur
only between V−V pairs. Our findings contribute to understanding the complex interplay between disorder and strong electronic correlations in 2D high entropy transition metal dichalcogenides and provide a pathway for tailoring the electronic and magnetic properties of 2D high entropy materials.

Exploring molecule-surface interactions of urania via IR spectroscopy and density functional theory.

Within the framework of surface-adsorbate interactions relevant to chemical reactions of spent nuclear fuel, the study of actinide oxide systems remains one of the most challenging tasks at both the experimental and computational levels. Consequently, our understanding of the effect of their unique electronic configurations on surface reactions lags behind that of d-block oxides. To investigate the surface properties of this system, we present the first infrared spectroscopy analysis of carbon monoxide (CO) interaction with a monocrystalline actinide oxide, UO2(111). Using a monocrystalline form largely avoids issues related to super-stoichiometries (UO2+x) and makes the experimental data suitable for further theoretical studies. Our findings reveal that CO adsorbs molecularly and shows a pronounced blue shift of the vibrational frequency to 2160 cm-1 relative to the gas-phase value. Interpreted through density functional theory (DFT) at different levels of computation, results indicate that to accurately describe the interaction between the CO molecule and the surface, it is essential to consider hybrid functionals, the non-collinearity of uranium's local magnetic moments, and spin-orbit coupling. Moreover, an intense IR absorption band at 978 cm-1 emerged upon CO adsorption, tentatively attributed to the frequency shift of the O-U-O asymmetric stretch of the UO2(111) surface in the presence of adsorbed oxygen. This new band, together with the observation of the importance of the relativistic effects in determining the nature of the chemical bonding of CO, is poised to broaden our understanding of actinide surface reactions.

On-Surface Synthesis and Characterization of Radical Spins in Kagome Graphene.

Flat bands in Kagome graphene might host strong electron correlations and frustrated magnetism upon electronic doping. However, the porous nature of Kagome graphene opens a semiconducting gap due to quantum confinement, preventing its fine-tuning by electrostatic gates. Here we induce zero-energy states into a semiconducting Kagome graphene by inserting π-radicals at selected locations. We utilize the on-surface reaction of tribromotrioxoazatriangulene molecules to synthesize carbonyl-functionalized Kagome graphene on Au(111), thereafter modified in situ by exposure to atomic hydrogen. Atomic force microscopy and tunneling spectroscopy unveil the stepwise chemical transformation of the carbonyl groups into radicals, which creates local magnetic defects of spin state S = 1/2 and zero-energy states as confirmed by density functional theory. The ability to imprint local magnetic moments opens up prospects to study the interplay between topology, magnetism, and electron correlation in Kagome graphene.

First-Principle Analysis of K2NaTiX6 (X=F, Cl and Br): Magnetic Stability and Half-Metallic Behavior

This study presents a detailed density functional theory (DFT) analysis of the double perovskite compounds K2NaTiX6 (X = F, Cl, Br). Our findings indicate that these materials are thermodynamically stable and exhibit intriguing electronic and magnetic properties. All three compounds show ferromagnetic ordering and half-metallic behavior, characterized by a spin gap in the minority spin channel. Notably, the half-metallic gap increases from F to Cl to Br, suggesting that the electronic structure can be tuned through halide substitution. The magnetic properties are largely attributed to the titanium (Ti) atom, which contributes a consistent magnetic moment of 1 μB per formula unit. To account for strong electron correlations, we applied the Hubbard U correction to the Ti d-orbitals, varying U from 1 to 4 eV. This correction led to an increase in both the half-metallic gap and the local magnetic moment on Ti, while preserving the total magnetic moment. Using the Heisenberg model, we estimated the Curie temperatures based on calculated exchange constants, providing insights into the thermal stability of the magnetic ordering. K2NaTiX₆ compounds are promising for spintronic applications due to their complete spin polarization at the Fermi level, supported by numerous studies on ferromagnetic half-metal halides.

Thermal-stimulated spin disordering accelerates water electrolysis

The thermal-stimulated paramagnetic interface is a potential route to reduce electrons scattering from local magnetic moments in anti-ferromagnetic materials.

Photoproduction of Loop Currents in Coronene Isomers Without Any Applied Magnetic Field

Applying an extended Peierls–Hubbard model to π electrons in a coronene isomer, we investigate their ground-state properties and photoinduced dynamics with particular interest in possible loop current states. Once we switch on a static magnetic field perpendicular to the coronene disk, diamagnetic (diatropic) and paramagnetic (paratropic) loop currents appear on the rim circuit and inner hub, respectively. Besides this well-known homocentric two-loop current state, heterocentric multiloop current states can be stabilized by virtue of possible electron–lattice coupling. These multiloop current states generally have a larger diamagnetic moment than the conventional two-loop one, and hence it follows that coronene, or possibly polycyclic conjugated hydrocarbons in general, may become more aromatic than otherwise with their π electrons being coupled to phonons. When we photoirradiate a ground-state coronene isomer without applying a static magnetic field, loop currents are induced in keeping with the incident light polarization. Linearly and circularly polarized lights induce heterocentric two-loop and multiloop currents, respectively, without and together with two homocentric loop currents of the conventional type, respectively. The heterocentric two-loop currents occur in a mirror-symmetric manner, which reads as the emergence of a pair of antiparallel magnetic moments, whereas the heterocentric multiloop ones appear at random in both space and time, which reads as the emergence of disordered local magnetic moments.

Compositional effect on pressure-induced polymorphism in high-entropy alloys

Recently, pressure-induced polymorphic phase transitions were discovered in several fragmented high-entropy alloys (HEAs), offering a valuable opportunity to deepen our understanding of these materials. However, the chemical and physical factors that govern these transitions are still unclear. Here, we combined in situ high-pressure synchrotron X-ray diffraction, X-ray emission spectroscopy (XES), and high-resolution transmission electron microscopy (HRTEM) to systematically study the evolution of the atomic and electronic structures in the Cantor alloy and its face-centered-cubic (fcc) subset alloys (CoCrFeMnNi, CoCrFeNi, CoCrMnNi, CoFeMnNi, CoCrNi, CoFeNi, CoMnNi, CrFeNi, and FeMnNi). Surprisingly, diverse behavior was observed among these closely related alloys during compression and decompression, which includes irreversible, reversible fcc to hexagonal close-packed (hcp) phase transitions, or even no detectable phase transitions up to ∼40 GPa. HRTEM measurements confirmed that the fcc and hcp phases abided by the classic Shoji-Nishiyama orientation relationship during the transitions. XES data indicated that high-pressure suppresses the local magnetic moments in all the studied alloys, suggesting that magnetic states do not significantly influence the polymorphic transitions. By comparing the effects of the atomic size difference, entropy, valence electron concentration, and stacking fault energy across all the compositions studied, only the stacking fault energy shows a strong correlation with the phase transitions, indicating it plays a key role in inducing polymorphism in HEAs.

Vibrational and Magnetic States of Point Defects in Bilayer MoSe2.

Defects in two-dimensional materials profoundly impact the physicochemical properties of the systems, whose characterization is highly desirable at the atomic scale. Here, using spectroscopic imaging scanning tunneling microscopy, we elucidate the vibrational and magnetic states of MoSe antisite and VMo vacancy with different charge states embedded in ultrathin MoSe2 bilayers supported on graphene substrate. Stringent vibronic states with multimode coupling are resolved on the defects. The spectral intensities are tunable with the electron tunneling rates and well-reproduced by theoretical modeling. Moreover, first-principles calculations suggest that the defects host a local magnetic moment of 2 μB in their neutral state, which is directly confirmed by our spin-flip inelastic electron tunneling spectroscopy. Our study deepens the understanding of defect properties and paves the way of defect-engineering material functionalities and spin-catalytic applications.

First-principles study and mesoscopic modeling of two-dimensional spin and orbital fluctuations in FeSe

We calculated the structural, electronic, and magnetic properties of FeSe within density-functional theory at the generalized gradient approximation level. First, we studied how the bandwidth of the d-bands at the Fermi energy is renormalized by adding simple corrections: Hubbard U, Hund's J, and by introducing long-range magnetic orders. We found that introducing either a striped or a staggered dimer antiferromagnetic order brings the bandwidths—which are starkly overestimated at the generalized gradient approximation level—closer to those experimentally observed. Second, for the ferromagnetic, the striped, checkerboard, and staggered dimer antiferromagnetic order, we investigate the change in magnetic formation energy with local magnetic moment of Fe at a pressure up to 6 GPa. The bilinear and biquadratic exchange energies are derived from the Heisenberg model and noncollinear first-principles calculations, respectively. We found a nontrivial behavior of the spin-exchange parameters on the magnetization, and we put forward a field-theory model that rationalizes these results in terms of two-dimensional spin and orbital fluctuations. The character of these fluctuations can be either that of a standard density wave or a topological vortex. Topological vortices can result in mesoscopic magnetization structures. Published by the American Physical Society 2024

DFT and Monte Carlo simulations of the intermetallic compound MnZnSb

The present work aims to investigate the structural, magnetic and electronic properties of ternary intermetallic MnZnSb using density functional theory (DFT) and Monte Carlo simulation (MCs). The obtained ground state results reveal that MnZnSb is stable in the ferromagnetic state with a metallic nature and high magnetic anisotropy. The calculated total and local magnetic moments are 2.96 μ B and 3.05 μ B for MnZnSb. Ferromagnetic behaviour was confirmed using the Stoner criterion. The elastic constants are used to verify the mechanical stability of the MnZnSb compound, demonstrating that the compound is mechanically stable and has a brittle character with low Debye temperature. The magnetic behaviour of MnZnSb is studied using MCs and the obtained results show that undergoes a transition from ferromagnetic to paramagnetic state at a critical temperature ( T C ) of 320 K. The computed magnetocaloric effect indicates that the compound exhibits large values of relative cooling power at 14 T, around 502.2 J/K.

Local magnetic moment oscillation around an Anderson impurity on graphene

Local magnetic moment oscillation around an Anderson impurity on graphene

5d transition-metal adsorption tailored valley splitting and magnetic anisotropy of Janus 2H-WSSe monolayer

Valley splitting and magnetic anisotropy of 5d TM (Hf, Ta, W, Re, Os, Ir, Pt, Au, and Hg) adsorbed Janus 2H-WSSe monolayers are investigated using first-principles calculations. The results show that 5d TM atoms adsorbed on the W-top site of Janus 2H-WSSe monolayer exhibit the lowest adsorption energy, regardless of the Se or S layer. The electronic states of Hf-SW and Re-SeW adsorbed systems cross the EF and show metallicity, while the other adsorbed systems still exhibit the semiconducting characteristics. According to the k·p model based on the effective Hamiltonian, the Zeeman effect arising from the local magnetic moment of 5d TM atoms lifts tunable valley splitting within 12.7–227 meV owing to different SOC strength. A maximum valley splitting of 227 meV is observed in the W adsorbed system, which is large enough to realize the valley Hall effect. Both perpendicular magnetic anisotropy (PMA) and in-plane magnetic anisotropy (IMA) can be adjusted by adsorbing different 5d TM atoms on the S/Se layer. Remarkably, the matrix elements differences (dx2–y2, dxy) decide the PMA and IMA behaviors, which are due to the competition of different TM-d orbitals based on the second-order perturbation theory. These results suggest that the 5d TM adsorbed Janus 2H-WSSe monolayer can be considered a good candidate for the spintronic and valleytronic devices.

First-principles calculations concerning ferromagnetism in Q-carbon

Spin-constrained first-principles calculations were performed to assess amorphous carbon systems having a density of 5.24, 5.46 or 5.63 g/cm3. The liquid quenching method was employed to produce suitable amorphous structures, and analyses of radial distribution functions, distributions of local magnetic moments and partial electronic densities of states were carried out. The system with a magnetic moment of 0.4 μB/atom and density of 5.63 g/cm3 was found to possess a high proportion (approximately 72.1 %) of sp3 hybridized carbon atoms. This result was in good agreement with recent experimental evaluations of ferromagnetic Q‑carbon. The simulations also indicated that unpaired electrons will be present in two types of sp2 hybridized carbon atoms and that these electrons are largely responsible for ferromagnetism in Q‑carbon. The present work provides an important starting point that will assist in understanding the nature of this material and promote the study of high-density Q‑carbon materials with novel physical properties.

Imaging magnetic transition of magnetite to megabar pressures using quantum sensors in diamond anvil cell

High-pressure diamond anvil cells have been widely used to create novel states of matter. Nevertheless, the lack of universal in-situ magnetic measurement techniques at megabar pressures makes it difficult to understand the underlying physics of materials’ behavior at extreme conditions, such as high-temperature superconductivity of hydrides and the formation or destruction of the local magnetic moments in magnetic systems. Here, we break through the limitations of pressure on quantum sensors by modulating the uniaxial stress along the nitrogen-vacancy axis and develop the in-situ magnetic detection technique at megabar pressures with high sensitivity (~1μT/Hz\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 1{{{\\rm{\\mu }}}}{{{\\rm{T}}}}/\\sqrt{{{{\\rm{Hz}}}}}$$\\end{document}) and sub-microscale spatial resolution. By directly imaging the magnetic field and the evolution of magnetic domains, we observe the macroscopic magnetic transition of Fe3O4 in the megabar pressure range from ferrimagnetic (α-Fe3O4) to weak ferromagnetic (β-Fe3O4) and finally to paramagnetic (γ-Fe3O4). The scenarios for magnetic changes in Fe3O4 characterized here shed light on the direct magnetic microstructure observation in bulk materials at high pressure and contribute to understanding magnetism evolution in the presence of numerous complex factors such as spin crossover, altered magnetic interactions and structural phase transitions.

Impact of composition on the properties of full-Heusler Ti2FexMn1-xAl alloys in spintronics

This study explores the structural, electronic, and magnetic characteristics of full-Heusler Ti2FexMn1-xAl alloys for spintronic applications. The regular Heusler structure is identified as the most stable across all x concentrations. The inverse Heusler structure exhibits half-metallic behavior with a finite energy band gap in the spin-up states, while the regular structure shows metallic behavior for both spin directions. Dirac-like points along the M→Γ direction are observed, particularly in alloys with x = 0 and 0.25 (inverse structure) and x = 0.5, 0.75, and 1 (regular structure), indicating advanced electronic properties. Magnetic analysis reveals that Ti atoms' local magnetic moments are antiparallel to those of Mn and Fe atoms. The total magnetic moment is highest for x = 1 (Ti2MnAl) and nearly zero for x = 0 (Ti2FeAl). Additionally, the inverse Heusler structure achieves 100 % spin polarization at the Fermi energy, underscoring its suitability for spintronic applications. This study highlights the potential of Ti2FexMn1-xAl alloys for future spintronic devices.

Molten Salt-Assisted Synthesis of Ferric Oxide/M-N-C Nanosheet Electrocatalysts for Efficient Oxygen Reduction Reaction.

Efficient, stable, and low-cost oxygen reduction catalysts are the key to the large-scale application of metal-air batteries. Herein, high-dispersive Fe2O3 nanoparticles (NPs) with abundant oxygen vacancies uniformly are anchored on lignin-derived metal-nitrogen-carbon (M-N-C) hierarchical porous nanosheets as efficient oxygen reduction reaction (ORR) catalysts (Fe2O3/M-N-C, M═Cu, Mn, W, Mo) based on a general and economical KCl molten salt-assisted method. The combination of Fe with the highly electronegative O induces charge redistribution through the Fe-O-M structure, thereby reducing the adsorption energy of oxygen-containing substances. The coupling effect of Fe2O3 NPs with M-N-C expedites the catalytic activity toward ORR by promoting proton generation on Fe2O3 and transfer to M-N-C. Experimental and theoretical calculation further revealed the remarkable electronic structure evolution of the metal site during the ORR process, where the emission density and local magnetic moment of the metal atoms change continuously throughout their reaction. The unique layered porous structure and highly active M-N4 sites resulted in the excellent ORR activity of Fe2O3/Cu-N-C with the onset potential of 0.977V, which is superior to Pt/C. This study offers a feasible strategy for the preparation of non-noble metal catalysts and provides a new comprehension of the catalytic mechanism of M-N-C catalysts.

Controlling 4f antiferromagnetic dynamics via itinerant electronic susceptibility

Optical manipulation of magnetism holds promise for future ultrafast spintronics, especially with lanthanides and their huge, localized 4f magnetic moments. These moments interact indirectly by spin polarizing the conduction electrons (the Ruderman-Kittel-Kasuya-Yosida exchange interaction), influenced by interatomic orbital overlap, and the conduction electron's susceptibility around the Fermi level. Here, we study this influence in a series of 4f antiferromagnets, GdT2Si2 (T = Co, Rh, Ir), using ultrafast resonant x-ray diffraction. We observe a twofold increase in the ultrafast intersublattice angular momentum transfer rate between the materials, originating from modifications in the conduction electron susceptibility, as confirmed by first-principles calculations. Published by the American Physical Society 2024

Observation of a V-Shape Superconductivity Evolution on Tungsten-Intercalated 2H-Type Niobium Diselenide.

van der Waals-layered niobium diselenide (NbSe2) intercalated by d-electron transition metals is an ideal test bed for the exploration of their diversiform evolution of ground states. These intercalations are mostly viewed as ordered structures aligned with periodicities of their host materials that enable control of the electronic phases via gradually changing of intercalation ratios. Here, we present the structure and superconductivity in tungsten (W)-intercalated 2H-NbSe2 crystals, which reveals an order to disorder distribution of W atoms with increasing confined intercalating amounts, leading to an approximate V-shape suppression of superconductivity. Aided by density functional theory calculations, we demonstrate that the local magnetic moment around W intercalants induced by the charge redistribution gives rise to the quick superconductivity suppression in 2H-NbSe2 below a certain dilute amount (W% = 0.06). Simultaneously, W intercalants also induce structural aberration due to aggregation effects and inhibit the generation of an ordered structure in 2H-NbSe2, resulting in a recovery of its superconductivity. The alteration of structure and electronic phases in 2H-NbSe2 via intercalation of nonmagnetic transition metals in the van der Waals gap enables the exploration of combined magnetic quantum criticality, superconductivity, and other related electronic correlations.

Modulating Optical and Magnetic Characteristics in Cu(1−x)DyxO Nanoparticles Through Doping

AbstractThis article reports the optical and magnetic properties of Cu(1−x)DyxO (x = 0.02 and 0.05) nanoparticles (NPs) synthesized via coprecipitation. An increase in Dy doping causes a red shift in the bandgap, and the X‐ray photoelectron spectroscopy reveals 2+ and 3+ valence states for both Cu and Dy ions, along with oxygen vacancies. The isothermal magnetizations fit well with a combination of linear antiferromagnetic and Langevin components. In particular, Dy‐doped CuO exhibits low‐temperature ferromagnetism driven by interactions between unpaired electrons from vacancies and Dy ions, promoting localized magnetic moments and enhancing ferromagnetic ordering.

Magnetic excitation in the hyperkagome antiferromagnet Mn3RhSi

In a paramagnetic state of a hyperkagome lattice antiferromagnet Mn3RhSi, magnetic diffuse scattering by magnetic short-range order is observed over a wide temperature range of approximately 500 K above the Néel temperature of 190 K. We have studied the magnetic excitation with a single crystal in a wide energy range from 0.3 to 140 meV by inelastic neutron scattering, where the prominent inelastic neutron-scattering signal is observed at Q=(2,0,0). The inelastic scattering intensity integrated for two of twelve magnon modes leads to a localized magnetic moment of about 5 μB per Mn site, although the long-range ordered magnetic moment is only 2.61 μB at 4 K. The result suggests that a large part of the magnetic moment does not order in Mn3RhSi, possibly due to the geometrical spin frustration of the hyperkagome lattice. Published by the American Physical Society 2024