Local Magnetic Moments Research Articles

High-harmonic generation in spin and charge current pumping at ferromagnetic or antiferromagnetic resonance in the presence of spin–orbit coupling

One of the cornerstone effects in spintronics is spin pumping by dynamical magnetization that is steadily precessing (around, for example, the z-axis) with frequency ω 0 due to absorption of low-power microwaves of frequency ω 0 under the resonance conditions and in the absence of any applied bias voltage. The two-decades-old ‘standard model’ of this effect, based on the scattering theory of adiabatic quantum pumping, predicts that component of spin current vector is time-independent while and oscillate harmonically in time with a single frequency ω 0 whereas pumped charge current is zero in the same adiabatic limit. Here we employ more general approaches than the ‘standard model’, namely the time-dependent nonequilibrium Green’s function (NEGF) and the Floquet NEGF, to predict unforeseen features of spin pumping: namely precessing localized magnetic moments within a ferromagnetic metal (FM) or antiferromagnetic metal (AFM), whose conduction electrons are exposed to spin–orbit coupling (SOC) of either intrinsic or proximity origin, will pump both spin and charge I(t) currents. All four of these functions harmonically oscillate in time at both even and odd integer multiples of the driving frequency ω 0. The cutoff order of such high harmonics increases with SOC strength, reaching in the one-dimensional FM or AFM models chosen for demonstration. A higher cutoff can be achieved in realistic two-dimensional (2D) FM models defined on a honeycomb lattice, and we provide a prescription of how to realize them using 2D magnets and their heterostructures.

Uncommon Magnetism in Rare-Earth Intermetallic Compounds with Strong Electronic Correlations

Rare-earth intermetallic compounds are characterised by the presence of a long-range magnetic order due to the interaction of local magnetic moments periodically located within the crystal lattice. This paper considers the possibility of forming an ordered state in cases where there is no opportunity to observe the local moment of the f-electronic shell in a traditional sense. These are, first of all, systems with a singlet ground state, as well as systems with fast spin fluctuations caused by a homogeneous intermediate-valence state of a rare-earth ion. Extensive experimental studies of these effects using neutron diffraction, neutron spectroscopy, and high-pressure studies of the magnetic phase diagram are presented and analysed, and the corresponding microscopic model representations are discussed. In particular, the possible origin of long-range magnetic order in mixed-valence compounds is analysed.

Localized magnetic moments variation for strengthening and tuning thermal expansion behavior of Mg alloys

We proposed the localized magnetic moments (LMMs) variation-induced strengthening for Mg alloys. Due to the strong interplay of magnetic and elastic, the introduction of merely 7 wt.% Mn3Ga0.7Ge0.3N in WE43 Mg alloy could not only reduce the coefficient of thermal expansion (CTE) but also enable an enhancement of ∼70% on compressive yield strength and less decrease of ∼2.1% on the elongation. We observed localized uniform lattice deformations and magnetic phase transitions for stress response and verified that the strain involves LMMs variation in the frustrated antiperovskite system, realizing simultaneously reinforcing and toughening in the compressive deformation process. Meanwhile, these magnetic transitions are accompanied by volume shrinkage and thus reduce the CTE of the WE43 Mg alloy. These results demonstrate that the introduction of the secondary phase with the variation of LMMs for stress response is an effective design strategy for alloys to achieve tunable thermal expansion behavior combined with high strength and moderate compressive deformability.

A doping strategy to achieve synergistically tuned magnetic and dielectric properties in MoSe2 for broadband electromagnetic wave absorption

Advanced electromagnetic (EM) wave absorbing materials are essential to tackle the even-increasing EM interference and pollution. Conventional methods usually combine magnetic and dielectric components for optimized impedance matching and attenuation. It is, however, challenging to simultaneously tune magnetic and dielectric properties with one wane and the other wax by adjusting the ratio between the corresponding components. Here, simultaneous modulation of both magnetic and dielectric properties has been achieved via a doping strategy in MoSe2. On the one hand, significant room-temperature ferromagnetism could be induced through the generation and coupling of local magnetic moments of Mn2+. On the other hand, Mn doping also enhances the dielectric properties by promoting the formation of amorphous and 1T phase of doped MoSe2. The synergistic magnetic and dielectric effects give rise to optimal absorption performance with a minimum reflection loss (RLmin) of −54.57 dB and a wide effective absorption bandwidth (EAB) of 8.24 GHz at 2.00 mm. Such comprehensive performance surpasses the majority of the transition metal dichalcogenide (TMD)-based composites and is the best among all the single-component TMD absorbers. Consequently, the study sheds light on synergistic modulation of EM properties in single-component materials, providing prospective solutions in the design of magnetic and dielectric devices for EM wave absorption and other fields, such as sensing, information storing, and quantum computing.

Manipulation of quantum spin transport of TM (Mn, Fe, Co, and Ni) (II) phthalocyanines coupled to carbon nanotube leads applicable in spintronic devices

We conducted a theoretical scrutiny on the quantum spin transport characteristics of a class of open-shell 3d-transition metal (TM = Mn, Fe, Co, and Ni) (II) phthalocyanines (TMPcs) molecules sandwiched between two semi-infinite armchair single-walled carbon nanotube probes. Herein, the spin density functional formalism is utilized in conjunction with state-of-the-art non-equilibrium Green's function technique. Molecular junctions composed of MnPc and FePc have higher local magnetic spin moments than CoPc and NiPc because of the strong exchange spin-splitting between the 3d-states of the Mn or Fe cation atoms and the p-states of the N anion atom. This indicates that TMPc molecules (TM = Mn, and Fe) bridged between two single-walled carbon nanotube (SWCNT) electrodes with a π-type geometry demonstrate a pronounced transmission near the Fermi level. Furthermore, the sign and robustness of spin filter efficiency are tailored by manipulating the central atom of 3d-transition metal and the contact SWCNT electrodes. Subsequently, FePc and MnPc molecular junctions have the advantage of operating practically as ideal spin filter devices compared to CoPc and NiPc molecular structures. Moreover, the tunneling magnetoresistance (TMR) of these molecular devices is assessed. For MnPc and FePc molecular junctions, the estimated total conductance exhibited a robust trend at zero bias voltage, whereas the magnitudes of spin-up and spin-dn quantum conductances increased significantly in CoPc and NiPc molecular configurations with bias voltages range between ±0.45 and ±0.95 Volt. Spin-down carriers essentially govern the quantum conductance of TMPc (Mn, Fe, Co, and Ni) molecular junctions at bias voltage around ±0.75 Volt. By examining their I-V characteristics, it has been established that MnPc, and FePc molecular junctions exhibit metal-like behavior, whereas CoPc and NiPc molecular devices are subject to negative differential resistance. As a result of our theoretical findings, it is found that TMPcs (TM = Fe, Mn) coupled to SWCNT probes yield a robust spin filter efficiency that may be of interest for prospective technological realizations in organic-inorganic molecular spintronic devices.

Comparative study between charge and spin thermoelectric figure of merits in an antiferromagnetic ring

Considering the unique and diverse characteristic features of antiferromagnetic (AFM) systems, here in our work, we explore spin-dependent thermoelectric behavior in an AFM ring geometry. A reasonably large ( ≫1 ) spin figure of merit, referred to as Z S T, is obtained under suitable input conditions. Two important prerequisites are (i) breaking the symmetry among up and down spin sub-Hamiltonians and (ii) generating different asymmetric transmission line shapes across a Fermi energy for two opposite spin electrons. Describing the physical system within a tight-binding framework, where spin-dependent scattering occurs due to the interaction of itinerant electrons with local magnetic moments via the usual spin-moment exchange interaction, we compute all the thermoelectric quantities based on Landauer integrals following the Green’s function technique. The behavior of charge figure of merit, denoted as Z C T, is also discussed along with Z S T. Though Z C T reaches above unity, it is much smaller compared to Z S T. This is the key finding of our investigation. To make the present communication a self-contained one, we compare the results with another arrangement of magnetic moments in a ring-like geometry and in a chain-like one. Our analysis gives a suitable hint that in the presence of spin-dependent scattering, much more favorable energy conversion can be substantiated.

Tunable d0 magnetism of hexagonal boron nitride introduced through an adjacent doping strategy

To meet the current requirements of diluted magnetic semiconductors (DMSs) resulting from continuous advancements in spintronics, designing d0 DMSs with high stability, spin polarization, and Curie temperature is essential. Present research on introducing d0 magnetism is limited to monatomic doping, lacking regulation measures for local magnetic moments and long range magnetic coupling. Herein, an adjacent doping strategy is employed to introduce degrees of freedom for tuning the magnetic properties of d0 DMSs. It is observed that by introducing Si and O atoms as central and adjacent dopants, respectively, the intrinsically nonmagnetic hexagonal boron nitride (h-BN) exhibits significant local magnetic moments. Furthermore, it is observed that the ionization energy, total magnetic moment, magnetic coupling, and Curie temperature of the doped h-BN are susceptible to the Si–O coordination. Subsequently, a magnetic half-metal (Si–O3-doped h-BN) with high thermal stability, 100% spin polarization, long range ferromagnetic coupling, and high Curie temperature is designed through high Si–O coordination doping. This study proposes a feasible approach for introducing tunable d0 magnetism using the design of Si–O adjacent-doped h-BN as an example.

The electronic, magnetic and optical properties of GaN monolayer doped with rare-earth elements

Structural, electronic, magnetic, and optical properties of rare earth (RE) metal element doped GaN monolayers are investigated using density functional theory, where RE = Nd, Sm, Eu, Gd, and Er. The introduction of RE atoms causes structural deformation of the GaN monolayer. The induced local magnetic moments are observed to be 4.00, 6.00, 7.00, 8.00, and 2.00 μB in Nd, Sm, Eu, Gd, and Er adsorbed GaN monolayers, respectively, while the values are 3.00, 5.00, 6.00, 7.00, and 3.00 μB in the substitution of Ga atoms. When Nd, Gd and Er adsorbed in GaN monolayer Fermi levels are shifted to the conduction band leading to metallicity, while in the case of Sm and Eu adsorption impurity states are found near the Fermi level. On the contrary, Eu, Gd and Er substitution of Ga atoms leads to the Fermi energy level being shifted below the VBM. In the Nd and Eu substitution system, the f-state is found near the Fermi level and the Nd-f state crosses the Fermi level. Optical absorption, transmission and refraction coefficients show that the addition of RE atoms can significantly enhance the prospects of GAN monolayers in visible as well as infrared applications. This research provides an outlook for the development of GaN monolayer-based optoelectronic as well as spintronics devices.

Giant Dzyaloshinskii-Moriya interaction, strong XXZ-type biquadratic coupling, and bimeronic excitations in the two-dimensional CrMnI6 magnet

Two-dimensional magnets have been discovered recently as a new class of quantum matter exhibiting a broad wealth of exotic phenomena, including notably various topological excitations rooted in emergent exchange couplings between the localized magnetic moments. By analyzing the anisotropies in the single-ion magnetization and two-body exchange couplings obtained from first-principles calculations, we reveal coexistence of both giant Dzyaloshinskii–Moriya interaction and strong anisotropic XXZ-type biquadratic coupling in a recently predicted monolayer CrMnI6 magnet. The former is induced by the spontaneous in-plane inversion symmetry breaking in the bipartite system, the latter is inherently tied to the distinct high-spin state of the Mn sublattice, while the large magnitudes of both stem from the significant spin-orbit coupling. Next, we use atomistic magnetics simulations to demonstrate the vital role of Dzyaloshinskii–Moriya interaction in harboring topological bimeronic excitations, and show that the biquadratic coupling favors a Berezinskii–Kosterlitz–Thouless-like transition as the system reduces its temperature from the paramagnetic phase. These findings substantially enrich our understanding of the microscopic couplings in 2D magnets, with appealing application potentials.

Interplay between Kondo and magnetic interactions in Pr0.75Gd0.25ScGeH

Combined experimental and density functional theory (DFT) study of Pr0.75Gd0.25ScGe and its hydride (Pr0.75Gd0.25ScGeH) reveals intricacies of composition-structure-property relationships in those distinctly layered compounds. Hydrogenation of the intermetallic parent, crystalizing in a tetragonal CeScSi-type structure, leads to an anisotropic volume expansion, that is, a(=b) lattice parameter decreases while the lattice expands along the c direction, yielding a net increase of cell volume. DFT calculations predict an antiparallel coupling of localized Gd and Pr magnetic moments in both materials at the ground state. While experiments corroborate this for the parent compound, there is no conclusive experimental proof for the hydride, where Pr moments do not order down to 3 K. DFT results also reveal that rare-earth – hydrogen interactions reduce spin-polarization of the Pr and Gd 5d and Sc 3d states at the Fermi energy, disrupt indirect exchange interactions mediated by conduction electrons, dramatically reduce the magnetic ordering temperature, and open a pseudo-gap in the majority-spin channel. Both experiments and theory show evidence of Kondo-like behavior in the hydride in the absence of an applied magnetic field, whereas increasing the field promotes magnetic ordering and suppresses Kondo-like behavior.

First-principles calculations of the electronic, optical properties and quantum capacitance of Mn doped Sc2CO2 monolayer under biaxial strain

The electronic structures and quantum capacitance of Mn atom doped Sc2CO2 under biaxial strain are investigated by first-principles calculations. The results indicate that pristine Sc2CO2 monolayer is nonmagnetic, while Mn doping makes the system have the magnetic character, which is mainly from Mn and C atoms neighboring to Mn atom. The total magnetic moment under strain ranges from 2.16 μB to 2.69 μB. The local magnetic moment of Mn atom under strains ranges from 2.59 μB to 4.33 μB, and the variation in local magnetic moment has the largest value of 1.84 μB. Spin-up band structures under strain exhibit the characteristic of the indirect semiconductor, and have the largest band gap of 0.3 eV at −2 % strain. The spin-down band structures under strain show the character of direct semiconductor and the largest band gap is 1.83 eV at 2 % strain. Sc2CO2-Mn under strains except +5 % are suitable for cathode material, while Sc2CO2-Mn with +5 % strain changes to cathode material in ionic/organic system. Effective mass, work function, and optical properties are further explored.

Modulation of the magnetization and Gilbert damping in Heusler-alloy Co3–xFexAl thin films

The saturated magnetization (mtot) and the Gilbert damping constant (α) are the two key factors that determine the critical current density of the magnetization reversal in the spin-transfer-torque magnetic memory devices. Here, this study demonstrates the efficient modulation of these two parameters by tunning the composition of the Heusler Co3–xFexAl thin films, utilizing the x-ray magnetic circular dichroism technique and ferromagnetic resonance measurements. With the increase in Fe concentration, the mtot shows a downward trend mainly resulting from the decrease in Fe local magnetic moment instead of Co. On the other hand, the ultralow α decreases from 0.004 to 0.0012. This has been attributed to the reduction in the spin–orbit coupling, which is corroborated by the decrease in the orbit-to-spin moment ratio. Our findings add a building block for the Heusler compounds with tunable Gilbert damping and appropriate magnetization and show great potential in spintronic applications.

First-principles study on the half-metallic properties of the VA group atoms adsorbed on WS2 monolayer

Adsorption of atoms on the surface of two-dimensional (2D) materials is one of the most effective ways to induce magnetic properties. In this study, the atomic structure, electronic structure, magnetic properties, and strain effects of VA group atoms (N, P, As, Sb and Bi) adsorbed on a WS2 monolayer are systematically studied using a first-principles method. After calculating the adsorption energy, it was determined that all of the VA group atoms showed a preference for being directly adsorbed above the S atoms. Based on the analysis of the orbital projection density of states and charge transfer, it appears that the group VA atoms chemisorb onto the WS2 layer. The adsorption of the VA group atoms on a WS2 monolayer will introduce 1 μB magnetic moment into the system. It is exciting that WS2 monolayer adsorbed with P, As, Sb or Bi is half-metallic with 100% spin polarization at the Fermi level. Furthermore, the magnetic properties are robust in the range of 10% strain and the magnetic moment of the system can be effectively controlled by tensile strain. In addition, when two or four atoms are adsorbed on a monolayer WS2 supercell, the adatoms show a tendency towards alignment in terms of their local magnetic moments, which may indicate a potential for ferromagnetic ordering in the system. After the adsorption of VA group atoms, monolayer WS2 exhibits structural stability, tunable magnetism under strain, 100% spin polarizability, and potential for ferromagnetism, making it a promising material for spintronic device applications.

Na3VAs2 monolayer: A two-dimensional intrinsic room-temperature ferromagnetic half-metal with large desired perpendicular magnetic anisotropy

Recent experiments have proved that the intrinsic two-dimensional (2D) magnetism has aroused strong interest in the application of advanced spintronics. However, low Curie temperature and low magnetic anisotropy energy (MAE) greatly limit their application range. By density functional theory calculation, we predict a stable 2D Na3VAs2 monolayer, which shows intrinsic ferromagnetic (FM) order and a large MAE (570 μeV per V atom). The Na3VAs2 monolayer has a local magnetic moment about 2 μB/V. Monte Carlo simulation shows that the Curie temperature (TC) of Na3VAs2 monolayer is up to 305 K based on anisotropic Heisenberg mode. More interestingly, the 2D Na3VAs2 monolayer shows an ideal half-metallic abundance, and it still maintains half-metallic characteristics under various strains. In addition, this monolayer has good dynamic, thermal, and mechanical stability. The excellent properties of the Na3VAs2 monolayer makes it great potential to be used in spintronic devices.

Electron correlation e ects in paramagnetic cobalt

We study the influence of Coulomb correlations on spectral and magnetic properties of fcc cobalt using a combination of density functional theory and dynamical mean-field theory. The computed uniform and local magnetic susceptibilities obey the Curie–Weiss law, which, as we demonstrate, occurs due to the partial formation of local magnetic moments. We find that the lifetime of these moments in cobalt is significantly less than in bcc iron, suggesting a more itinerant magnetism in cobalt. In contrast to the bcc iron, the obtained electron self-energies exhibit a quasiparticle shape with the quasiparticle mass enhancement factor m*/m ~ 1.8, corresponding to moderately correlated metal. Finally, our calculations reveal that the static magnetic susceptibility of cobalt is dominated by ferromagnetic correlations, as evidenced by its momentum dependence.

Photoelectric structure and magnetic changes caused by niobium disulfide adsorbing (non)-metal atoms under defects.

The property transition between metal and semiconductor is the key to improving the properties of transition metal dichalcogenides (TMDCs). The adsorption of the NbS2 compound in the defect state was adjusted for the first time. The hybrid system overwrites the original surface mechanism of NbS2 and induces indirect band gaps. This modulation mode makes NbS2 convert into a semiconductor and effectively improves the catalytic activity of the material in the system. In addition, the original local magnetic moment of the compound is concentrated in the vacancy region and is improved. The optical properties of the adsorption system indicate that NbS2 compounds can be effectively applied in visible and low-frequency ultraviolet regions. This provides a new idea for the design of the NbS2 compound as a two-dimensional photoelectric material. In the study, we assume that only one atom is adsorbed on the NbS2 supercell of the defect, and the distance between the two adjacent atoms exceeds 12.74 Å, so the interaction between atoms is ignored in the study. Adsorbed atoms include nonmetallic elements (H, B, C, N, O, F), metallic elements (Fe, Co), and noble metal elements (Pt, Au, Ag). The density functional theory (DFT) was used in the experiment. The non-conservative pseudopotential method was used in the calculation to optimize the crystal structure geometrically. The approximate functional is Heyd-Scuseria-Ernzerhof (HSE06). The calculation method includes the spin-orbit coupling (SOC) effect. The crystal relaxation optimization uses a 7 × 7 × 1 k point grid to calculate niobium disulfide's photoelectric and magnetic properties. A vacuum space of 15Å is introduced in the direction outside the plane, and the free boundary condition is adopted to avoid the interaction between atomic layers. For the convergence parameter setting, the interatomic force of all composite systems is less than 0.03 eV/Å, and the lattice stress is less than 0.05 Gpa.

A Structurally Flexible Halide Solid Electrolyte with High Ionic Conductivity and Air Processability

AbstractIn this work, a structurally revivable, chloride‐ion conducting solid electrolyte (SE), CsSn0.9In0.067Cl3, with a high ionic conductivity of 3.45 × 10−4 S cm−1 at 25 °C is investigated. The impedance spectroscopy, density functional theory, solid‐state 35Cl NMR, and electron paramagnetic resonance studies collectively reveal that the high Cl− ionic mobility originates in the flexibility of the structural building blocks, Sn/InCl6 octahedra. The vacancy‐dominated Cl− ion diffusion encompasses co‐ordinated Sn/In(Cl) site displacements that depend on the exact stoichiometry, and are accompanied by changes in the local magnetic moments. Owing to these promising properties, the suitability of the CsSn0.9In0.067Cl3, as an electrolyte is demonstrated by designing all‐solid‐state batteries, with different anodes and cathodes. The comparative investigation of interphases with Li, Li–In, Mg, and Ca anodes reveals different levels of reactivity and interphase formation. The CsSn0.9In0.067Cl3 demonstrates an excellent humidity tolerance (up to 50% relative humidity) in ambient air, maintaining high structural integrity without compromises in ionic conductivity, which stands in contrast to commercial halide‐based lithium conductors. The discovery of a halide perovskite conductor, with air processability and structure revival ability paves the way for the development of advanced air processable SEs, for next‐generation batteries.

Tuning the spin texture of graphene with size-specific Cu n clusters: a first-principles study

The size dependent interaction of Cu n (n = 1‒5) clusters with pristine and defective (C-vacancy) graphene is studied by employing density functional theory. The computed binding energies are in the range of ∼0.5 eV for pristine graphene and ∼3.5 eV for defective graphene, indicating a much stronger interaction in the later system. The induced spin–orbit coupling interaction, due to the proximity of the Cu n cluster, is studied with non-collinear spin-polarized simulations. The clusters cause a spin splitting in the order of few meV. The resultant low energy bands spin textures are also computed, and a spin–valley coupling in the case of even atom clusters on pristine graphene is predicted, leading to the emergence of a spin lifetime anisotropy. For defective graphene, a complete out-of-plane spin texture and a large spin splitting of 40–100 meV is obtained for Cu n (n = 1, 2, 3, 5) clusters due to local magnetic moment. On the other hand, for Cu4/defective graphene, having no net magnetic moment, the spin–valley coupling prevails close to the band edges.

Anomalous delocalization of resonant states in graphene & the vacancy magnetic moment

Carbon atom vacancies in graphene give rise to a local magnetic moment of origin, whose magnitude is yet uncertain and debated. Partial quenching of π magnetism has been ubiquitously reported in periodic first principles calculations, with magnetic moments scattered in the range 1.0–2.0 µ B, slowly converging to the lower or the upper end, depending on how the diluted limit is approached. By contrast, (ensemble) density functional theory calculations on cluster models neatly converge to the value of when increasing the system size. This stunning discrepancy has sparked a debate about the role of defect–defect interactions and self-doping, and about the importance of the self-interaction-error in the density-functional-theory description of the vacancy-induced states. Here, we settle this puzzle by showing that the problem has a fundamental, mono-electronic origin which is related to the special (periodic) arrangement of defects that results when using the slab-supercell approach. Specifically, we report the existence of resonant states that are anomalously delocalized over the lattice and that make the π midgap band unphysically dispersive, hence prone to self-doping and quenching of the π magnetism. Hybrid functionals fix the problem by widening the gap between the spin-resolved π midgap bands, without reducing their artificial widths. As a consequence, while reconciling the magnetic moment with expectations, they predict a spin-splitting which is one order of magnitude larger than found in experiments.

Chemical design of electronic and magnetic energy scales of tetravalent praseodymium materials

Lanthanides in the trivalent oxidation state are typically described using an ionic picture that leads to localized magnetic moments. The hierarchical energy scales associated with trivalent lanthanides produce desirable properties for e.g., molecular magnetism, quantum materials, and quantum transduction. Here, we show that this traditional ionic paradigm breaks down for praseodymium in the tetravalent oxidation state. Synthetic, spectroscopic, and theoretical tools deployed on several solid-state Pr4+-oxides uncover the unusual participation of 4f orbitals in bonding and the anomalous hybridization of the 4f1 configuration with ligand valence electrons, analogous to transition metals. The competition between crystal-field and spin-orbit-coupling interactions fundamentally transforms the spin-orbital magnetism of Pr4+, which departs from the Jeff = 1/2 limit and resembles that of high-valent actinides. Our results show that Pr4+ ions are in a class on their own, where the hierarchy of single-ion energy scales can be tailored to explore new correlated phenomena in quantum materials.