Density Of Magnetic Moments Research Articles

First-principle calculations of magnetic properties of Ho6(Fe, Mn)Bi2 compounds

The magnetic properties of Ho6MnBi2 and Ho6FeBi2 crystals are investigated by means of density functional theory. These materials are currently an active subject of research in the context of magnetic refrigeration applications since they exhibit a remarkable magnetocaloric effect. In this work, the equation of state, density of states and magnetic moments are calculated and compared with previous experimental results for these materials. Also, the Curie temperatures for the paramagnetic to ferromagnetic phase transition observed in these systems are calculated from first principles. All the calculated quantities are in reasonable agreement with experimental data, which suggests that density functional theory could provide a reliable framework to theoretically investigate the magnetic properties of intermetallic ternary compounds.Graphical abstract

Air-Stable Wafer-Scale Ferromagnetic Metallo-Carbon Nitride Monolayer.

The pursuit of robust, long-range magnetic ordering in two-dimensional (2D) materials holds immense promise for driving technological advances. However, achieving this goal remains a grand challenge due to enhanced quantum and thermal fluctuations as well as chemical instability in the 2D limit. While magnetic ordering has been realized in atomically thin flakes of transition metal chalcogenides and metal halides, these materials often suffer from air instability. In contrast, 2D carbon-based materials are stable enough, yet the challenge lies in creating a high density of local magnetic moments and controlling their long-range magnetic ordering. Here, we report a novel wafer-scale synthesis of an air-stable metallo-carbon nitride monolayer (MCN, denoted as MN4/CNx), featuring ultradense single magnetic atoms and exhibiting robust room-temperature ferromagnetism. Under low-pressure chemical vapor deposition conditions, thermal dehydrogenation and polymerization of metal phthalocyanine (MPc) on copper foil at elevated temperature generate a substantial number of nitrogen coordination sites for anchoring magnetic single atoms in monolayer MN4/CNx (where M = Fe, Co, and Ni). The incorporation of densely populating MN4 sites into monolayer MCN networks leads to robust ferromagnetism up to room temperature, enabling the observation of anomalous Hall effects with excellent chemical stability. Detailed electronic structure calculations indicate that the presence of high-density metal sites results in the emergence of spin-split d-bands near the Fermi level, causing a favorable long-range ferromagnetic exchange coupling through direct exchange interactions. Our work demonstrates a novel synthesis approach for wafer-scale MCN monolayers with robust room-temperature ferromagnetism and may shed light on practical electronic and spintronic applications.

Indirect effect of Si on the ferromagnetic transformation of Mn in FeCoMnAlSi high-entropy soft magnetic alloy

In this study, we examined the indirect impact of Si on the ferromagnetic transformation of Mn in (Fe0.8Co0.2)80-xMn10Al10Six (x = 0, 1, 2, 3, 4, & 5 at%) high-entropy soft magnetic alloys. Our findings indicate strong correlation between Si and the ferromagnetic transformation, substantiated by simulation calculations. With Si content increasing the saturation magnetization of the system exhibits a distinctive trend of an initial increase followed by a subsequent decrease. Notably, the coercivity consistently decreases with x increases. Remarkably, when x equals 3, the soft magnetic properties of the system reach an optimal state, characterized by a saturation magnetization of 186.5 emu·g−1 and a coercivity of 1.6 Oe. The density of states and magnetic moment of the system are obtained by simulation calculation. Notably, the average atomic magnetic moment of Mn (1.642 μB) surpasses that of Co (1.434 μB), suggesting the excellent soft magnetic properties inherent in the system.

Electronic, magnetic and optical properties of CoNi spinel ferrites doped by rare earth atoms: a density functional theory study

Density Functional Theory is employed to investigate how specific rare earth (RE) dopants (Pr, Y, Dy) affect the electronic, magnetic, and optical characteristics of Co0.5Ni0.5Fe2O4 (CN) spinel ferrites. The impact of RE atoms, which replace Fe ions in the supercell, is elucidated by analyzing the spin-resolved density of states, magnetic moment, and optical parameters. The findings demonstrate that both single and collective RE doping play a vital role in manipulating the key properties of these materials. Notably, we reveal that incorporating RE dopants into CN structures leads to a reduction in the energy band gap. Additionally, this work underscores substantial spin-dependent behavior and provides valuable insights into optical properties, suggesting that RE-doped CNs hold significant potential in the fields of spintronics and optoelectronics.

Enhanced longitudinal spin Seebeck effect in Au/ Y3Fe5O12

We report enhanced longitudinal spin Seebeck coefficient in Au/polycrystalline Y3Fe5O12 (YIG) system in polycrystalline pellets with higher density of surface magnetic moments and a good surface finish. The surface finish, with low artifacts, of the YIG pellet was achieved by using the oxide polishing suspension for a single long-duration of time (1 h) with controlled pressure, in addition to the normal practice of diamond polishing. The longitudinal spin Seebeck effect (LSSE) voltage (VISHE) for the Au/polycrystalline YIG system is found to vary sinusoidally as a function of external magnetic field direction (θ), and increases linearly with the applied temperature difference (ΔT). The observed values of longitudinal spin Seebeck coefficient S are 0.163 and 0.181 μV/K at ΔT = 11 and 19 K, respectively. The observed LSSE coefficient in the present study is ∼ 11.1 times of the reported value of 0.016 μV/K for Au/ polycrystalline YIG system (at ΔT = 10 K). The observed enhancement in VISHE has been ascribed to higher surface moment density and good surface quality.

The effect of different transition metal dopants on the magnetic properties of armchair antimonene, phosphorene, and their binary nanoribbons

Binary antimonene-phosphorene, a novel two-dimensional material with a medium band gap, high carrier mobility, and environmental stability, has attracted growing attention due to its wide applications in spintronics. Using density functional theory, we comprehensively investigate and compare the structural and magnetic properties of armchair Sb, P, and binary SbP nanoribbons doped with 3d transition metal atoms (Ti, Cr, Mn, Fe, Co, and Ni) with various concentrations of dopant atoms. We investigated magnetic properties, including magnetic moment, spin polarization, and spin density. It was found that the magnetic properties for single P and Sb nanoribbons do not depend on the substitution position of TM atoms. In contrast, for binary SbP nanoribbons, the substitution of TM atoms in P or Sb positions leads to different behaviors. For example, the magnetization for substitution of Fe and Co atoms in the P position of the binary SbP nanoribbon will be greater than those of the Sb position, but for other impurity atoms, the magnetization of the binary SbP nanoribbon does not strongly depend on the substitution position. The results show that with increasing the number of impurity atoms in the nanoribbon, the magnetic moment increases, except for the nickel atom in the Sb position of the SbP nanoribbon, in which the nanoribbon prefers the ferrimagnetic state. Also, the spin density results show that the magnetism of the doped systems is mainly attributed to the dopant atoms, and the neighboring atoms have a negligible contribution. The findings of antimonene-phosphorene nanoribbons doped with TM atoms may have potential applications in spintronic nanodevices.

Symmetry breaking and self-interaction correction in the chromium atom and dimer.

Density functional approximations to the exchange-correlation energy can often identify strongly correlated systems and estimate their energetics through energy-minimizing symmetry-breaking. In particular, the binding energy curve of the strongly correlated chromium dimer is described qualitatively by the local spin density approximation (LSDA) and almost quantitatively by the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA), where the symmetry breaking is antiferromagnetic for both. Here, we show that a full Perdew-Zunger self-interaction-correction (SIC) to LSDA seems to go too far by creating an unphysical symmetry-broken state, with effectively zero magnetic moment but non-zero spin density on each atom, which lies ∼4eV below the antiferromagnetic solution. A similar symmetry-breaking, observed in the atom, better corresponds to the 3d↑↑4s↑3d↓↓4s↓ configuration than to the standard 3d↑↑↑↑↑4s↑. For this new solution, the total energy of the dimer at its observed bond length is higher than that of the separated atoms. These results can be regarded as qualitative evidence that the SIC needs to be scaled down in many-electron regions.

Highly Luminescent Chiral Double π-Helical Nanoribbons.

Unveiling the mechanism behind chirality propagation and dissymmetry amplification at the molecular level is of significance for the development of chiral systems with comprehensively outstanding chiroptical performances. Herein, we have presented a straightforward Cu-mediated Ullmann homocoupling approach to synthesize perylene diimide-entwined double π-helical nanoribbons encompassing dimer, trimer, and tetramer while producing homochiral or heterochiral linking of chiral centers. A significant dissymmetry amplification was achieved, with absorption dissymmetry factors (|gabs|) increasing from 0.009 to 0.017 and further to 0.019, and luminescence dissymmetry factors (|glum|) rising from 0.007 to 0.013 and eventually to 0.015 for homochiral double π-helical oligomers. The disparity of magnetic transition dipole moment (m) densities in homochiral and heterochiral tetramers by time-dependent density functional theory calculations confirmed that homochiral oligomerization can maximize the total m, which is favorable for achieving ever-increasing g factors. Notably, these double π-helices exhibited exceptional photoluminescence quantum yields (ΦPL) ranging from 83 to 95%. The circularly polarized luminescence brightness (BCPL) eventually reached a remarkable 575 M-1 cm-1 for the homochiral tetramer, which is among the highest values reported for chiral small molecules. This kind of linearly extended double π-helices offers a platform for a comprehensive understanding of the mechanism behind chirality propagation and dissymmetry amplification.

A Magnet‐Driven Soft Bistable Actuator

AbstractBistable morphing structures are widely used as actuation mechanisms in soft actuators, soft robotics, energy absorbers, mechanical computers, optical lenses, metamaterials, and flexible electronics. However, untethered actuators, repetitive actuators, and hybrid‐assembly (containing in‐plane‐assembly and out‐of‐plane‐assembly) actuators remain challenging to realize using existing bistable structures, which hinders the novel application of such actuators in research, engineering, and daily life. This problem is solved by fabricating a magnet‐driven soft bistable actuator (MSBA) unit. The self‐buckling of the circular polydimethylsiloxane (PDMS) sheet ensures the bistability of the actuator and allows it to operate as an independent unit, free of external constraints. The reorientation of neodymium‐iron‐boron (NdFeB) microparticles embedded in the PDMS sheet enables the dome‐shaped actuators to exhibit repetitive snapping under the stimulus of a direction‐switching magnetic field. The potential of this MSBA unit in bionics, electronics, and biomechanics applications is demonstrated in systematic studies involving modification of the buckling deflection and magnetic moment density. The MSBA unit exhibits excellent performance in hybrid designs and intelligent systems.

Ground state structural, electronic and magnetic properties of Mg-doped LaMnO[formula omitted] perovskite: GGA+U study

The effect of Mg doping on the ground state structural, electronic and magnetic properties has been investigated by means of GGA+U calculations. The inspection of the structure, magnetic moment data and density plots, show that by substituting La by Mg at the La site at a concentration of 0.25, the compound exhibits a Pbnm orthorhombic structure and a half metallic ferromagnetic behaviour with degenerated Mn-eg states. The corresponding average A-site cation radius 〈rA〉 value does not allow metallic-insulator phase transition in the compound. However, by doping Mg on Mn site with the same proportion, the resulting compound shows a Pbnm orthorhombic structure and a half metallic behaviour with degeneracy lifting in Mn-eg states. Our results obtained for LaMn0.75Mg0.25MnO3 agree more with experimental data than those obtained for La0.75Mg0.25MnO3, suggesting that Mg will prefer to substitute at the Mn site due to their close ionic radii.

Giant magnetocaloric effects of MnNiSi-based high-entropy alloys near room temperature

The materials combining high-entropy alloys with good mechanical properties and magnetic materials with a significant magnetocaloric effect are among the best candidates for magnetic refrigeration. For this purpose, the high-entropy alloys (MnNiSi)1-x(FeCoGe)x (x = 0.4, 0.42, 0.45, and 0.47) were designed in this study. The effects of the Si/Ge ratio on the magnetocaloric effect and mechanical properties were also investigated. The results show that high-entropy alloy (MnNi)0.6Si0.62(FeCo)0.4Ge0.38 possesses a Vickers hardness value of 580.6HV2 and compressive strength of 267 MPa with respect to a significant entropy change of 48.5 J·kg−1K−1 under a magnetic field of 5 T near the Curie temperature of 308.8 K. In order to clearly understand the origin of this excellent comprehensive performance, the influence mechanism of crystal structure and configurational entropy were explored. Furthermore, the first-principles calculations indicated that the magnetic properties mainly originate from Mn atoms located in 3d sites and are influenced by the interaction between the Mn and Fe atoms. Additionally, the total magnetic moment and electron density are taken into account to reveal their influence on magnetocaloric properties.

A first principles study on electronic structure, magnetic and optical characteristics of Se doped CoNiFe2O4 spinel ferrites

Employing first principles calculations within the Density Functional Theory, the influence of Se dopants on electronic structure, magnetic and optical characteristics of CoNiFe2O4 (CNFO) spinel ferrites (SFs) is elucidated. The role of Se dopants replacing the O atoms in the supercell is revealed by means of spin-asymmetric density of states, magnetic moment, and optical spectra. The findings suggest that electronic structure, magnetism and optical properties are determined by both Se content and atomic arrangment due to doping sites. It is shown that the energy band gap declines with increasing Se concentration and that, based on site occupation, a half metallic characteristic can be achieved. An appreciable spin polarization and relevant data on optical characteristics demonstrate that Se doped CNFO SFs are promising candidates for prospective spin dependent applications and visible light photocatalysis.

Oxygen Nonstoichiometry as a Source of Ferromagnetism in GdCoO3: First-Principle Investigation

By using the density functional theory, we have studied the electronic structure and magnetism of the oxygen vacancy defect in GdCoO3 perovskite compound. We have carried out a magnetic stability calculation; between many possible magnetic configurations, the ferromagnetic ordering was predicted in both gadolinium and cobalt sublattices. We have demonstrated that the oxygen vacancy defect causes a ferromagnetic order via a mixture of octahedral HS Co[Formula: see text], tetrahedral HS Co[Formula: see text] and octahedral IS Co[Formula: see text]. From the spin magnetic moment and partial density of state investigation, we have predicted an induced long-range ferromagnetic ordering in nonstoichiometric GdCoO[Formula: see text].

Tunable magnetic phase transition and magnetocaloric effect in the rare-earth-free Al-Mn-Fe-Co-Cr high-entropy alloys

In the present study, the structure, magnetic phase transition, and magnetocaloric performance of Al20Mn20Fe20Co14.5+xCr25.5-x (x = 0, 1, and 2), a series of typical rare-earth-free high-entropy (HE) alloys, were experimentally determined and theoretically analyzed based on first-principle calculation. Ferromagnetic behavior was established and their Curie temperature could be tuned in a wide temperature range, which correlated with Co content; specifically, it increased from 282.8 K for Co14.5Cr25.5 to 345.8 K for Co16.5Cr23.5. In addition, the electronic density of states and magnetic moments associated with each consistent element were analyzed in the HE alloys. Maximum magnetic entropy changes of 1.135, 1.150, and 1.096 J/kgK were obtained for an x of 0, 1, and 2 under a magnetic field change of 5 T. These values are comparable or superior to those of reported 3d metal-based HE alloys. Furthermore, the experimental characterizations and theoretical predications agreed well, which provided an in-depth understanding of the rare-earth-free HE Al-Mn-Co-Fe-Cr alloy system, which is expected to be expanded to other systems in the future.

Tunable Spin Seebeck Thermopower in Nonlocal Perovskite MAPbBr3‐Based Structure

AbstractRecently organic–inorganic hybrid perovskite (OIHP) has risen suddenly to become a superstar for spintronic applications. Fundamental understanding of its spin properties is of great importance for future advances of its applications in spin‐optoelectronic devices. In this work, the authors pioneer modulation of the spin Seebeck thermopower (SSTP) in a Pt/MAPbBr3/NiFe nonlocal structure via longitudinal spin Seebeck effect measurements. They unravel the effective magnon injection into the perovskite film from the NiFe layer, and the tunability of SSTP in this structure is easily realized through the optimization of morphology and component engineering of the OIHP film. First, upon biasing samples with a magnetic flux density during the preparation process, a high‐quality OIHP film can be obtained, in which the spin scattering can be reduced leading to the SSTP enhancement. Second, by doping Cr into the OIHP lattice, the increased magnetic moment density and the additional in‐plane inverse Rashba–Edelstein effect synergistically improve spin accumulation in the Pt layer and thus tune its SSTP. Unveiling those phenomena is important to understanding magnon transport in the OIHP interlayer, which is instructive for designing magnonic devices based on OIHP film.

Tuning two-dimensional electron gas at LaAlO<sub>3</sub>/KNbO<sub>3</sub> interface by strain gradient

The superlattices composed of polar/polar perovskites have two-dimensional electron gas (2DEG) at the interface, which has broad applications in nano devices, super sensitive sensor devices, high electron mobility transistor, etc. Tuning the electronic properties of the 2DEG at the interface perovskite superlattice, such as the coupling between strain gradient and the electronic properties of the 2DEG in correlated electronic systems, is of great significance. In this paper, the properties of (LaAlO<sub>3</sub>)<sub>4.5</sub>/(KNbO<sub>3</sub>)<sub>8.5</sub> superlattice, which is composed of KNbO<sub>3</sub> and LaAlO<sub>3</sub>, are systematically investigated through first-principles density functional theory calculations. The band structure of (LaAlO<sub>3</sub>)<sub>4.5</sub>/(KNbO<sub>3</sub>)<sub>8.5</sub> superlattice exhibits 2DEG at the interface, which is consistent with the result in the literature. The band structure, density of states, magnetic moments and carrier concentration at the interface are fully investigated by using compressive gradient and tensile strain gradient, respectively. The results show that compressive strain gradient can effectively reduce the 2DEG concentration at the interface. When the compressive strain gradient coefficient reaches 12%, the 2DEG concentration decreases by 76.4%, and the interface magnetic moment disappears. The total magnetic moment of the superlattice decreases by 88.44%. When the tensile strain gradient is 12%, the electron gas concentration at the interface is increased by about 23.9%, and the interface magnetic moment is reduced by about 90.7%. At the same time, an obvious magnetic moment appears in the layer near the interface. Hence, the strain gradient can be a new approach to regulating the electron gas concentration at interface of perovskite superlattice. The tensile strain gradient increases the electron gas concentration at the interface, while the compressive strain gradient reduces the electron gas concentration. Therefore, it opens up a new way to exploring the regulation of high-performance spin polarized carrier gas at the oxide interface.

(Digital Presentation) Oxygen Evolution Reaction in Ru1-XMxO2 (M = Ti, Zr, Nb, Ta, Cr) Doped Catalysts: Activity and Stability Effects

The complex reaction mechanisms and dissolution pathways that drive oxygen evolution reaction on metal and metal oxide surfaces under acidic conditions challenge the development of a highly active, highly stable, and low cost catalyst for proton-exchange membrane (PEM) water electrolyzers. Currently, ruthenium-based materials present lower overpotentials, lower cost and higher global supply compared to iridium-based materials, though they are also considerably less stable. In order to elucidate a way to improve RuO2 stability under the harsh conditions during water electrolysis, we use a density functional theory (DFT) approach to investigate the effects on catalyst structure, OER activity and stability of transition metal substitution within Ru1-xMxO2 (M = Ti, Zr, Nb, Ta, Cr) at different atomic concentrations. Calculations show that M substitution within rutile RuO2 affects the electronic structure resulting in regions of electron accumulation and depletion at the surface and shifts the Ru d-band and O2p band centers, which are highly dependent on dopant characteristics and doped site. Moreover, dissolution calculations on the Ru1-xTixO2 surfaces show that Ti substitution alter the metal dissolution pathway energetics and thermodynamics bringing stability to the catalyst in terms of reducing material loss. Theoretical XRD patterns, Ru d-band and O p-band center calculations, and activation and reaction energy trends are in excellent agreement with experimental results that includes X-ray diffraction analysis, high resolution transmission electron microscopy images, X-ray photoelectron spectroscopy of core and valence bands, and rotating disk electrode measurements. In this presentation, we focus on the theoretical and computational aspects. The evaluation of the thermodynamics and kinetics of the water splitting, and oxygen evolution mechanism is done with spin-polarized plane-wave DFT calculations, while Ab Initio Molecular Dynamics (AIMD) simulations allow following and complementing the understanding of the dynamics of the initial steps of water dissociation on the various M and Ru sites. Electronic structure changes on the surface due to the presence of M before and during the reaction are analyzed based on the density of states and local magnetic moments, and the activation energies are obtained from the climbing Nudge Elastic Band method. Dissolution calculations are carried out with constrained-AIMD simulations using the slow-growth approach within the “Bluemoon ensemble” as implemented in VASP.

First-principles study of K, Ni and P doped superconductor AFe2As2 (A=Ba, Sr)

The characteristics of structural changes, Fe magnetic moment and electron density of K, Ni and P-doped AFe2As2 (A = Ba, Sr) were studied by first-principles calculations. After the introduction of the dopants, the Fe–As bond length and As–Fe–As angle became smaller. Although K substitution involved hole doping, Ni substitution involved electron doping, and P substitution was isovalent doping, K, Ni and P doping makes Fe and As more closely related, and the electron density near As atoms were strengthened. After doping, the distribution tendency of electron density near the As ions leads to coupling constants J1, J2 decreases eventually, which mean AFM order is suppressed. This redistribution of electron density is the inducement of superconductivity.

CO2 gas sensor based on Pt-, Ag-, Au- and Pd-doped InSe monolayer: a first-principles study

Through a first-principles study based on density functional theory, a physical model of CO2 gas molecule adsorption on an InSe monolayer was designed and built. The geometric structures of the InSe monolayer doped with different transition metal elements were optimized, and the spin-polarized energy band structure and magnetoelectric properties, such as the density of states, semimetallicity and magnetic moment of nanosheets with stable adsorbed phases, were calculated. The microscopic mechanism of these properties was analyzed by crystal field theory, and it was found that InSe monolayers are typical semiconductors, but transition metal–InSe monolayers can conduct electricity and are typical semi-metallic nanosheets. Second, the adsorption mechanism of CO2 gas molecule adsorption on InSe monolayers is studied. The calculation results show that when the O atom of the CO2 gas molecule is adsorbed on the surface of an InSe monolayer, the adsorption structure is relatively stable. The surface adsorption is mainly due to the transition of electrons on the InSe monolayer surface to the CO2 gas molecule. The doping of metal atoms gives CO2 gas molecules strong adsorption energy, promotes the magnetic properties of the adsorption system, and completes the transition from semiconductor to metal. The change of the work function of the adsorption system before and after doping indicates that this doping method can also enhance the sensitivity of the InSe monolayer to CO2 gas molecules.

Enhancing glass forming ability and magnetic properties of Co–Fe–Si–B metallic glasses by similar element substitution: Experimental and theoretical investigations

The poor glass forming ability (GFA) and magnetic properties of Co-based metallic glasses limit their applications. Herein the effects of similar element substitution of Co by Fe on the GFA and magnetic properties of Co72.5-xFexSi12.5B15 (x = 0, 4.5, 20, and 36) amorphous alloys were studied by experiments and density functional theory (DFT) calculations. The experimental results indicate that Fe substitution for Co improves the GFA of the studied metallic glasses, but in a concentration dependent manner, where Co52.5Fe20Si12.5B15 shows the best GFA. Meanwhile, the saturated mass magnetization (Ms) enhances with increasing Fe content. The atomic structures were analyzed by pair distribution function, coordination number, chemical short-range order, Voronoi polyhedron, and locally five-fold symmetry, and it was found that the highest average degree of the locally five-fold symmetry in Co52.5Fe20Si12.5B15 is a critical factor for enhancing GFA, which improves the stability of the local atomic structures by increasing the packing density and reducing the potential energy. Density of states and magnetic moments calculated for the four metallic glasses manifest that the larger magnetic moment of Fe atoms and gradual increase in the contribution of the Fe 3d state are responsible for the increase in the Ms with Fe content. This work links the GFA and magnetic properties of Co–Fe–Si–B metallic glasses with their atomic/electronic structures, providing reference for designing Co-based metallic glasses with good GFA and enhanced magnetism.