Magnetic Moments Of Fe Atoms Research Articles

An Ab Initio Study of Vacancies in Disordered Magnetic Systems: A Case Study of Fe-Rich Fe-Al Phases.

We have performed quantum-mechanical calculations to examine the impact of disorder on thermodynamic, structural and electronic (magnetic) properties of Fe-Al systems with vacancies. A series of supercells was used and their properties were computed employing density-functional theory (DFT) as implemented in the VASP package. Our case study is primarily aimed at a disordered solid solution FeAl but we have compared our results also with those obtained for the ordered FeAl intermetallic compound for which experimental data exist in literature. Both phases are found in Fe-Al-based superalloys. The Fe-18.75at.%Al solid solution was simulated using special quasirandom structures (SQS) in three different disordered states with a different distribution of Al atoms. In particular, we have considered a general disordered case (an A2-like variant), the case without the first nearest neighbor Al-Al pairs (a B2-like distribution of atoms) and also the case without both the first and second nearest neighbor Al-Al pairs (the D0-like variant, in fact, an Fe-rich FeAl phase). The vacancy formation energies as well as the volumes of (fully relaxed) supercells with vacancies showed a large scatter for the disordered systems. The vacancy formation energies decrease with increasing concentration of Al atoms in the first coordination shell around the vacancy (an anti-correlation) for all disordered cases studied. The computed volumes of vacancies were found significantly lower (by 25–60%) when compared with the equilibrium volume of the missing atoms in their elemental states. Lastly, we have analyzed interactions between the vacancies and the Fe atoms and evaluated vacancy-induced changes in local magnetic moments of Fe atoms.

Temperature dependent magnetic properties of Dy-doped Fe16N2: Potential rare-earth-lean permanent magnet

Using the full potential linearized augmented plane wave method, we explored the magnetocrystalline anisotropy of Dy impurity doped α″-Fe16N2. Here, we considered two different Dy concentrations (1.56 and 3.125%). We also investigated the temperature dependence of the magnetic properties. The Dy site occupancy was found to be concentration dependent. The magnetic moments of Fe atoms near the Dy impurity was suppressed. However, we found a large magnetic moment in Dy atom (4.85–4.87 μB). Compared with the magnetic anisotropy constant in the pristine α″-Fe16N2 (0.57 MJ/m3), we obtained an enhanced perpendicular magnetic anisotropy. The calculated anisotropy constants in 1.56% Dy doping was 0.97 MJ/m3 while it became 1.27 MJ/m3 in 3.125% Dy doping. The Curie temperature in the pristine structure was about 820 K and it was suppressed to 765 K and 605 K in 1.56% and 3.125% Dy doping. At room temperature (300 K), the coercive field of 3.125% Dy doped system was enhanced almost 50% compared with that of the pure Fe16N2. We also found a maximum energy product of 50.9 MGOe for 3.125% Dy doping at 300 K. Our finding may suggest that the α″-Fe16N2 can be a potential permanent magnet with a lean Dy doping.

Spin Gapless Semiconductor–Nonmagnetic Semiconductor Transitions in Fe-Doped Ti2CoSi: First-Principle Calculations

Employing first-principle calculations, we investigated the influence of the impurity, Fe atom, on magnetism and electronic structures of Heusler compound Ti2CoSi, which is a spin gapless semiconductor (SGS). When the impurity, Fe atom, intervened, Ti2CoSi lost its SGS property. As TiA atoms (which locate at (0, 0, 0) site) are completely occupied by Fe, the compound converts to half-metallic ferromagnet (HMF) TiFeCoSi. During this SGS→HMF transition, the total magnetic moment linearly decreases as Fe concentration increases, following the Slate–Pauling rule well. When all Co atoms are substituted by Fe, the compound converts to nonmagnetic semiconductor Fe2TiSi. During this HMF→nonmagnetic semiconductor transition, when Fe concentration y ranges from y = 0.125 to y = 0.625, the magnetic moment of Fe atom is positive and linearly decreases, while those of impurity Fe and TiB (which locate at (0.25, 0.25, 0.25) site) are negative and linearly increase. When the impurity Fe concentration reaches up to y = 1, the magnetic moments of Ti, Fe, and Si return to zero, and the compound is a nonmagnetic semiconductor.

First principles study of the ground state magnetic properties of hypothetical B2-Fe50Al50 multilayers

The ground state structural, electronic and magnetic properties of multilayers of B2 Fe-Al stoichiometric (Fe50Al50) phase were studied by first principles calculations in the framework of the density functional theory as a function of the number of layers. From the cohesive energy point of view all, the studied multilayers are more stable in the ferromagnetic state. The calculated equilibrium lattice parameter of the B2-Fe50Al50 phase in the multilayers is smaller than that in bulk. The calculations of the magnetic properties show an enhancement of the total magnetic moment of the B2-Fe50Al50 phase on going from bulk to multilayers structures. The total and partial density of states curves indicate changes in the electronic configuration of surface Fe atoms (relative to that observed in bulk Fe50Al50) in all the studied multilayers. For Fe atoms located at sites in the inner layers, there were observed significant modifications in their electronic distribution only for multilayers with low number of bcc unit cell layers (2 and 4). The increase of the magnetic moment of the multilayers is discussed in terms of the observed enhancement of the local magnetic moment of Fe atoms.

Local Chemical Strain in PtFe Alloy Nanoparticles.

Precise deciphering of the atomic-insight chemical strain and distribution in nanosolids, especially in nanoalloys with disordered structure, is significant to correlate the structure and chemical properties. In this report, we found an abnormally enhanced negative thermal expansion (NTE) along the c axis ina 3.5 nm PtFe magnetic L10 alloy, which was usually weakened by the reduced contribution of invar behavior ina nanosized magnetic material. 57Fe Mössbauer spectra and extended X-ray absorption fine structure studies indicated the local disordered state of chemical coordination of the L10 phase. Pair distribution function and reverse Monte Carlo simulation revealed a surface A1-phase coating and atomic-level stretching chemical strain on the heteroatomic pair along the c axis. Local tensile chemical strain and increasing magnetic moments of Fe atoms on the surface of the L10 core made a dominant contribution to the enhanced NTE behavior. The present work brings an atomic reveal for the local chemical strain in bimetallic nanomaterials and provides an innovative perspective to tailoring the functional properties according to the local chemical environment.

Effect of Fe and Ti Substitution Doping on Magnetic Property of Monolayer CrSi2: a First-Principle Investigation

First-principle calculations based on spin-polarized density functional theory were performed to investigate the effect of Fe and Ti substitution doping on magnetic property of monolayer CrSi2. The electronic structures, binding energy, magnetic property, total and partial density of states, and spin density of monolayer CrSi2 are scientifically studied. Calculated binding energy reveals that Fe-doped monolayer CrSi2 is more stable than Ti-doped monolayer CrSi2. The local magnetic moment of Fe and Ti atom all decrease compared with atomic moment in free gas phase due to variation of bond interaction and charge transfer. The density of states and spin-density results indicated that local magnetic moment of Fe atom is larger than Ti atom, leading to total magnetic moment of Fe-doped monolayer CrSi2 is bigger than Ti-doped monolayer CrSi2.

Tuning nitrogen reduction reaction activity via controllable Fe magnetic moment: A computational study of single Fe atom supported on defective graphene

The design and development of low-cost and highly efficient electrocatalysts for nitrogen reduction reaction is crucial for NH3 synthesis. Herein, by means of comprehensive density functional theory (DFT) calculations and thermodynamic analysis, the catalytic mechanisms of N2 to NH3 conversion on single Fe atom embedded in single vacancy and N-doped graphene are systematically investigated. Significantly, the magnetic moment of Fe atom varies with the coordination of neighboring N atom. An approximately linear scaling is found between the adsorption energy of N atom (ΔEN*) and Fe magnetic moment. The increased magnetic moment of Fe atom can strengthen the ΔEN*, and promote the charge transfer between the N2 molecule and substrates, resulting in a substantially lower overpotential for N2 electroreduction. Our results highlight an important role of Fe magnetism in N2 activation, which offers a novel strategy for inspiring the rational design of highly efficient carbon-based materials for N2 fixation and conversion.

Local probe of irradiation-induced structural changes and orbital magnetism in Fe60Al40 thin films via an order-disorder phase transition

Hard x-ray absorption and magnetic circular dichroism spectroscopy have been applied to study the consequential changes of the local environment around Fe atoms and their orbital polarizations in 40 nm thick ${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$ thin films along the order-disorder $(B2\ensuremath{\rightarrow}A2)$ phase transition initiated by 20-keV ${\mathrm{Ne}}^{+}$ ion irradiation with fluences of $(0.75--6)\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{4pt}{0ex}}\text{ions}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. The analysis of the extended x-ray absorption fine structure spectra measured at the Fe K edge at room temperature revealed an increased number of Fe-Fe nearest neighbors from 3.47(7) to 5.0(1) and $\ensuremath{\sim}1%$ of volume expansion through the transition. The visualization of the Fe and Al nearest-neighbor rearrangement in the first coordination shell of Fe absorbers via the transition was carried out by wavelet transformations. The obtained changes in Fe coordination are evidently reflected in the x-ray magnetic circular dichroism spectra which show an increased orbital magnetic moment of Fe atoms and a pronounced magnetic multielectronic excitations peak at $\ensuremath{\sim}60$ eV above the edge. The amplitudes of both peaks demonstrated similar dependencies on the irradiation fluence. The results of self-consistent density functional calculations on relaxed ${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$ model structures for the ordered $(B2)$ and the disordered $(A2)$ phases are consistent with the experimental findings and point to the formation of Fe-rich regions in the films studied.

Revealing the novel fracture mechanism of the interfaces of TiB2/Fe composite from a first principles investigation

We investigate the atomic structures, chemical bonding, stability and fracture mechanism of B- and Ti-terminated incoherent TiB2 (0001)/Fe (111) and semi-coherent TiB2 (0001)/Fe (100) interfaces using first-principles calculations. It is found that all Ti-terminated interfaces (Ti-HCP, Ti-MT and Ti-OT) as well as B-HCP type TiB2 (0001)/Fe (100) interface are non-diffusive type. Meanwhile, B-HCP, B-MT and B-OT configurations of TiB2 (0001)/Fe (111) interface are diffusive type due to the formation of additional FexB intermetallic compound at the original Fe/TiB2 interface. The calculated works of adhesion and interfacial energies indicate that Ti-HCP and B-HCP are the most stable structures for both incoherent and semi-coherent interfaces. We find that the magnitude of interfacial elastic energy is comparable to that of the chemical energy for the semi-coherent TiB2 (0001)/Fe (100) interface. The electronic structures of TiB2/Fe interfaces reveal the formation of Fe-B, Ti-B and Fe-Ti bonds at or next to the interface. Using Griffith's theory, it is predicted that the mechanical failure of TiB2/Fe composite would initiate at the interface between TiB2 and Fe. The first principles tensile experiment performed on all Ti-HCP interfaces agrees with the prediction. In the case of B-HCP interfaces, due to the formation of either FexB diffusive layer or strong covalent Fe-B bonds, the mechanical failure eventually occurs in the Fe slab rather than that predicted by Griffith's theory. We also find the formation of diffusive FexB layer could significantly suppress the local magnetic moment of Fe atom at TiB2/Fe interface due the formation of strong covalent Fe-B bond.

Enhancement of the Magnetic Anisotropy in Rare-Earth-Free Multilayer Fe16N2/Ag/Fe16N2 and Fe16N2/Au/Fe16N2

Using the first principles method, we investigated the interface effect on the electronic structure and the magnetic properties of multilayer Fe16N2/Ag/Fe16N2 and Fe16N2/Au/Fe16N2. The thicknesses of Ag (100) and Au (100) were fixed to three monolayers, and the lattice mismatch was about 1%. The magnetic moment of Fe atoms at the interface was suppressed due to hybridization with non-magnetic Ag and Au atoms. Due to this reduction in the magnetic moments and also because of the non-magnetic volume of the Ag and Au layer, an overall 40% suppression of the magnetization was found in both systems. The hybridization between interface Ag (Au) and Fe atoms and the spin-orbit coupling associated with Ag (Au) atoms mainly contributed to the enhancement of the magnetocrystalline anisotropy. The magnetocrystalline anisotropy constant was enhanced from 0.57 MJ/m3 in pure Fe16N2 to 1.58 MJ/m3 and 0.89 MJ/m3 in Fe16N2/Ag/Fe16N2 and Fe16N2/Au/Fe16N2 multilayer systems, respectively. This enhancement in magnetocrystalline anisotropy results in an enhancement of the coercive field. The coercive fields were about 30 and 16.7 kOe in the Ag and the Au multilayer systems, respectively. Overall, we found substantial enhancements in the magnetocrystalline anisotropy constant and the coercive field due to the interface effect. This finding may suggest that the Fe16N2/Ag/Fe16N2 and the Fe16N2/Au/Fe16N2 structures can be utilized for potential rare-earth-free permanent magnets.

Composition-dependent properties and phase stability of Fe-Pd ferromagnetic shape memory alloys: A first-principles study

The composition-dependent properties and their correlation with the phase stability of Fe75+xPd25−x (−10.0≤x≤10.0) alloys are systematically investigated by using first-principles exact muffin-tin orbitals (EMTO)-coherent potential approximation (CPA) calculations. It is shown that the martensitic transformation (MT) from L12 to body-centered-tetragonal (bct) occurs in the ordered alloys with about −5.0≤x≤10.0. In both the L12 and bct phases, the evaluated a and c/a agree well with the available experimental data; the average magnetic moment per atom increases whereas the local magnetic moments of Fe atoms, dependent on both their positions and the structure of the alloy, decrease with increasing x. The tetragonal shear elastic constant of the L12 phase (C′) decreases whereas that of the bct phase (Cs) increases with x. The tetragonality of the martensite (|1−c/a|) increases whereas its energy relative to the austenite with a negative value decreases with Fe addition. All these effects account for the increase of MT temperature (TM) with x. The MT from L12 to bct is finally confirmed originating from the splitting of Fe 3d Eg and T2g bands upon tetragonal distortion due to the Jahn-Teller effect.

Magnetic behavior of novel alloyed L10-phase Co1-xFexPt nanoparticles

In this work, alloying of CoPt and FePt nanoparticles (NPs), i.e. the Co1-xFexPt NPs (x = 0, 0.25, 0.5, 0.75, 1), were synthesized by the polyol process. These as-synthesized NPs show the A1 phase with a particle size less than 5 nm. After annealing at 700 °C, the A1 (cubic) phase was transformed to L10 (tetragonal) phase in all samples. The lattice parameters varied as a function of the composition. The particle size grew larger after annealing and the size distribution was wide ranging from <10 nm to >100 nm. The size and distribution was however independent of the Co(Fe) concentration. X-ray absorption spectroscopy indicated that there was a random distribution of Co and Fe atoms in the layered structure. Magnetic measurements of the annealed NPs showed that the magnetic hysteresis loop depends on the composition. The coercivity (Hc) was very high for the CoPt and FePt NPs, whereas the Ms value was maximized for the Co0.5Fe0.5Pt NPs. The variation of Hc was attributed to the change in lattice parameters which could alter the exchange interaction, and thus the magnetocrystalline anisotropy. On the other hand, higher polarization and increased magnetic moments of Fe atoms were believed to be the reason for the enhanced Ms in the Co(Fe)Pt NPs. In addition, all NPs were magnetically stable against temperature variation with changes in Ms of less than 10%. The Curie temperature was expected to be as high as 800–900 K. Given these properties, these new forms of magnetic nanoparticles may find use in advanced magnetic recording technology.

Electronic and magnetic properties of Zn1−xFexSe alloys

The spin polarized density functional theory along with self consistent plane wave pseudopotential method is used to investigate electronic and magnetic properties of ternary Zn1−xFexSe alloys with x = 0.125, 0.25, 0.5 and 0.75. The exchange-correlation potential treated within generalized gradient approximation is used. The calculated spin-polarized band structures, partial and total density of states reveal that Zn0.875Fe0.125Se and Zn0.75Fe0.25Se exhibit half metallic ferromagnetic characteristics and Zn0.50Fe0.50Se is nearly half metallic in nature. The half metallic band gaps for x = 0.125 and 0.25 are 0.69 and 0.39eV respectively, while the corresponding band gaps are 0.86 and 0.81eV. The p-d hybridization reduces the magnetic moment of Fe atoms from its free space charge value of 4 μB and induces the small magnetic moments on Zn and Se sites. The results are compared with available experimental data.

Magnetism and energetics for vacancy and helium impurity in Fe-9Cr alloy: A first-principles study

Under the extreme fusion environment, helium impurities and vacancies play an important role in the microstructure evolution in Fe-9Cr based (wt.%) reduced activation ferritic/martensitic (RAFM) steels. In this work, vacancy and helium (He) impurity formation and related magnetism change in Fe-9Cr alloys are investigated by first-principles calculations. In Fe-9Cr alloys, Cr atom has an antiferromagnetic moment of around −1.85µB, as compared to that of Fe atoms in matrix. Contrary to the case for the first nearest neighboring (1nn) and 2nn Cr atom around Fe atom, 3–5nn Cr atoms increase the magnetic moment of Fe atom. The formation energy of vacancy and He impurities at various sites in Fe-9Cr are discussed. In Fe-9Cr alloys, He atoms at octahedral interstitial sites (O-sites) are unstable and eventually move to tetrahedral interstitial sites (T-sites) upon relaxation. The formation of defects can strongly affect the magnetic moment. T-site He atom leads to an obvious magnetic moment of 1nn Cr decrease. The electronic structure and local strain field analyses are performed to understand the origin of energy difference of He at different interstitial sites in Fe-9Cr alloys. The present theoretical results are helpful for a better understanding of the behavior of vacancy, He impurity and their complexes in Fe-9Cr alloys.

Thickness-dependent electronic and magnetic properties of γ′−Fe4N atomic layers on Cu(001)

Growth, electronic and magnetic properties of $\gamma'$-Fe$_{4}$N atomic layers on Cu(001) are studied by scanning tunneling microscopy/spectroscopy and x-ray absorption spectroscopy/magnetic circular dichroism. A continuous film of ordered trilayer $\gamma'$-Fe$_{4}$N is obtained by Fe deposition under N$_{2}$ atmosphere onto monolayer Fe$_{2}$N/Cu(001), while the repetition of a bombardment with 0.5 keV N$^{+}$ ions during growth cycles results in imperfect bilayer $\gamma'$-Fe$_{4}$N. The increase in the sample thickness causes the change of the surface electronic structure, as well as the enhancement in the spin magnetic moment of Fe atoms reaching $\sim$ 1.4 $\mu_{\mathrm B}$/atom in the trilayer sample. The observed thickness-dependent properties of the system are well interpreted by layer-resolved density of states calculated using first principles, which demonstrates the strongly layer-dependent electronic states within each surface, subsurface, and interfacial plane of the $\gamma'$-Fe$_{4}$N atomic layers on Cu(001).

Iron-based magnetic superhalogens with pseudohalogens as ligands: An unbiased structure search

We have performed an unbiased structure search for a series of neutral and anionic FeL4 (L = BO2, CN, NO2, NO3, OH, CH3, NH2, BH4 and Li2H3) clusters using the CALYPSO (Crystal structure Analysis by Particle Swarm Optimization) structure search method. To probe the superhalogen properties of neutral and anionic FeL4 clusters, we used density-functional theory with the B3LYP functional to examine three factors, including distribution of extra electron, pattern of bonding and the nature of the ligands. Theoretical results show that Fe(BO2)4, Fe(NO3)4 and Fe(NO2)4 can be classified as magnetic superhalogen due to that their electron affinities even exceed those of the constituent ligands. The magnetic moment of Fe atom is almost entirly maintained when it is decorated with various ligands except for neutral and anionic (Li2H3)4. Moreover, the current work is also extended to the salt moieties formed by hyperhalogen/superhalogen anion and Na+ ion. It is found that these salts against dissociation into Na + FeL4 are thermodynamic stable except for Na[Fe(OH)4]. These results provides a wealth of electronic structure information about FeL4 magnetic superhalogens and offer insights into the synthesis mechanisms.

Interdiffusion in epitaxial ultrathin Co2FeAl/MgO heterostructures with interface-induced perpendicular magnetic anisotropy

The interfacial atomic structure of epitaxial ultrathin Co2FeAl/MgO(001) heterostructures, which is related to the interface-induced perpendicular magnetic anisotropy (PMA), was investigated using scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, and X-ray magnetic circular dichroism. Al atoms from the Co2FeAl layer significantly interdiffused into MgO, forming an Al-deficient Co–Fe–Al/Mg–Al–O structure near the Co2FeAl/MgO interface. This atomic replacement may have enhanced the PMA, which is consistent with the observed large perpendicular orbital magnetic moments of Fe atoms at the interface. This work suggests that control of interdiffusion at ferromagnet/barrier interfaces is critical for designing an interface-induced PMA system.

Stacking fault energy of C-alloyed steels: The effect of magnetism

First-principles calculations have been performed to study the effect of C on the stacking fault energy (SFE) of paramagnetic γ-Fe and FeCrNi austenitic steel. In these systems, the local magnetic structure is very sensitive to the volume in both fcc and hcp structures, which emphasizes the importance of the magnetovolume coupling effect on the SFE. The presence of C atom suppresses the local magnetic moments of Fe atoms in the first coordination shell of C. Compared to the hypothetical nonmagnetic case, paramagnetism significantly reduces the effect of C on the SFE. In the scenario of C being depleted from the stacking fault structure or twin boundaries, e.g., due to elevated temperature, where the chemical effect of C is dissipated, we calculate the C-induced volume expansion effect on the SFE. The volume induced change in the SFE corresponds to more than ∼ 50% of the total C effect on the SFE obtained assuming uniform C distribution.

Structures, stabilities, and magnetic properties of the FenAu (n = 1−12) clusters**Project supported by the National Natural Science Foundation of China (Grant No. 21301112) and the Ph. D. Program Foundation of the Education Ministry of China (Grant No. 20131404120001).

The configurations, stabilities, electronic, and magnetic properties of FenAu (n = 1−12) clusters are investigated systematically by using the relativistic all-electron density functional theory with the generalized gradient approximation. The substitutional effects of Au in Fen+1 (n = 1, 2, 4, 5, 10−12) clusters are found in optimized structures which keep the similar frameworks with the most stable Fen+1 clusters. And the growth way for FenAu (n = 6−9) clusters is that the Au atom occupies a peripheral position of Fen cluster. The peaks appear respectively at n = 6 and 9 for FenAu clusters and at n = 5 and 10 for Fen+1 clusters based on the size dependence of second-order difference of energy, implying that these clusters possess relatively high stabilities. The analysis of atomic net charge Q indicates that the charge always transfers from Fe to Au atom which causes the Au atom to be nearly non-magnetic, and the doped Au atom has little effect on the average magnetic moment of Fe atoms in FenAu cluster. Finally, the total magnetic moment is reduced by 3 μB for each of FenAu clusters except n = 3, 11, and 12 compared with for corresponding pure Fen+1 clusters.

Interfacial structure, ferroelectric stability, and magnetoelectric effect of magnetoelectric junction FeCo/BaTiO3/FeCo with alloy electrode

The interfacial structures, ferroelectric instability, and magnetoelectric coupling effect of FeCo/BaTiO3/FeCo junction with alloy electrode have been studied systematically using the first-principles calculations within density functional theory. Owing to different interfacial coupling between the electrode and BaTiO3 barrier layer, there are four possible interfacial structures, including TiO2/Fe, BaO/Fe, TiO2/Co, and BaO/Co interfaces. Among the four interfacial structures, the TiO2/Fe interface is the most stable one. With the decreasing thickness of BaTiO3 barrier layer in the junction, there exists a critical size of ferroelectricity with a thickness of ~4.5 formula unit cells. However, the ferroelectric polarization of magnetoelectric junction with the alloy FeCo electrode is larger than that of the junction with the Co electrode using a same size due to the stronger screening ability of alloy FeCo electrode. Meanwhile, we find that the induced magnetic moments of Ti atoms are nearly from the contribution of the first TiO2 layer adjacent to the interfaces irrespective of the thickness of ferroelectric barrier. In other words, the induced magnetic moments of Ti atoms only maintain an atom-layer thickness. Moreover, the induced magnetic moments of Ti atoms and local magnetic moments of Fe atoms at the two interfaces of tunneling junction depend on the polarization orientation, leading to a sizable ME coupling effect.