Fe 3d States Research Articles

Antiferromagnetism, superparamagnetism, and effects of A-cation distribution on magnetic properties of LaSrFeO[formula omitted] Ruddlesden–Popper oxides: A combined experimental and theoretical study

Understanding the structure–property relationship in complex magnetic oxides is crucial for developing new magnetic materials. In this study, we synthesized LaSrFeO4 Ruddlesden–Popper oxide using spray-pyrolysis method. Electron paramagnetic resonance (EPR) spectra revealed two Fe-associated lines: a broad line indicating short-range antiferromagnetic (AFM) order, disappearing at 375 K, and a narrow line due to the presence of superparamagnetic phase. The magnetic moment from the linewidth’s temperature dependence was 10−19 A m2. EPR intensity analysis estimated the superparamagnetic to antiferromagnetic phase ratio at 0.014 ± 0.004.Using density-functional theory with Agapito–Curtarolo–Buongiorno–Nardelli (ACBN0) pseudo-hybrid functional including self-consistent Hubbard U correction, we calculated the density of states for two La/Sr atomic distributions. The calculations show that a uniform La/Sr distribution perturbs 2D symmetry of Fe 3d states, which can contribute to the observed lowering of the critical exponent β for the magnetically ordered phase. These results highlight the impact of La/Sr distribution on LaSrFeO4’s properties.

Interlinking the Fe doping concentration, optoelectronic properties, and photocatalytic performance of ZnO nanostructures

Doping is one of the effective strategies to modulate the optoelectronic properties and photocatalytic activity of photocatalysts. In this study, the effect of Fe doping (0–10 %) on morphology, optical and electronic properties, and photocatalytic activity of ZnO nanostructures is studied. The X-ray diffraction analysis shows that >5 % Fe doping, ZnFe2O4 is segregated as a secondary phase. The crystalline size decreases from 50.8 nm to 21.4 nm and the micro-strain increases with increasing the Fe concentration. The Fe doping-induced electronic restructuring facilitates visible light absorption through the O 2p → Fe 3d transition and the suppression of charge recombination by efficiently trapping conduction band electrons. Density functional theory (DFT) calculations are employed to unravel the underlying electronic changes induced by Fe doping in ZnO. The formation of shallow donor levels below the conduction band originates from the Fe 3d state. Photoluminescence spectra of pristine and Fe-doped ZnO nanostructures show characteristic emission peaks at approximately 384 nm and 570 nm, indicating the recombination of free excitons and oxygen interstitial defects, respectively. The results of the photocatalytic activity tests confirm that the 1 % Fe-doped ZnO nanostructures can exhibit the highest efficiency compared to the heavily doped ZnO nanostructures. The high efficiency in photocatalytic activity of 1 % Fe-doped ZnO nanostructures is ascribed to the modulated electronic structure and defect density. The adsorption affinity of methylene blue and water molecules to the surfaces of pristine and Fe-doped ZnO is simulated using the Monte-Carlo method. This study emphasizes the importance of controlling the dopant concentration to enhance the photocatalytic activity of various photocatalysts.

Structural, electronic, mechanical and magnetic properties of AFeAs (A: Li, Na and K) pnictides: A density functional approach

In this comprehensive investigation, the physical (electronic, structural and magnetic) and mechanical characteristics of AFeAs (A = Li, Na, K) are examined by using first principle approximations. Employing optimized cell parameters, bulk modulus, phase stability and correlation of them with magnetic order is also reported. The equation of state described the existence of an antiferromagnetic ground state. The effects of structural alterations on electronic characteristics for all nonmagnetic and magnetic systems are also covered here. The mechanical stability and metallic nature of all AFeAs is revealed from the mechanical features and the electronic band diagram. The orbital partial density of state having mixed orbital containing degenerate 3dzx and 3dzy orbitals of the Fe-3d state are also discovered. Additionally, as many features of the stable KFeAs system were not previously quantified so, a comparative picture of KFeAs with the other two representatives (NaFeAs and LiFeAs) of A-111 groups is also provided.

Phase stability and physical behaviour of Fe3Pd, FePd and FePd3 binary intermetallic compounds

The FP-LAPW method is used to examine the physical properties of Fe–Pd binary intermetallic compounds. The calculated formation enthalpies indicate that FePd and FePd3 compounds are thermodynamically stable in L10 and L12 phases, respectively, and the ferromagnetic Fe3Pd can be formed in a tetragonal Z1 structure with a small negative formation enthalpy. The elastic coefficients and their related parameters show that all compounds are mechanically stable and ductile in nature. The FePd-L10 is the harder compound and Fe3Pd-Z1 exhibits the highest anisotropy. The band structure, the TDOS profile and the charge density distribution show that these compounds are ferromagnetic with a metallic character and covalent-metallic bonds. The PDOS shows that the Pd-4d and Fe-3d states are dominant, and the asymmetry of Fe-3d states is behind the system strong spin polarization. The calculated magnetic moments agree well with previous theoretical results. The thermodynamic parameters are determined using the quasi-harmonic Debye model.

Ferromagnetism with above-room-temperature Curie temperature in Fe-doped β-Ga2O3 studied by first-principles calculations

Induced by the spin-polarized double-exchange interaction between the 3d orbitals of Fe atoms and O-2p orbitals near the O/Ga vacancy complexes, the origin of magnetism in β-Ga2O3 is investigated by using geometry optimization and energy calculation based on the first-principles generalized gradient approximation Hubbard + U method of density-functional theory. The results show that the formation energy of Fe2+ in the β-Ga2O3 matrix is lower than that of Fe3+, and the system is more stable when the Ga or O vacancies (VGa or VO) are adjacent to the Fe lattice position. It is found that the β-Ga31Fe2+/3+O47 and β-Ga30Fe2+/3+O48 systems all exhibit ferromagnetism with above-room-temperature Curie temperature. β-Ga30Fe2+/3+O48 (i.e., with Fe2+/3+ and VGa) has more spin-polarized O atoms in the same direction. The magnetism originates from the double-exchange interaction of the coupling and hybridization of the Fe-3d state of Fe3+ and the O-2p state, which increases the magnetic moment of the β-Ga30Fe3+O48 system. This study provides a theoretical reference for designing and preparing new β-Ga2O3-based dilute magnetic semiconductors and related spintronic and optoelectronic devices.

Intertwined electronic and magnetic structure of the van-der-Waals antiferromagnet Fe2P2S6

Many unusual and promising properties have been reported recently for the transition metal trichalcogenides of the type MPS3 (M = V, Mn, Fe, Ni ...), such as maintaining magnetic order to the atomically thin limit, ultra-sharp many-body excitons, metal-insulator transitions and, especially for Fe2P2S6, giant linear dichroism among others. Here we conduct a detailed investigation of the electronic structure of Fe2P2S6 using angle-resolved photoemission spectroscopy, q-dependent electron energy loss spectroscopy, optical spectroscopies and density functional theory. Fe2P2S6 is a Mott insulator with a gap of Egap ≈ 1.4 eV and zigzag antiferromagnetism below TN = 119 K. The low energy excitations are dominated by Fe 3d states. Large and sign-changing linear dichroism is observed. We provide a microscopic mechanism explaining key properties of the linear dichroism based on the correlated character of the electronic structure, thereby elucidating the nature of the spin-charge coupling in Fe2P2S6 and related materials.

Thermodynamics and robust n-type charge carrier density in Co-doped FeTe2: strain strategy

Herein, we investigated the combined effect of Co-doping and strain (biaxial [110] and hydrostatic [111]) on the thermodynamics and electronic structure of the FeTe2 motif using ab-initio calculations by considering the strong correlation effects. The pristine one has a non-magnetic semiconducting nature with an indirect band gap (E g ) of 0.384 eV. Interestingly, our results revealed that the Co-doping at the Fe site induced an n-type conductivity (i.e. few states are crossing the Fermi level from the valence to conduction band) in the system having a substantial charge carrier density magnitude of 0.14 × 1021 cm−3. The metallicity mainly comprises the Co-3d orbitals along with a significant contribution from Fe-3d states. Thermodynamic, mechanical, and dynamical stability of the Co-doped FeTe2 structure is confirmed by computing the formation energetic, elastic constants, and phonon band structure, respectively. Generally, an increasing and decreasing trend in E g value is evident against the applied compressive and tensile strains having ranged from −5% to +5% for the case of the undoped system, respectively. On the other hand, the Co-doped structure maintained its n-type conduction against considered both types of strains. Moreover, it is demonstrated that compressive strains strengthen the charge carrier density amplitude, while tensile strains show a negative impact. Hence, the present work displays that robust n-type conductivity and stable structure of Co-doped FeTe2 system, makes it a desirable candidate for device applications.

Influence of Pr doping in non-stoichiometric NiFe2O4 on magnetic response and electronic properties: Magnetic Compton scattering and ab-initio studies

Magnetic Compton profiles of 5 % and 15 % Pr-doped non-stoichiometric NiFe2O4 have been measured using synchrotron radiations of energy 182.65 keV at SPring-8, Japan. An analysis of MCPs measured at 6 and 300 K show that the total spin moment decreases with increasing temperature and Pr concentration in Ni0.9-xPrxFe2.1O4 which is consistent with the magnetic moment (spin + orbital) as observed from magnetization measurements data. Spin moments at different sites are deduced using decomposition of MCPs in the elemental profiles of Ni, Pr-4f, Fe and diffused electrons. Spin moments at Ni-site have been found to play crucial role in emergence of absolute spin moments. Spin moments at Pr-site is aligned antiparallel to that of Ni and Fe sites. The orbital magnetic moment is also deduced from the subtraction of MCP based spin moments from the magnetization data. Theoretical FP-LAPW based total and atom specific spin moments are found in consonance with the MCP based data. Total and spin projected density of states are also computed using the FP-LAPW method to validate role of Pr-4f and Fe-3d (Oh) states in amending the total magnetic moment with Pr concentration. The M−H based saturation magnetization derived at 6 and 300 K temperature show that the saturation magnetic moments reduce from 2.21 → 2.05 μB/f.u. at 6 K and 2.10 → 1.98 μB/f.u. at 300 K with increase in Pr concentration from 5 % → 15 %. On the basis of hysteresis curve and coercive force, both the doped compounds show soft magnetism.

Anomalous metallic oxygen band in the potential superconductor KCa[formula omitted]Fe[formula omitted]As[formula omitted]O[formula omitted]: A DFT study

The electronic structure, magnetism and Fermi surface of a hole self-doped potential iron-based superconductor KCa2Fe4As4O2 (12442) were studied by a first-principles DFT/GGA approach. The self-doping value is obtained from DFT is 0.75 hole on Fe-3d states for KCa2Fe4As4O2. Surprisingly, for the KCa2Fe4As4O2 it was found that except the Fe-3d bands the O-2p bands also cross the Fermi level, which is quite anomalous behavior for O-2p states. In general, the O-2p states are usually fully occupied in transition metal oxides. Here it is not the case due to the fact that CaO layer gives some of the electrons to the FeAs layer, thus leaving O-2p states partially occupied. The O-2p states form additional Fermi surface sheet with propeller-like shape around Γ-point and modify the Fe-3d Fermi surface sheets due to the hybridization. To prove the presence of anomalous metallic O-2p band in KCa2Fe4As4O2 the comparative study with the same family systems KCa2Fe4As4F2 and RbGd2Fe4As4O2 (where it is not the case) was performed. Finally, we discuss the main differences between the emergence of the metallic O-2p band in case of KCa2Fe4As4O2 and those in case of cuprate superconductors LaBaCuO4 and YBa2Cu3O7. Therefore, we expect that the KCa2Fe4As4O2 might be a potential superconductor with unusual properties.

Electronic and magnetic properties of FeCr2O4 nanoparticles by advanced synchrotron based soft X-ray magnetic circular dichroism

Multifunctional materials are now being considered for a variety of technical uses. The iron-chromite spinel FeCr2O4 NPs generated by the sol-gel process has been studied experimentally in this paper. The material crystallized into a cubic form, according to structural analysis based on x-ray diffraction. The Vibrating Sample Magnetometer, Soft-x-ray absorption spectroscopy (XAS) and soft-x-ray magnetic circular dichroism (XMCD) were used to explore the electronic and magnetic structures of FeCr2O4 NPs. Cr ions have trivalent Cr3+ and Fe ions have divalent Fe2+ states present here. The Fe 3d states have a high degree of hybridization with the O 2p states. The Fe and Cr 2p XMCD analysis shows antiparallel alignment of Cr and Fe spins, as well as remarkable hybridization of Fe 3d and O 2p electrons.

The structural, mechanical and electronic properties of Ti-Al-based compounds by first-principles calculations.

The first-principles calculations with density functional theory were performed to investigate the effects of transition metal elements (Mo, Cu, Fe, Ni and Nb) on the physical properties of the Ti-Al-based compounds. Our optimized crystal parameters are in good agreement with the previous theoretical and experimental values. The mechanical stability is verified by the independent elastic constants. The B/G and Poisson's ratio ν both show that Al6TiMo is brittle, while other compounds exhibit ductility. The values of compression anisotropy of the compounds are small, but the shear anisotropy of AlCu2Ti and AlNi2Ti is much more intense than that of other compounds. The anisotropy in elastic properties of AlFe2Ti and AlNbTi2 is smaller than that of the others. It can be seen that the capacity to compress along c-axis is smaller than that along a-axis and b-axis for AlNbTi2. For AlNbTi2, the anisotropy of the bulk modulus along a-axis relative to b-axis is more insignificant than that along c-axis relative to b-axis. The hardness and Debye temperature verify that AlFe2Ti has the greatest resistance to the plastic deformation and more intense inter-atomic bonding force, respectively. Band structures and DOS are used to investigate the electronic properties. The band structures without band gaps show that these ternary Ti-Al-based compounds are conductors. DOS shows the interactions between elements and gives the bond properties. Density of states and charge density both show the strong covalent properties of AlFe2Ti by the hybridization between Fe-3d and Ti-3d states.

Study on the magnetic origin in p-type ferromagnetic semiconductor (Ga,Fe)Sb: ab initio calculations

The electronic and magnetic properties of Ga 1−x Fe x Sb (x=0.125, 0.25, 0.5) have been investigated using the first-principles calculations based on density functional theory (DFT) and DFT plus on-site repulsion scheme (DFT+U). We found that the Coulomb U parameter plays an important role in predicting the correct stable magnetic order. Using the DFT+U method, the ferromagnetic (FM) ordering is found to be the magnetic ground state for both concentrations 12.5% and 25%, where the DFT failed without on-site repulsion U-term correction. This failure of semi-local DFT functionals is the fingerprint of the correlated nature of Fe-3d electrons. This result is in good agreement with experiment. Increasing Fe concentration up to 50% changing the magnetic ground state from FM to antiferromagnetic (AF) phase. We have used the Heyd-Scuseria-Ernzerhof (HSE) method to calculate the unfolded band structure of supercell for Ga 1−x Fe x Sb for x=0.125, in both FM and AF phases. The location of Fe-3d states in the calculation of the unfolded band structure showed a good accordance with recent resonance photoemission spectroscopy (RPES) experimental results. We have found that hole carriers have an important role at Fermi level E F . The rules of thumb to predict the stability of the ferromagnetism in Fe doped GaSb FMS have been discussed.

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.

Importance of the Hubbard U parameter to explore accurate electronic and optical behaviour of BiFeO3

Based upon Hubbard U corrected density functional theory (DFT), the electronic and optical responses of multiferroic Bismuth ferrite (BiFeO3; BFO) have been critically explored. Treating BFO as a strongly correlated insulating system, a Hubbard U parameter is considered for accurate description of localised Fe-3d states. However, the U parameter for the Fe-3d state is not sufficient as 2p states of O atoms are greatly influenced by Fe-3d states. Similar U parameters for Fe-3d as well as O-2p states are considered. An effective U value around 4.3 eV not only solves the band gap mismatch of BFO from both a theoretical and experimental aspect, but also predicts the exact nature of the band gap. Moreover, within this framework, the optical responses of BFO are critically investigated. Multiple transitions appearing in both theoretical and experimental absorption spectra are well explained by crystal field transitions of Fe3+ ions in a d5 high spin state, supported by the density of states curve of BFO. Therefore, based upon the DFT + U approach, accurate electronic and optical characteristics of BFO are critically explored, which is essential for its multidirectional application perspectives.

First-Principles Study on Optoelectronic Properties of Fe-Doped Montmorillonite Clay.

A theoretical investigation is conducted to describe optoelectronic properties of Fe-doped montmorillonite nanoclay under spin states of low spin (LS), intermediate spin (IS), and high spin (HS). Ground state electronic properties are studied using spin-polarized density functional theory calculations. The nonradiative and radiative relaxation channels of charge carriers are studied by computing nonadiabatic couplings (NACs) using an "on-the-fly" approach from adiabatic molecular dynamics trajectories. The NACs are further processed using a reduced density matrix approach with the Redfield formalism. The computational results are presented for electronic density of states, absorption spectra, charge carrier dynamics, and photoluminescence (PL) by comparing various spin multiplicities. Results on spin α and spin β components are independent and quite different because of the partial occupation of Fe 3d states. Overall, HS is the most stable with the largest Fe-O distances. One finds different nonradiative relaxation pathways in space and on the time scale for electrons and holes. The Redfield PL reveals obvious Fe 3d-3d transitions for LS and IS.

First-principles study of magnetic and optical properties in dopant-doped two-dimensional SnS2

We use GGA and GGA + U methods to calculate the magnetic and optical properties of pure, Fe doped, Mn doped, (Fe, Mn) co-doped and (Fe, Mn, VSn) co-doped SnS2 based on first principles. The results show that both Fe doping and Mn doping have spin polarization. The most stable (0, 3) configuration of (Fe, Mn) co-doped SnS2 exhibits magnetism, which mainly comes from the magnetic coupling between Fe-3d state, Mn-3d state and the nearest neighbor S-3p state of doped atoms. The most stable (0, 3, 1) configuration in (Fe, Mn, VSn) co-doped SnS2 also exhibits magnetism, which comes from the magnetic coupling between Fe-3d state, Mn-3d state and the nearest neighbor S-3p state of doped atoms. We also find that the introduction of doping atoms enhances the electromagnetic absorption capacity of the ɛ1(ω) of the dielectric function and increases the electrical conductivity. Mn doped and (Fe, Mn) co-doped SnS2 have low transmittance properties in visible and ultraviolet light ranges, while Fe doped and (Fe, Mn, VSn) co-doped SnS2 have high transmittance properties in visible and ultraviolet light ranges. Our findings indicate that Fe doped, Mn doped, (Fe, Mn) co-doped and (Fe, Mn, VSn) co-doped SnS2 systems have potential applications in spin semiconductors and optoelectronic devices.

Effect of Fe2+/3+ doping on the new magnetic mechanism and itinerant electronic characteristics of ZnO

Fe2+/3+ doping has been experimentally proven to have ferromagnetic effects on ZnO. However, the source and mechanism of magnetic properties are still explained by using the traditional double exchange theory with oxygen atoms as the intermediary. Therefore, the theoretical explanation of these phenomena needs to be expanded urgently. In this study, first principles calculations are used to investigate the effect of Fe2+/3+ doping on the magnetic mechanism of ZnO on the basis of the generalized gradient approximation plane wave ultrasoft pseudopotential method within the framework of spin density functional theory. This study shows that the Zn34Fe2O36(Fe3+) system exhibits the characteristics of ferromagnetism above room temperature and that the total magnetic moment is not an integer when the distance between two Fe ions is 4.790 Å. The Zn34Fe2O36(Fe3+) system has a part of O1− ions in addition to O2− ions, that is, it contains the characteristics of itinerant electrons (donors) and local electrons (acceptors). This study is the first to discover that Fe3d states also produce local and itinerant electrons. It also reveals a new magnetic mechanism for sp–d double exchange with itinerant electrons as the media in the Zn34Fe2O36(Fe3+) system.

Electronic structure modification in Fe-substituted β-Ga2O3 from resonant photoemission and soft x-ray absorption spectroscopies

Oriented thin films of β-(Ga1−x Fe x )2O3 were deposited by radio frequency magnetron sputtering on c-Al2O3 and GaN substrates. The itinerant character of the Fe 3d states forming the top of the valence band (VB) of the Fe-substituted β-Ga2O3 thin films has been determined from resonant photoelectron spectroscopy. Further, the admixture of the itinerant and localized characters of these Fe 3d states has been obtained for larger binding energies; i.e. deeper in the VB. The bottom of the conduction band (CB) for β-(Ga1−x Fe x )2O3 has been also found to have strongly hybridized states involving Fe 3d and O 2p states compared to that of Ga 4s in pristine β-Ga2O3. This suggests that β-Ga2O3 transforms from a band-like system to a charge-transfer system with Fe substitution. Furthermore, the bandgap red shifts with Fe composition, which has been found to be primarily related to the shift of the CB edge.

Effect of Fe on Bi2Te3: Structure, magnetic properties, and XPS valence band

The crystal structure and magnetic properties of Fe doped Bi2Te3 are presented. The samples were studied by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and magnetization measurements. Rietveld refinement results show a decrease in volume due to the decrease in c lattice parameter as iron is introduced. By XPS measurements, the Te 3d, Bi 4f, Bi 5d, Te 4d, and Fe 3d core levels were identified, and the XPS valence band (VB) was measured. The deconvolution process of high resolution XPS spectra in Fe 3d orbital shows that the Fe3+ ion is present in the samples. Theoretical calculations by DFT reveal that the Bi2Te3 compound is a semiconductor with a narrow energy band gap in agreement with experimental results. DOS analysis shows that the main contributions to the valence band are the Te 5p, Bi 6p, and Fe 3d states. XPS VB shows a decrease in the intensity at zero eV due to increased Fe 3d states and good correspondence with DOS calculated for Bi2Te3. Finally, magnetic susceptibility measurements on Bi2Te3 do not show a magnetic behavior; however, for the doped sample with x = 0.2, a weak ferromagnetic behavior (WFM) below 14 K is observed.

Correlating ligand-to-metal charge transfer with voltage hysteresis in a Li-rich rock-salt compound exhibiting anionic redox.

Anionic redox is a double-edged sword for Li-ion cathodes because it offers a transformational increase in energy density that is also negated by several detrimental drawbacks to its practical implementation. Among them, voltage hysteresis is the most troublesome because its origin is still unclear and under debate. Herein, we tackle this issue by designing a prototypical Li-rich cation-disordered rock-salt compound Li1.17Ti0.33Fe0.5O2 that shows anionic redox activity and exceptionally large voltage hysteresis while exhibiting a partially reversible Fe migration between octahedral and tetrahedral sites. Through combined in situ and ex situ spectroscopic techniques, we demonstrate the existence of a non-equilibrium (adiabatic) redox pathway enlisting Fe3+/Fe4+ and O redox as opposed to the equilibrium (non-adiabatic) redox pathway involving sole O redox. We further show that the charge transfer from O(2p) lone pair states to Fe(3d) states involving sluggish structural distortion is responsible for voltage hysteresis. This study provides a general understanding of various voltage hysteresis signatures in the large family of Li-rich rock-salt compounds.