Fe Magnetic Moments Research Articles

Hard x-ray photoemission spectroscopy of the ferrimagnetic series Gd6(Mn1−xFex)23

We study the evolution of the electronic structure of the intermetallic series ${\mathrm{Gd}}_{6}{({\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})}_{23}, x=0.0--0.75$, which shows nonmonotonic ferrimagnetic ordering temperatures ${T}_{C}$ but with a systematic reduction of the total bulk magnetization upon increasing Fe content, $x$. We have carried out hard x-ray photoemission spectroscopy to elucidate the relation between electronic structure and properties of the series. The Gd $3d$ and Gd $4d$ core-level spectra indicate trivalent ${\mathrm{Gd}}^{3+}$ multiplets in the intermediate-coupling scheme with features due to $L\text{\ensuremath{-}}S$ and $j\text{\ensuremath{-}}J$ coupling. The Fe $2p$ core levels show asymmetric single peak metal-like spectra, while the Mn $2p$ core levels show asymmetric doublet peaks. The relative intensities of the Mn $2p$ doublets as a function of $x$ indicate occupancy changes of distinct crystallographic sites associated with Mn up-spin and down-spin states. The valence band spectra identify the Gd $4f$ states at high binding energies ($\ensuremath{\sim}7.4$ eV). The Mn $3d$ states occur at the Fermi level and as a broad feature between 2 and 5 eV binding energy in ${\mathrm{Gd}}_{6}{\mathrm{Mn}}_{23}$. Upon substitution, the Fe $3d$ states show up as small shifts to higher binding energies compared to Mn $3d$ states. The Fe $3s$ and Mn $3s$ spectra show exchange split peaks, allowing an estimate of the Mn and Fe magnetic moments using a Van Vleck analysis, which also provides a quantification of occupancy changes with $x$. The overall results are consistent with the bulk net magnetization, indicating that Mn up-spin sites become Fe down-spin sites on substitution, while the nonmonotonic ${T}_{C}$ originates in a change from Mn sublattice to Fe sublattice derived ordering.

Electronic structure of YbFe4Al8 antiferromagnet: A combined X-ray photoelectron spectroscopy and first-principles study

Depending on their chemical composition, Yb compounds often exhibit different valence states. Here we investigate the valence state of YbFe4Al8 using X-ray photoelectron spectroscopy (XPS) and first-principles calculations. The XPS valence band of YbFe4Al8 consists of two contributions coming from divalent (Yb2+) and trivalent (Yb3+) configurations. The determined value of the valence at room temperature is 2.81. Divalent and trivalent contributions are also observed for core-level Yb 4d XPS spectra. We study several collinear antiferromagnetic models of YbFe4Al8 from the first-principles and for comparison we also consider LuFe4Al8 with a fully filled 4f shell. We predict that only Fe sublattices of YbFe4Al8 carry significant magnetic moments and that the most stable magnetic configuration is AFM-C with antiparallel columns of magnetic moments. We also present a Mulliken electronic population analysis describing charge transfer both within and between atoms. In addition, we also study the effect of intra-atomic Coulomb U repulsion term applied for 4f orbitals on Yb valence and Fe magnetic moments.

Synthesis, characterization and structural study of Fe(III) complex derived from sodium dodecyl benzene sulfonate

ABSTRACT. Fe(III) complex derived from sodium dodecyl benzene sulfonate (SDBS) has been synthesized and characterized using FT-IR, TGA, UV-Vis and VSM analysis. The complex structure was determined and illustrated. The study of thermogravimetric, optic and magnetic properties are reported and discussed. The bands in electronic spectrum involved a fully allowed transitions characteristic of Charge Transfer absorption from ligand to metal. The structural study revealed an octahedral geometry around the iron(III) ion. The complex obtained has the following formula [Fe(SDB)3]11H2O , where Fe(III) is linked to three bidentate ligands. This complexation is confirmed by layering FT-IR Ligand/Complex spectra. Moreover, the VSM analysis collected at room temperature exhibits antiferromagnetic behaviour referred to the anti-alignment of the magnetic moment of Fe3+.
 
 KEY WORDS: Sodium dodecyl benzene sulfonate, Iron(III) complex, Spectral analysis, TGA, Magnetic properties
 
 Bull. Chem. Soc. Ethiop. 2022, 36(2), 353-361. 
 DOI: https://dx.doi.org/10.4314/bcse.v36i2.9

Study on FeCr thin film for a spintronic material with negative spin polarization

Negative spin polarization materials generate a spin current whose spin direction is opposite to the magnetization direction. This characteristic is technologically important because it increases the freedom in the design of spintronic devices. In this paper, we studied FeCr from the viewpoints of band structure, atomic magnetic moments, and local atomic arrangement to explore its potentiality as a negative spin polarization material. First-principles calculations on the disordered FeCr alloy predicted a high negative spin polarization, which was attributed to the band matching of the minority spin. We experimentally fabricated Fe1-xCrx thin films in the Cr concentration (x) range from 0.2 to 0.4 by sputter deposition and demonstrated inverse magnetoresistance using current-perpendicular-to-plane giant magnetoresistance (GMR) devices, which provided evidence of negative spin polarization. The spin polarization of Fe0.7Cr0.3 estimated from the GMR device measurements was −0.28 and fell below the calculated value. Synchrotron X-ray magnetic circular dichroism measurements showed a ferrimagnetic configuration of the Fe and Cr magnetic moments. The Fe moment showed a large moment over a wide Cr concentration range, whereas the Cr moment decreased with the Cr concentration, which agreed with the calculation result. Synchrotron Mössbauer spectroscopy measurements indicated the inhomogeneity of the FeCr composition even in the as-deposited state. Fe-rich regions were formed after annealing, reflecting the phase separation nature of the Fe-Cr system. Inhomogeneity was not considered in the calculation and could be the origin of the smaller negative spin polarization observed in the experiment.

Theory‐Guided Regulation of FeN4Spin State by Neighboring Cu Atoms for Enhanced Oxygen Reduction Electrocatalysis in Flexible Metal–Air Batteries

AbstractIron, nitrogen‐codoped carbon (Fe−N−C) nanocomposites have emerged as viable electrocatalysts for the oxygen reduction reaction (ORR) due to the formation of FeNxCycoordination moieties. In this study, results from first‐principles calculations show a nearly linear correlation of the energy barriers of key reaction steps with the Fe magnetic moment. Experimentally, when single Cu sites are incorporated into Fe−N−C aerogels (denoted as NCAG/Fe−Cu), the Fe centers exhibit a reduced magnetic moment and markedly enhanced ORR activity within a wide pH range of 0–14. With the NCAG/Fe−Cu nanocomposites used as the cathode catalyst in a neutral/quasi‐solid aluminum–air and alkaline/quasi‐solid zinc–air battery, both achieve a remarkable performance with an ultrahigh open‐circuit voltage of 2.00 and 1.51 V, large power density of 130 and 186 mW cm−2, and good mechanical flexibility, all markedly better than those with commercial Pt/C or Pt/C‐RuO2catalysts at the cathode.

Theory-Guided Regulation of FeN4 Spin State by Neighboring Cu Atoms for Enhanced Oxygen Reduction Electrocatalysis in Flexible Metal-Air Batteries.

Iron, nitrogen-codoped carbon (Fe-N-C) nanocomposites have emerged as viable electrocatalysts for the oxygen reduction reaction (ORR) due to the formation of FeNx Cy coordination moieties. In this study, results from first-principles calculations show a nearly linear correlation of the energy barriers of key reaction steps with the Fe magnetic moment. Experimentally, when single Cu sites are incorporated into Fe-N-C aerogels (denoted as NCAG/Fe-Cu), the Fe centers exhibit a reduced magnetic moment and markedly enhanced ORR activity within a wide pH range of 0-14. With the NCAG/Fe-Cu nanocomposites used as the cathode catalyst in a neutral/quasi-solid aluminum-air and alkaline/quasi-solid zinc-air battery, both achieve a remarkable performance with an ultrahigh open-circuit voltage of 2.00 and 1.51 V, large power density of 130 and 186 mW cm-2 , and good mechanical flexibility, all markedly better than those with commercial Pt/C or Pt/C-RuO2 catalysts at the cathode.

Coordination-tuned Fe single-atom catalyst for efficient CO2 electroreduction: The power of B atom

• CO 2 RR performances on Fe single-atom catalysts coordinated by B atoms are explored. • Introduction of B atoms can promote multi-electron reduction reactions. • Linear relationship between limiting potential and O adsorption energy is revealed. • Fe magnetic moment and d-band center are key factors to improve CO 2 RR performance. Designing of effective electrocatalysts for electrocatalytic CO 2 reduction into value-added chemicals is the key to reducing CO 2 concentration and achieving carbon neutrality. However, achieving high activity and product selectivity simultaneously remains a significant challenge. Herein, a series of Fe single-atom catalysts coordinated by B atoms, namely FeB x C y (x + y = 3 or 4), are constructed to systematically investigate the electrocatalytic CO 2 reduction reaction (CO 2 RR) based on density functional theory computations. Eight catalysts, including FeO 4 , are identified that can effectively activate CO 2 molecules and significantly inhibit competitive hydrogen evolution reaction (HER). Among them, FeB 2 C and FeB 2 C 2 h ( h represents a cis structure) show the higher CO 2 RR activity with the less negative limiting potentials of −0.24 and −0.40 V toward production of CH 4 , indicating the optimal content for doping B atoms. The activity mechanism shows that d-band center and magnetic moment of central Fe atom can be manipulated by rational modulating the coordinated B atoms to improve the CO 2 RR performance. By the coordinated B atom, an optimal adsorption strength of the reaction intermediates can be achieved on the FeB x C y surface, and thereby increasing CO 2 RR catalytic activity and product selectivity. FeB 2 C with more negative d-band center and the optimal Fe atomic magnetic moment shows the best CO 2 RR performance. These results reveal a great potential of coordination tuning for CO 2 RR, and provide a new theoretical perspective for rational design of high activity, selective CO 2 RR catalysts.

Re-entrant ferromagnetism at ultrahigh temperatures in epsilon–iron as possible origin of the geomagnetic field

A recent theory of band ferromagnetism in 3d metals predicts ‘re-entrant’ ferromagnetism at temperatures far above the boiling point of these metals in normal conditions (Teodorescu, C.M., 2021. Spin asymmetry originating from densities of states: Criterion for ferromagnetism, structures and magnetic properties of 3d metals from crystal field based DOSs. Results in Physics 25, 104,241). Metals, and in particular iron rich alloys, are still solid at such extremal temperatures in the Earth’s inner solid core. It follows that this piece of the Earth may become ferromagnetic. This hypothesis is investigated in this work in more details, by using densities of states derived by ab initio density functional theory calculations for hexagonal close-packed iron and applying the basic theory of band ferromagnetism derived in the above Reference. The temperature for ‘re-entrant’ ferromagnetism increases with the pressure, ranging between about 6530 and 6640 K for pressures between 330 and 360 GPa; these temperatures are in the range of most estimates for the temperature of the inner solid core of our planet. The dimension of the ferromagnetic “innermost inner core” (IMIC) derived from the estimated Fe magnetic moment are within the dimensions of a IMIC with different anisotropy in the propagation of seismic waves. For body centered cubic Fe no ‘re-entrant’ ferromagnetism is predicted based on the actual model. It follows that the Earth’s inner solid core with hexagonal close-packed structure is the main responsible for the geomagnetic field, and also most probably the reversal of this field proceeds by simple rotation of the magnetization of this core, while keeping a non-vanishing magnetic field during the reversal. This might prevent the Earth’s surface bombardment with energetic charged particles during the reversals, with beneficial effects for complex lifeforms and for mankind civilization.

Inducing Fe moment in LaFeSi with p-block element substitution

We studied the LaFeSi phase using density functional theory based full-potential linearized augmented plane wave (FP-LAPW) method. Specifically we examined the effect of p-element substitution on the stability and magnetic properties focusing on pathways to induce Fe magnetic moments. We demonstrate that either partial or complete substitutions at the Si 2c-site by several p-block elements lead to non-zero Fe moments. Our theoretical study shows that partially substituted LaFeSi1-yGay and LaFeSi1-yAly are more thermodynamically stable than the fully substituted LaFeGa and LaFeAl. We also found that the P substitutions, either partial or complete, have the most negative formation energies, however the compounds containing phosphorus are non-magnetic. Our work highlights a pathway toward manipulations of the otherwise quenched Fe-moments in LaFeSi-related compounds and help in advancing the potential magnetic functionalities of LaFeX compounds.

Temperature dependence of site-resolved Fe magnetic moments in ThMn12-type Sm(Fe1−xCox)12 compounds studied via synchrotron Mössbauer spectroscopy

The relationship between the magnetization and magnetic moment at each Fe site in Sm(Fe1−xCox)12 compounds and their temperature dependence were investigated via synchrotron Mössbauer spectroscopy. The enhancement in the Fe magnetic moments in the Sm(Fe1−xCox)12 compounds with increasing Co content was observed at a similar rate as the increment in their saturation magnetization at 300 K. We found that the magnitudes of the Fe magnetic moments vary in a descending order: 8i > 8j > 8f, and they are independent of the Co content because the variation is caused by the magneto-volume effect. The curvatures of the temperature dependence of the Fe-sublattice magnetic moments are larger and also vary in the order of 8i > 8j > 8f because the enhancements in the exchange coupling strength decrease in the same order.

WITHDRAWN: Effect of magnetostrictive coupling on the magnetic properties of TbFe2: ab initio study

• Elastic and magnetoelastic constants governing the amorphous TbFe 2 have been computed, and strains on crystal lattice have been studied. • Ferromagnetic interaction between Tb atoms has been found and the 111-direction is revealed to be the easy axis of the magnetization. • XMCD and XAS spectra show a large sensitivity on magnetic and elastic properties. Density functional theory (DFT) within the GGA approach is applied to investigate the electronic, elastic and magnetic properties of the amorphous TbFe 2 alloy. We found that TbFe 2 exhibits a trigonal R3̅m structure at T=0 K with lattice constants a = b = c = 5.15 Å, and α = β = γ = 60.14°. This structure can undergo elastic deformations that affect the TbFe 2 properties. The electronic properties analysis revealed a magnetic behavior and a metallic character. Therefore, to investigate the elastic and magnetic properties of TbFe 2 , the elastic and magnetoelastic constants governing the system are determined before studying the effects of applied strains on the crystal lattice. The results reveal a redistribution of Tb and Fe magnetic moments along the a-and c-axis. Moreover, a ferromagnetic interaction between the Tb atoms is found, and the [111] direction is the easy axis of the magnetization. Furthermore, the XMCD and XAS spectra of TbFe 2 in normal and deformed cases show significant magnetic and elastic properties sensitivity.

Effect of Doping on the Electronic Structure of the Earth's Lower Mantle Compounds: FeXO3 with X = C, Al, Si.

The effect of the mutual doping of C, Si, and Al atoms on the electronic structure and magnetic properties of FeXO3 (X = C, Al, Si) compounds, which are constituent compounds of the Earth’s lower mantle, was studied. In our first principles calculations, it was found that doping with carbon for both FeSiO3 and FeAlO3 leads to the transition of the compound from a half-metallic state to a metallic one. The values of the magnetic moments of Fe were obtained for pure and doped compounds. For the doped compounds, there is a tendency of the Fe magnetic moment to increase with the growth in the number of substituted ions in the case of replacing Si with C and Si for Al; on the contrary, in the case of replacing Al with C and Si, a decrease in the magnetic moment was revealed. For FeXO3 (X = C, Al, Si), the obtained magnetic moment values were found to be in a good agreement with the known experimental data.

Temperature dependent magnetization in Co@Fe Nanoparticles

We have performed atomistic simulations to investigate effect of nanoparticle size, and core–shell ratio on magnetization of Co@Fe spherical nanoparticles (NPs). Here, Fe forms the core and Co forms the shell. Mean temperature-dependent magnetization and magnetic-ordering temperature are found to be sensitive to particle size as well as core–shell ratio. A pronounced display of depression in Tc with increase in NP size is observed. The magnetization of the Co@Fe core–shell NPs has also been studied as a function of interfacial exchange energy (JFe−Co) between Fe and Co magnetic moments. An increase in ordering temperature with the increase in JFe−Co is observed.

The structural evolution and superconductivity under pressure for superconductor AFe2As2 (A = Ba, Sr)

AbstractIn collinear antiferromagnetic (AFM) order, the structural evolution and electronic structure of AFe2As2 (A = Ba, Sr) under hydrostatic and uniaxial pressures have been studied. The Fe‐Fe distance is closely related to the AFM suppression as well as the Fe‐As bond. Through analysis of Fermi surface and band diagram, the uniaxial pressure along c‐axis can make the band near the Γ point to produce Lifshitz transition under the condition of lower pressure, and it is easier to suppress Fe magnetic moment. If the pressure component along c‐axis is larger in the experiment, it is easier to cause the superconducting. The superconductivity of BaFe2As2 and SrFe2As2 under pressure may be due to the increasing of Fe 3d‐ and As 4p‐hybridized bands width near the Fermi level.

Electronic structure and chemical bonding in smart anti-corrosion coatings

Nitrilotris-(Methylenephosphonato)-Three-aqua-Iron(II) (FeNTP) is a linear organic-inorganic coordination polymer forming an anti-corrosion coating on the steel surface in aqueous medium with NTP-based corrosion inhibitors. The Cd-containing NTP-based inhibitor has higher protective properties than the inhibitor without cadmium. At the same time, the Fe–O bonds are more covalent, and the data from X-ray photoelectron and Mössbauer spectroscopy allow one to suppose that the Fe atoms within the polymer with Cd are in a different magnetic state. However, the causes of these phenomena and their relationship were not studied before. Here, for the first time, the electronic structure of FeNTP is calculated using density functional theory. A stable solution with a large magnetic moment at Fe atoms (4 μB) is found for FeNTP. The calculated properties correspond to the experimental ones obtained earlier: the Fe magnetic moment (estimated by XPS), the quadrupole splitting (Mössbauer spectroscopy), and the character of the chemical bond between Fe and O atoms (X-ray diffraction). Also, a second, nonmagnetic, solution is found, the calculated properties of which are identical to those of the Cd-containing FeNTP. The difference in electronic structure between these two states manifests itself in different Fe 3d occupancy, angular electron density distribution around the Fe atoms, and magnetic state. The interrelation between the magnetic state of the Fe atoms and their chemical bonds is described. This made it possible to reveal the mechanism for the effect of Cd doping of the protective layers on their anti-corrosive properties. Thus, an approach to developing the more effective and efficient corrosion inhibitors in the neutral aqueous media is proposed.

Magnetic and optical characteristics of Fe doped SrTiO3 perovskite compound: a first principle study

In this study, we theoretically probed the electronic, magnetic and optical characteristics of Sr1-xFexTiO3 and SrTi1-xFexO3 perovskite materials with the implementation of DFT based Wein2K code. In both doping atom is partially removed by (Fe) dopant. The substitution of Fe at A-site in SrTiO3 consequences the half-metallic ferromagnetic (FHM) character, while the substitution of Fe at B-site in SrTiO3 resulted into metallic character of the studied material. Moreover, In Sr0.5Fe0.5TiO3 the local magnetic moment of Fe is slightly decreased, whereas in SrTi0.5Fe0.5O3 the Fe magnetic moment is highly decreased from its free space value 4 µB. This reduction is due to p-d hybridization. In case of Sr0.5Fe0.5TiO3, the exchange constants N0α for s-d and N0β for p-d interaction are computed. The optical response of studied materials is described by jn.hat Sr0.5Fe0.5TiO3 is a prominent candidate for spintronics applications.

Element-specific contributions to improved magnetic heating of theranostic CoFe2O4 nanoparticles decorated with Pd

Decoration with Pd clusters increases the magnetic heating ability of cobalt ferrite (CFO) nanoparticles by a factor of two. The origin of this previous finding is unraveled by element-specific X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) combined with atomic multiplet simulations and density functional theory (DFT) calculations. While the comparison of XAS spectra with atomic multiplet simulations show that the inversion degree is not affected by Pd decoration and, thus, can be excluded as a reason for the improved heating performance, XMCD reveals two interrelated responsible sources: significantly larger Fe and Co magnetic moments verify an increased total magnetization which enhances the magnetic heating ability. This is accompanied by a remarkable change in the field-dependent magnetization particularly for Co ions which exhibit an increased low-field susceptibility and a reduced spin canting behavior in higher magnetic fields. Using DFT calculations, these findings are explained by reduced superexchange between ions on octahedral lattice sites via oxygen in close vicinity of Pd, which reinforces the dominating antiparallel superexchange interaction between ions on octahedral and tetrahedral lattice sites and thus reduces spin canting. The influence of the delocalized nature of Pd 4d electrons on the neighboring ions is discussed and the conclusions are illustrated with spin density isosurfaces of the involved ions. The presented results pave the way to design nanohybrids with tailored electronic structure and magnetic properties.

Unusual anisotropic magnetic orbital moment obtained from x-ray magnetic circular dichroism in a multiferroic oxide system

The electric-field control of $d$-electron magnetism in multiferroic transition metal oxides is attracting widespread interest for the underlying fundamental physics and for next generation spintronic devices. Here, we report an extensive study of the $3d$ magnetism in magnetoelectric Ga$_{0.6}$Fe$_{1.4}$O$_3$ (GFO) epitaxial films by polarization dependent x-ray absorption spectroscopy. We found a non-zero integral of the x-ray magnetic circular dichroism, with a sign depending upon the relative orientation between the external magnetic field and the crystallographic axes. %By reliably enlarging the limit of the spin and orbital sum rules, which usually holds for materials where the magnetic ions exhibit a unique crystal field symmetry This finding translates in a sign-reversal between the average Fe magnetic orbital and spin moments. Large Fe-displacements, among some of the octahedral sites, lower the symmetry of the system producing anisotropic paths for the Fe-O bondings giving rise to a large orbital-lattice interaction akin to a preferential crystallographic direction for the magnetic orbital moment. The latter may lead to a partial re-orientation of the magnetic orbital moment under an external magnetic field that, combined to the ferrimagnetic nature of the GFO, can qualitatively explain the observed sign-reversal of the XMCD integral. The results suggest that a control over the local symmetry of the oxygen octahedra in transition metal oxides can offer a suitable leverage over the manipulation of the effective orbital and spin moments in magnetoelectric systems.

Strain-induced modulation of temperature characteristics in ferrimagnetic Tb\u2013Fe films

This study investigates the effect of strain on the compensation temperature of ferrimagnetic Tb–Fe films formed on a flexible substrate. The compensation temperature is determined by the anomalous Hall measurement, and an application of 1.2% tensile strain reduces the compensation temperature by 12 K. X-ray magnetic circular dichroism reveals that approximately 5% of Fe magnetic moment and approximately 1% of Tb magnetic moment are reduced by an application of 0.9% tensile strain at the room temperature. To understand the greater reduction in Fe magnetization compared with that in Tb and the compensation temperature reduction simultaneously, a model applying molecular field theory is analyzed. Changes in three types of exchange coupling between Fe and Tb atoms are speculated to be caused by the strain.

NMR study of magnetic structure and hyperfine interactions in the binary helimagnet FeP

We report a detailed study of the ground state helical magnetic structure in monophosphide FeP by means of ${}^{31}$P NMR spectroscopy. We show that the zero-field NMR spectrum of the polycrystalline sample provides strong evidence of an anisotropic distribution of local magnetic fields at the P site with substantially lower anharmonicity than that found at the Fe site by M\"{o}ssbauer spectroscopy. From field-sweep ${}^{31}$P NMR spectra we conclude that a continuous spin-reorientation transition occurs in an external magnetic field range of 4 - 7 T, which is also confirmed by specific-heat measurements. We observe two pairs of magnetically inequivalent phosphorus positions resulting in a pronounced four-peak structure of the single crystal ${}^{31}$P NMR spectra characteristic of an incommensurate helimagnetic ground state. We revealed a spatial redistribution of local fields at the P sites caused by Fe spin-reorientation transition in high fields and developed an effective approach to account for it. We demonstrate that all observed ${}^{31}$P spectra can be treated within a model of an isotropic helix of Fe magnetic moments in the ($ab$)-plane with a phase shift of 36$^{\circ}$ and 176$^{\circ}$ between Fe1-Fe3 (Fe2-Fe4) and Fe1-Fe2 (Fe3-Fe4) sites, respectively, in accordance with the neutron scattering data.