Magnetic Moments Of Iron Atoms Research Articles

Separating the contributions of the magnetic subsystems in antiferromagnetic ferrihydrite nanoparticles by analyzing the magnetization in fields of up to 250 kOe

Contributions of different magnetic subsystems formed in the systems of synthetic ferrihydrite nanoparticles (characterized previously) with an average size of < d>≈2.7 nm coated with polysaccharide arabinogalactan in different degrees have been separated by measuring the dependences of their magnetization M on magnetic field H of up to 250kOe on vibrating sample and pulsed magnetometers. The use of a wide measuring magnetic field range has been dictated by the ambiguity in identifying a linear M(H) portion for such antiferromagnetic nanoparticle systems within the conventional field range of 60–90 kOe. The thorough analysis of the magnetization curves in the temperature range of 100–250 K has allowed the verification of the contributions of (i) uncompensated magnetic moments µun in the superparamagnetic subsystem, (ii) the subsystem of surface spins with the paramagnetic behavior, and (iii) the antiferromagnetic susceptibility of the antiferromagnetically ordered ferrihydrite particle core. As a result, a model of the magnetic state of ferrihydrite nanoparticles has been proposed and the numbers of spins corresponding to magnetic subsystems (i)–(iii) have been estimated. An average magnetic moment μunof ∼ 145μB (μB is the Bohr magneton) per particle corresponds approximately to 30 decompensated spins of iron atoms in a particle (about 3 % of all iron atoms), which, according to the Néel’s hypothesis μun∼<d>3/2, are localized both on the surface and in the bulk of an antiferromagnetically ordered particle. The fraction of free (paramagnetic) spins is minimal in the sample without arabinogalactan coating of the nanoparticle surface (7 %) and is attained 20 % of all iron atoms in the sample with the highest degree of spatial separation of particles. According to this estimation, paramagnetic spins are located mainly on the edges and protruding areas of particles. Most magnetic moments of iron atoms are ordered antiferromagnetically and the corresponding magnetic susceptibility of this subsystem behaves as in an antiferromagnet with the randomly distributed crystallographic axes, i.e., increases with temperature.

Hydrogen-Induced Order–Disorder Effects in FePd3

Binary intermetallic compounds, such as FePd3, attract interests due to their physical, magnetic and catalytic properties. For a better understanding of their hydrogenation properties, both ordered FePd3 and disordered Fe0.25Pd0.75 are studied by several in situ methods, such as thermal analysis, X-ray powder diffraction and neutron powder diffraction, at moderate hydrogen pressures up to 8.0 MPa. FePd3 absorbs small amounts of hydrogen at room temperature and follows Sieverts’ law of hydrogen solubility in metals. [Pd6] octahedral voids are filled up to 4.7(9)% in a statistical manner at 8.00(2) MPa, yielding the hydride FePd3H0.047(9). This is accompanied by decreasing long-range order of Fe and Pd atoms (site occupancy factor of Fe at Wyckoff position 1a decreasing from 0.875(3) to 0.794(4)). This trend is also observed during heating, while the ordered magnetic moment decreases up to the Curie temperature of 495(8) K. The temperature dependences of the magnetic moments of iron atoms in FePd3 under isobaric conditions (p(D2) = 8.2(2) MPa) are consistent with a 3D Ising or Heisenberg model (critical parameter β = 0.28(5)). The atomic and magnetic order and hydrogen content of FePd3 show a complex interplay.

Interparticle magnetic interactions in synthetic ferrihydrite: Mössbauer spectroscopy and magnetometry study of the dynamic and static manifestations

Samples of synthetic ferrihydrite with an average nanoparticle size of 2.7 nm have been examined by magnetometry and Mössbauer spectroscopy. Ferrihydrite is characterized by the antiferromagnetic interactions between the magnetic moments of iron atoms. In ferrihydrite nanoparticles, as in any other antiferromagnetic ones, structural defects induce the formation of an uncompensated magnetic moment, which determines the magnetic properties typical of single-domain ferro- and ferrimagnetic particles. The manifestation of the magnetic interactions between ferrihydrite nanoparticles in the magnetic properties of the material and in the temperature evolution of Mössbauer spectra has been in focus. The results obtained on synthetic ferrihydrite have been compared with the data for the biogenic ferrihydrite sample with a similar average size of particles surrounded by a polysaccharide shell, which weakens and screens the interparticle magnetic interactions. A clear manifestation of the effect of the interparticle magnetic interactions on the transition to the blocked state is the presence of a significant contribution of the relaxation component in the Mössbauer spectra at temperatures of the transition from the superparamagnetic to blocked state. The temperature dependence of the particle relaxation time obtained from the Mössbauer spectra points out the collective effect of freezing of the magnetic moments of particles due to the magnetic interactions between them.

Metallic nature and site-selective magnetic moment collapse in iron oxide Fe4O5 at the extreme conditions of Earth's deep interior

Properties of iron oxides at the extreme conditions are of essential importance in condensed matter physics and Geophysics. The recent discovery of a new type of iron oxide, Fe4O5, at high pressure and high temperature of Earth's deep interior attracts great interests. In this paper, we report the electronic structure and the magnetic properties of Fe4O5 employing the density functional theory plus dynamic mean field theory (DFT+DMFT) approach. We find that Fe4O5 stays metallic from ambient pressure to high pressure. The magnetic moments of iron atoms at the three different crystallographic positions of Fe4O5 undergo position-dependent collapse as being compressed. Such site-selective magnetic moment collapse originates from the shift of energy levels and the consequent charge transfer among the Fe-3d orbits under compression.

Site-Selective Magnetic Moment Collapse in Compressed Fe5O6 Supported by the National Natural Science Foundation of China (Grant Nos. 11604255 and U1930401), and the Natural Science Basic Research Program of Shaanxi (Grant No. 2021JM-001).

Iron oxide is one of the most important components in the Earth’s mantle. The recent discovery of the stable presence of Fe5O6 in the Earth’s mantle environment has stimulated significant interests in understanding of this new category of iron oxides. We report the electronic structure and magnetic properties of Fe5O6 calculated by the density functional theory plus dynamic mean field theory (DFT + DMFT) approach. Our calculations indicate that Fe5O6 is a conductor at ambient pressure with dominant Fe-3d density of states at the Fermi level. The magnetic moments of iron atoms at three non-equivalent crystallographic sites in Fe5O6 collapse at significantly different rates under pressure. This site-selective collapse of magnetic moments originates from the shifting of energy levels and the consequent charge transfer among the Fe-3d orbits when Fe5O6 is being compressed. Our simulations suggest that there could be high conductivity and volume contraction in Fe5O6 at high pressure, which may induce anomalous features in seismic velocity, energy exchange, and mass distribution in the deep interior of the Earth.

Magnetic Anisotropy of Graphene-Coated Ultrathin Iron Layers on a Substrate of Hexagonal Boron Nitride Gr/Fe/h-BN

The theoretical results of the computer simulation of the magnetic properties of ultrathin iron films on a nonmagnetic substrate of hexagonal boron nitride Gr/Fe/h-BN have been obtained in terms of the spin density functional theory (SDFT). The main attention is focused on numerical studies of the magnetic anisotropy of iron films with the hexagonal structure similar to the crystal structure of intercalated iron films in the Gr/Fe/Ni(111) system. For this purpose, the total energies are calculated for ferromagnetic structures with different directions of the magnetic moments of iron atoms with respect to the film surface. The effect of graphene on the magnetic properties of the Gr/Fe/h-BN system is analyzed. The results of the theoretical simulation of Fe/h-BN and Gr/Fe/h-BN show that it is possible to obtain in this system quite “thick” iron films (dFe ≥ 10 A) that will have the perpendicular magnetic anisotropy.

Magnetic anisotropy and core-shell structure origin of the biogenic ferrihydrite nanoparticles

Ferrihydrite is a low-crystalline nanoscale matter. The uncompensated magnetic moment of the ferrihydrite caused by the antiferromagnetic ordering of the magnetic moments of iron atoms and leads to the magnetic properties very similar to those of ferro- and ferrimagnetic nanoparticles. In this study, we investigated the biogenic ferrihydrite nanoparticles with the narrow size distribution and an average diameter of ≈2 nm obtained by the bacteria life cycle. The features caused by the surface effects and the inhomogeneous structure of ferrihydrite have been examined in the temperature range of 4–300 K using Mössbauer spectroscopy and magnetometry. Based on the Mössbauer data, we identified the superparamagnetic blocking temperature at the temperature of 30 K for the largest ferryhidrite particles. We established that the exceptional magnetic anisotropy of ferrihydrite (KV=1.2⋅105 erg/cm3 and KS=0.1 erg/cm2) is reached because of the highly developed ferrihydrite nanoparticles’ surface. According to the Mössbauer data, we propose a core-shell structural model of the biogenic ferrihydrite particles. We found that the size of the dense core depends on the particle size. The well-crystallized core is formed only for nanoparticles larger than ≈2 nm, whereas smaller particles consist entirely of a matter with a lower density of iron atoms.

Magnetic susceptibility, EPR, NEXAFS and XPS spectra of Fe-doped CaBi2Nb2O9

In order to study the electronic state of Fe atoms and interatomic exchange interactions in solid solutions of Bi2CaNb2−2xFe2xO9−δ with perovskite structure, methods of magnetic dilution, NEXAFS, XPS and EPR spectroscopy were used. Increased values of the magnetic moment of iron atoms in solid solutions compared to pure spin values for Fe(III) are explained by the presence of clusters of Fe(III) atoms predominantly with antiferromagnetic type of exchange. Exchange parameters and cluster distribution in Bi2CaNb2−2xFe2xO9−δ solid solutions depending on the Fe content were calculated according to the Heisenberg–Dirac–van Vleck model. The effect of calcium atoms on the clusterization degree and intensity of Fe interatomic exchange interactions was displayed. XPS spectra of Bi2CaNb2-2xFe2xO9-δ, were identified to enable estimation of the effective charge of the atoms composing it. The analysis of the NEXAFS Fe2p-spectra of iron oxides and the iron-containing solid solutions revealed that Fe atoms were mainly in an oxidation state of +3. EPR spectra of Fe containing solid solution powder show an intense asymmetric line in the low-field part of the spectra at g=4.27 with weakly manifested shoulder g∼8–10 and a broad component in the g∼2 range with a high-field wing.

Computer Simulation of Energy Parameters and Magnetic Effects in Fe-Si-C Ternary Alloys

The paper presents ab initio simulation with the WIEN2k software package of the equilibrium structure and properties of silicon and carbon atoms dissolved in iron with the body-centered cubic crystal system of the lattice. Silicon and carbon atoms manifest a repulsive interaction in the first two nearest neighbors, in the second neighbor the repulsion being stronger than in the first. In the third and next-nearest neighbors a very weak repulsive interaction occurs and tends to zero with increasing distance between atoms. Silicon and carbon dissolution reduces the magnetic moment of iron atoms.

Electron paramagnetic resonance and magnetic properties of α-BiNb1−xFexO4−δ

Iron-containing ceramic materials based on BiNbO4 of orthorhombic modification were obtained by solid-phase synthesis. The solid solutions α-BiNb1-xFexO4-δ were synthesized of х ≤ 0.03. The single-phase structure of these samples was established by X-ray diffraction and SEM- analyses. In the EPR spectra of BiNb1-xFexO4-δ, a wide band with a center at g = 2.16–2.37 were observed, which had a low-intensity signal with a g-factor of ~ 4.3 on the low-field wing. Along with the broad spectrum component, a "noise-like" signal was observed in the magnetic field range of 190-400 mT, which was formed by a large number of narrow lines related to the integral signal of ferromagnetic resonance and its fine structure. The isotherms of the paramagnetic component of the magnetic susceptibility of iron atoms in BiNb1-xFexO4-δ are typical for antiferromagnets. The value of the effective magnetic moment of iron atoms increases along with the increase in temperature from 5.84 MB (90 K) to 6.43 MB (320 K). This can be explained by the presence of exchange-bound aggregates of Fe (III) atoms with the antiferro- and ferromagnetic types of exchange in the solid solutions. Magnetic behavior of BiNb1-xFexO4-δ was investigated taking into account the presence of possible types of clusters of iron atoms by theoretical calculation of the magnetic susceptibility within the framework of the model of dilute solid solutions. The best agreement between the experimental and calculated data was obtained for the dimer parameters of Jdim - 17 cm-1 and Jdim 45 cm-1, for antiferro-and ferromagnetic type of exchange.

XPS spectra, electronic structure, and magnetic properties of RFe5Al7 intermetallics

The results of X-ray photoelectron spectroscopy measurements (core levels and valence bands) of RFe5Al7 (R = Lu, Tm, Er, Ho, Dy, Tb, Gd) single crystals are presented in comparison with the results of bulk magnetization studies and electronic structure calculations. It is shown that the increase of the Curie temperature in RFe5Al7 from Tm to Gd is associated with an increase of the indirect R 4f – Fe 3d exchange interaction at the expense of the multiplicity 2S + 1 (statistical weight) in the ground state 2S+1LJ of R3+ ions. The nonmonotonic behavior of the ferrimagnetic compensation temperature, Tcomp, as well as the values of the spontaneous magnetic moment, Ms, and formation energy, Eform, of the 4fn levels in R metals in a series from ErFe5Al7 to GdFe5Al7 are explained by the difference in the quantum numbers L, J and S of the ground state of R3+ ions, leading to a maximum value of Tcomp, Ms and Eform for the Dy-containing compound. The electronic structure of Gd/LuFe5Al7 is calculated using the GGA+U approach, on the basis of which the physical mechanism and relative strength of the interatomic R-Fe and Al-Fe interactions are considered, and also the difference in the magnetic moments of iron atoms in different structural positions is explained.

Magnetic susceptibility of solid solutions Bi2SrNb2−2xFe2xO9−δ

The state of iron atoms and their interatomic interactions were investigated by the method of magnetic dilution in Bi2SrNb2−2xFe2xO9−δ with a layered perovskite-like structure. It was found that the solid solutions are characterized by increased values of the magnetic moment of iron atoms, compared with purely spin values of Fe(III). This was explained by the presence of exchange-bound aggregates of Fe(III) atoms with antiferro- and ferromagnetic types of exchange. The dependencies of the exchange parameters and cluster distribution on the iron content of Bi2SrNb2−2xFe2xO9−δ solid solutions were calculated.

Characterisation of the Fe-10 wt% Si nanocrystalline powder obtained by mechanical alloying and annealing

Nanocrystalline Fe-10wt% Si powders has been obtained by mechanical alloying. The synthesis conditions as well as their influence on the structural and magnetic properties of this soft magnetic alloy have been investigated in the light of several experimental techniques (X-ray diffraction, neutron diffraction, DSC, magnetic measurement, SEM). By X-ray diffraction (XRD) investigations have been highlighted that Fe-Si alloy is formed after 4h of mechanical milling and the mean crystallite size is down to 14nm (8h milled sample). The heat treatment leads to the formation of the Fe3Si compound with DO3-type superstructure for low milling times. The saturation magnetisation of the as-milled powders decreases in the first 4h of milling due to the formation of the Fe-Si alloy. The behaviour of the saturation magnetisation versus milling time and annealing was explained by the different magnetic moment of iron atoms in Fe3Si structure. The Curie temperature decreases from 770°C (the Curie temperature for pure iron) to about 650°C after 8h of milling.

Ab initio моделирование межфазной границы феррит/цементит

The article presents results of first-principles modelling of the ferrite/cementite interface with the Isaichev orientation relationship. Periodicity and close arrangement of the atoms in the two arrays (ferrite and cementite) for this orientation relationship indicates that the surface has low formation energy. Structural model of the interface was developed. The best conjugation of the two lattices is provided when the (101)c plane consisting of iron atoms in S-type positions is located at the interface. The supercell selected for ab initio modelling contained 64 atoms: 56 iron (28 in both ferrite and cementite parts) and 8 carbon atoms (in the cementite part). Simulation was performed in the framework of density functional theory method of the full-potential linearised augmented-plane wave with generalised gradient approximation in WIEN2k package. The optimum parameters of the system simulation were selected and volume optimization of structure was carried out. Calculated formation energy of ferrite/cementite interface is 0.594 J/m2, which is good agreement with both the theoretical and experimental data. There is a good correlation between the values of the magnetic moment and the interplanar distance for the iron atoms at the interface. Magnetic moment of iron atoms

Mössbauer study of amorphous alloys Fe85–x Cr x B15 (x = 0–20)

Mossbauer spectra of amorphous alloys Fe85–x Cr x B15 (x = 0–20) are measured at 91 K. Distributions of 57Fe hyperfine fields are recovered from the experimental spectra. The data are analyzed using a nanocrystal model. It is found that atoms of chrome replace iron atoms in nanocrystals of type α-Fe with orthorhombic o-Fe3B and tetragonal Fe3B phases. The local magnetic moments of iron atoms in the nanocrystal phases are estimated. Replacing iron atoms with atoms of chrome sharply reduces the local magnetic moments of iron atoms in nanocrystals of amorphous alloys.

Structure and phase composition of films formed by ultradispersed particles of iron and carbon

Ion-plasma sputtering and the codeposition of ultradispersed iron and carbon particles are used to create their solid solutions containing 17.6 at % carbon. Solid-solution alloys formed immediately during deposition confirm the theory of the thermal-fluctuation melting of small particles. Coating deposition is accompanied by the generation of an X-ray amorphous carbide phase, the amount of which correlates with the initial C concentration in the sample. The coating possesses an amorphous structure if the carbon content is 23.9 at %. By means of nuclear gamma-resonance spectroscopy, it is found that the magnetic moments of iron atoms are anisotropically oriented due to the formed textured coatings. The deviation of crystallite orientations from the average direction of the texture axis depends on the degree of carbonization. Pearlite eutectic (α-Fe(C) + Fe3C) arises from the amorphous phase without the formation of intermediate carbides at a temperature of 500°C. The relative cementite content correlates with the amount of carbon embedded into the coating.

Structures, energetics and magnetic properties of Au&lt;sub&gt;n&lt;/sub&gt;SFe&lt;sub&gt;m&lt;/sub&gt; and Au&lt;sub&gt;n&lt;/sub&gt;Fe&lt;sub&gt;m&lt;/sub&gt; clusters for m = 1-5 and n = 1-5, 12 and 32

Simulations of nanoscale systems are important from the understanding point of view of the physics and chemistry involved in describing observed phenomenon. This paper presents the results of systematic theoretical investigation of the structural and magnetic properties of gold-iron complexes at small size scale. Ab initio calculations are performed for AunFem with and without the presence of a sulfur atom, i.e., the clusters AunSFem, where n, m = 1 - 5. The study also includes Au6SFe and Au12Fe in order to investigate how the fully wrapped Fe atom responds to the Au atoms .The study mainly focuses on the geometrical and magnetic changes with a step by step removal of the sulfur atom as a function of the cluster size. It is found that average Au-Fe bond length is increased with increasing number of atoms in the cluster. An increase of Au-Fe bond length would have increased the magnetic moment of the Fe atom, but due to the hybridization of Au and Fe orbitals, the moment converges to about 3.00 μB. This value is higher than the magnetic moment of Fe atom in bulk gold. An enhanced magnetic moment is found on Fe atom even if it is fully wrapped by the Au12 octahedral cluster. From this study it is found that, the cluster stability is increased on the addition of a single sulfur atom to the AunFem clusters. A special stability is observed in Au4SFe, Au6SFe, Au12SFe and Au4SFe2 clusters as the sulfur atom in these clusters is found to be doubly bonded. Generally, the systematic studies on the small sized clusters show an enhanced magnetic moment on the iron atoms bonded to the gold atoms as compared to the corresponding bare iron clusters. This indicates that, the magnetic moment of iron atoms can be enhanced by a complete coating with gold atoms for practical applications. This complete coating can prevent iron from oxidation and may also prevent coalescence of iron clusters and formation of thromboses. The coupling of iron atoms in this work remains ferromagnetic irrespective of the number of gold atoms in the cluster.Keywords: Cluster, Magnetic moment, Coating, Nanoparticles.

Three-sublattice model that takes into account the spin density anisotropy, short-range order, and dimensioned factor for the binary Fe-Cr(V, Mo) systems

The diffusion properties of the elements entering into the composition of austenitic and ferritic steels are analyzed as functions of the temperature and the time evolution of induced radioactivity. As compared to austenitic steels, ferritic steels are shown to have better diffusion properties and swelling resistance during operation at temperatures up to 650°C under irradiation conditions and can be used as structural materials in the core of a fast neutron reactor. Experimental and calculated (Calphad method) data on the Fe-Cr phase diagram and the thermodynamic properties and the experimental data on the short-range order in Fe-Cr alloys are analyzed, and it is concluded that they are conflicting and that models taking into account the short-range order should be developed. The results of quantum-mechanical calculations of the average magnetic moment for ferromagnetic (FM) iron as a function of the volume are analyzed and used to introduce a concept of partial magnetic moments of the iron atoms located in the first four coordination spheres (1–4 CS). The values of these moments are calculated. The concept of the partial magnetic moments of iron atoms agrees qualitatively with the experimental data on the spin-density anisotropy of the bcc lattice of pure iron. This concept is used to formulate a three-sublattice model for binary FM alloys of Fe-M systems (M is an alloying paramagnetic element). An extended cell whose sites contain 8 bcc cells and 16 atoms per cell and that is isomorphic to a DO3-type crystal lattice is considered. A functional is constructed for the internal energy on the extended 16-atom cell. The dimensioned factor is taken into account in a self-consistent manner, by the expansion of the interaction energies of atoms of both components located in different CSs in the atom displacement with respect to ideal lattice sites. The dimensioned factor is taken into account in the three-sublattice model to obtain a set of equations of state for bcc FM binary iron-rich alloys in a 1–3 CS approximation. The obtained estimates demonstrate that the anisotropy of the distribution of magnetic moments in the bcc iron lattice is responsible for the appearance of a short-range order in bcc FM iron-rich Fe-Cr (V, Mo) binary alloys.

Atomic and magnetic structures of Fe70Al5B25 amorphous alloys

The short-range order around boron, aluminum, and iron atoms in Fe75B25 and Fe70Al5B25 amorphous alloys has been studied by 11B and 27Al nuclear magnetic resonance at 4.2 K and 57Fe Mossbauer spectroscopy at 87 and 295 K. The average magnetic moment of iron atoms μ(Fe) in these alloys has been measured by a vibrating sample magnetometer. It has been revealed that the substitution of aluminum atoms for iron atoms does not disturb μ(Fe) in the Fe70Al5B25 alloy, gives rise to an additional contribution to the 11B NMR spectrum in the low-frequency range, and shifts maxima of the distribution of hyperfine fields at the 57Fe nuclei. In the Fe70Al5B25 amorphous alloy, the aluminum atoms substitute for iron atoms in the nearest coordination shells of boron and iron atoms. This alloy consists of nanoclusters in which boron and iron atoms have a short-range order of the tetragonal Fe3B phase type.

Geometry, electronic properties and magnetism of FenO+m(n+m=4) clusters

The initial geometries of FenO+m(n+m=4) clusters are built by using the "binomial" scheme. And 12 stable isomers are obtained by optimization and frequency analysis based on the density-functional theory GGA (PW91) and the all-electron numerical basis set. The binding energies, the energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, and the magnetic moments of all the stable isomers are determined and analyzed. It is found that Fe—O bonds are important for stabilizing the FenO+m clusters, and that the total magnetic moments of the isomers are mainly dependent on the magnetic moment of iron atoms and the arrangement of the magnetic moments of the atoms.