Nearest-neighbor Fe Atoms Research Articles

Mössbauer analysis of (FeNiCrSiB) amorphous alloy ribbons

The fabrication of iron-based amorphous alloys was executed through the utilization of the melt-spinning technique, followed by a characterization employing X-ray diffraction (XRD), scanning electron microscopes (SEM) and transmission Mössbauer spectroscopy (MS). The studies were performed on Fe76-xNixCr4Si8B12 (where x = 0, 4, 12, and 30) metallic glasses in the form of ribbons. Mössbauer spectroscopy enables the exploration of the localized surroundings of iron (Fe) atoms within the glassy state, revealing alterations in the amorphous structure attributed to variations in the addition of Nickel (Ni). The broad lines in the Mössbauer spectra of ferromagnetic amorphous ribbons are a result of the distribution of non-equivalent iron sites and interatomic distances. The line intensity ratio suggests a preferential orientation of iron moments within the ribbon plane. The results derived from Mössbauer analysis indicate that the amorphous alloys feature bimodal distributions of the hyperfine fields. Changes in the nickel content within the alloys impact the disorder in the as-cast state and concurrently influence parameters such as the average hyperfine field <Hhyp>, the average radius R of the first coordination shell, and the number of nearest neighbor Fe atoms.

Effects of strain and defects on hydrogen diffusion and trapping in [formula omitted]-iron using a developed Fe-H reactive force field

Strain and defects significantly affect the diffusion and trapping of hydrogen (H), which plays a critical role in the initiation of hydrogen embrittlement phenomena in α-iron. In this work, based on a newly developed Fe-H reactive force field, the molecular dynamic models with perfect lattice and defects across various strain regions are presented for studying diffusion and aggregation of H atoms in α-iron. At first, the effect of gradient strain on H diffusion and trapping are elucidated. Then, H diffusion and trapping behaviors at defects (including vacancies, edge dislocations, Σ3[11̄0](112) and Σ5[001](310) grain boundaries) undergoing various strain are investigated. The simulation results demonstrate that the gradient strain leads to a gradient distribution of H solution energies, which drives a directional diffusion and aggregation of H atoms from compressive to tensile strain region. The charge transfer from nearest neighbor Fe atoms to H atoms weakens the Fe-Fe interactions, consequently resulting in the local stress relaxation within the H-enrich region. Regarding the H diffusion and trapping at defects, the results show that tensile strain decreases the difference in H solution energies between the bulk structure and defects, which facilitates H diffusion and escape. Conversely, compressive strain amplifies the energy difference, leading to the enhancement of H trapping and inhibition of H transport. This study contributes to an enhanced comprehension of interactions between hydrogen and defects under strain from an atomic scale.

Study on the flotation mechanism of cobalt-bearing pyrite: A DFT calculation

The adsorption mechanism of flotation collector methyl xanthate on the pure and Co-doped FeS2 (100) crystal plane was investigated by density functional theory (DFT) calculation. For the purpose of comprehensive calculation, the top sites on the two nearest neighbor Fe atoms (TFe-Fe-I) of the FeS2 (100) crystal plane, and on the one Co atom and one nearest neighbor Fe (TFe-Co) as well as on the top sites on the two nearest neighbor Fe atoms (TFe-Fe-II) of the Co-doped FeS2 (100) crystal plane are taken into account, respectively. The adsorption energy analysis showed that the TFe-Co sites on the Co-doped FeS2 (100) crystal plane were more favorable for methyl xanthate adsorption. Furthermore, From the charge density difference and Bader charge analysis results of the most stable adsorption configuration of methyl xanthate on TFe-Co sites of the Co-doped FeS2 (100) crystal plane, it can be clearly seen that the direction of charge transfer is S to Fe and Co after methyl xanthate adsorption on mineral surfaces. The results of projected density of states (PDOS) and crystal orbital Hamilton population (COHP) in terms of the complex interactions atomic orbitals and chemical bonds further reveal that Co substitution of Fe in FeS2 promoted the adsorption of methyl xanthate onto the Co-doped FeS2 (100) surface.

Effects of RE (La, Ce) on fcc-bcc martensitic transformation of iron via Bain transformation path

The fundamental understanding of the effects of rare earth (RE) on the phase transformation of Fe-based alloys is essential in the development of advanced RE-containing high strength steels. In this work, we investigated the effects of La and Ce on the martensitic phase transformation of Fe in terms of the driving force and minimum energy path (MEP) using the first-principles calculations. The calculations of formation energy show that the RE substitution increases the stabilization of fcc-Fe and decreases that of bcc-Fe, thus decreasing the driving force of phase transformation. Meanwhile, the analysis based on the generalized solid-state nudged elastic band (G-SSNEB) method indicates that in the RE-containing systems, an additional energy barrier appears when the phase transformation proceeds along the Bain transition path to c/a ratio of 1.14 in the FM state, which is associated with a sudden decrease in magnetization due to the hybridization between the orbits of the RE atoms and those of the nearest neighbor Fe atoms. The results of potential-energy surfaces (PES) confirm that magnetic ordering and lattice volume change simultaneously with the Bain transition, and in comparison to pure Fe, the variation between the onset and the end structural volumes of the phase transformation is slight for the RE-containing Fe systems.

NiFe local ordering in segregated Ni3Fe alloys: A simulation study using angular dependent potential

In this work, we analyze the compositional disordering in the Ni3Fe alloy (with surface layers exposed to vacuum) at elevated temperatures in simulations using a Modified Embedded Atom potential with the Angular Dependent terms (ADP). We find that the Fe enrichment primarily occurs in the sub-surface layers of the alloy samples with (1 0 0), (1 1 0), and (1 1 1) orientated surfaces exposed to vacuum at elevated temperatures. Nickel enrichment occurs primarily in the surface layers with the (1 0 0) and (1 1 0) orientations. In the case of (1 1 1) orientated surfaces exposed to vacuum, the surface layer composition (25% Fe, 75% Ni) is not significantly altered even at a relatively high temperature of 1100 K. Our findings are qualitatively in agreement with the experimental and density functional theory (DFT) studies in the literature on the segregation in Ni3Fe alloys. We find that, irrespective of the orientation of the surface layers, Fe segregation (in the bulk of the alloy) predominantly occurs so as to achieve a local ordering similar to that in the NiFe alloy. In particular, the Fe atoms form planar 5-Fe atom clusters in the (1 0 0) planes, which are sandwiched between pure Ni layers with the (1 0 0) orientations. Thus, the central Fe atom of the 5-Fe atom cluster in the (1 0 0) plane has a local environment (in terms of nearest neighbor atoms) similar to that of a Fe atom in the NiFe alloy. We also find that a significant proportion of disordered Ni atoms in the bulk are associated with the Fe segregation in the form of 5-Fe atom clusters (i.e., NiFe local ordering). Furthermore, in the disordered Ni3Fe alloy, the Ni atoms with pure Ni nearest neighbor environment (i.e., with no nearest neighbor Fe atoms) exist primarily in the bulk, while in the surface layers the percentage of such Ni atoms is relatively insignificant.

Evolution of the structural and multiferroic properties of PbFe2/3W1/3O3 ceramics upon Mn-doping

The perovskite system Pb(Fe1-xMnx)2/3W1/3O3 (0 ≤ x ≤ 1, PFMWO) has been prepared by conventional solid-state reaction under different sintering conditions. Structures and phase composition as well as thermal, magnetic and dielectric properties of the compounds have been systematically investigated experimentally and by first-principles density functional calculations.A clean perovskite phase is established at room temperature for compositions 0 ≤ x ≤ 0.4. Rietveld refinements of X-ray and neutron powder diffraction patterns demonstrate that the compounds crystallize in space group Pm-3m (0 ≤ x ≤ 0.4). The degree of ordering of the Fe and W/Mn cations was found to depend on the concentration of Mn. First-principles calculations suggest that the structural properties of PFMWO are strongly influenced by the Jahn-Teller effect.The PFMWO compounds behave as relaxor ferroelectrics at weak Mn-doping with a dielectric constant that rapidly decreases with increasing Mn content. A low temperature antiferromagnetic G-type order with propagation vector k = (1/2,1/2,1/2) is derived from neutron powder diffraction data for the samples with x ≤ 0.4. However with increasing doping concentration, the magnetic order is perturbed. First-principles calculations show that the dominant exchange coupling is antiferromagnetic and occurs between nearest neighbor Fe atoms. When the system is doped with Mn, a relatively weak ferromagnetic (FM) interaction between Fe and Mn atoms emerges. However, due to the presence of this FM interaction, the correlation length of the magnetic order is greatly shortened already at rather low doping levels.

The Structure and Magnetic Moment Study of Fe–Si–Al by First-Principles Calculation

In general, the contribution to the reinforcement of the magnetism in the order–disorder transition of FeAl alloys depends on their Fe content and the order–disorder transition. However, the Fe75Si15Al10 alloys were investigated by means of first-principles calculation and experimental verification. We found that the mechanism of the magnetic moment ( $\mu _{m}$ ) reinforcement is the exchange interaction of Fe atom with its nearest neighbor Fe atoms. In addition, the exchange interaction of Fe–Fe atoms is affected by internal stress, lattice distortion, the Fe content, and the position of nearest neighbor nonferrous atoms (Si and Al atoms). At the annealing temperature of 600 °C , the $\mu _{m}$ is the maximum value, as the ordered DO3 structure is transformed extremely.

Extended x-ray absorption fine structure study of MnFeP0.56Si0.44 compound**Project supported by the National Natural Science Foundation of China (Grant Nos. 51461035, 51161017, and 11404176), the Scientific Research Projects of the Higher Educational Department of Inner Mongolian Autonomous Region (Grant No. NJZZ14033). The XAFS measurement was performed under the approval of Photon Factory Program Advisory Committee (Proposal Nos. 2012G095 and 2014G047).

The MnFeP0.56Si0.44 compound is investigated by x-ray diffraction, magnetic measurements, and x-ray absorption fine structure spectroscopy. It crystallizes in Fe2P-type structure with the lattice parameters a = b = 5.9823(0) Å and c = 3.4551(1) Å and undergoes a first-order phase transition at the Curie temperature of 255 K. The Fe K edge and Mn K edge x-ray absorption fine structure spectra show that Mn atoms mainly reside at 3g sites, while 3f sites are occupied by Fe atoms. The distances between the absorbing Fe atom and the first and second nearest neighbor Fe atoms in a 3f-layer shift from 2.65 Å and 4.01 Å in the ferromagnetic state to 2.61 Å and 3.96 Å in the paramagnetic phase. On the other hand, the distance between the 3g-layer and 3f-layer changes a little as 2.66 Å–2.73 Å below the Curie temperature and 2.68 Å–2.75 Å above it.

Comparative study of pure and Co-dopedBaFe2As2

We present a comparative calculation of the electronic structure of the high critical temperature superconductor Co-doped ${\text{BaFe}}_{2}{\text{As}}_{2}$ and its parent compound at the electron correlation level by the embedded cluster method; the electron correlation is calculated through the second-order M\o{}ller-Plesset perturbation theory. The superconducting doped material is represented by the ${\text{Ba}}_{4}{\text{CoFe}}_{4}{\text{As}}_{8}$ cluster. The analysis of the orbital populations in this cluster reveals the formation of an antiferromagnetic order in the Fe plane with a spin-density increase on the central Co atom with respect to the spin density of the central Fe atom of the undoped case. This increase is associated with an increase of the ${d}_{{z}^{2}}$ orbital population of the central atom. However, the formation mechanism of the local magnetic moment implies also a spin transfer from the nearest-neighbor Fe atoms and from the next-nearest-neighbor As atoms to the central Co atom, and it corresponds to a ${J}_{1}\ensuremath{-}{J}_{2}$ antiferromagnetic Heisenberg model. Some particular features of ${d}_{yz}$ and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals in the triplet and in the singlet cluster states are interpreted to correspond to a spinless fermion. This result, as well as the result relative to the formation mechanism of the magnetic moments, can be connected with a model of resonating-valence-bond (RVB) superconductors suggested recently by Poilblanc et al. [Phys. Rev. B 89, 241106 (2014)] and based on the Anderson RVB theory.

First-principles Calculation of Effects of Carbon on Tetragonality and Magnetic Moment in Fe–C System

The effects of carbon content on the tetragonality and magnetic moment of the Fe–C system have been evaluated using the first-principles calculation. Three types of supercell, Fe54C1, Fe54C2, and Fe128C1 (which correspond to Fe-0.40C, Fe-0.79C, and Fe-0.17C mass%, respectively) are used for the calculation. The main results are as follows: (1) The total and mechanical energies of the Fe–C system with carbon atoms at the octahedral sites are smaller than those of the system with carbon atoms at the tetragonal sites. The carbon atom at the octahedral site produces a relatively large expansion in one direction; (2) The tetragonality of the Fe–C system obtained using the first-principles calculation increases linearly with increasing carbon content and agrees well with experimental results. The average magnetic moment of the Fe atom increases with increasing carbon content; (3) The magnetic moment of the Fe atom at the carbon atom nearest neighbor site is lower than that of pure iron and increases with increased distance between the Fe and carbon atoms. The projected density of states exhibits a hybridization with the main contributions being from Fe d and C p states, which leads to the abovementioned decrease in the magnetic moment of the Fe atom. (4) In Fe54C2, the tetragonality and magnetic moment of the Fe atom change with the distance between two carbon atoms, with the tetragonality being 0.981, 1.036, or 1.090. When the Fe–C–Fe pair, which consists of the first carbon atom and its two nearest neighbor Fe atoms, is perpendicular to the second pair, which consists of the second carbon atom and its two nearest neighbor Fe atoms, the tetragonality is 0.981 and does not agree with the experimental value. The mechanical energy is relatively large. On the other hand, when the first pair is parallel to the second pair, the tetragonality is 1.036, which agrees well with experimental data. In this case, the mechanical energy is relatively small. When a straight C–Fe–C pair is formed, the tetragonality is 1.090; (5) In an Fe54C2 supercell, the formation enthalpy is relatively low when the calculated tetragonality is 1.036, and the existence probability under the assumption of a Boltzmann distribution is high. In other cases, the formation enthalpy is relatively high and the existence probability is almost zero; (6) The average magnetic moment of an Fe atom is proportional to volume, but it is not clearly related to the tetragonality. It is believed that the increase in the magnetic moment of the Fe atom by the addition of a carbon atom is primarily due to the magnetovolume effect, and is not due to the tetragonality effect.

Effect of hydrogen adsorption on the magnetic properties of a surface nanocluster of iron

The effect of hydrogen adsorption on the magnetic properties of an Fe${}_{3}$ cluster immersed in a Cu(111) surface has been calculated using density functional theory and the results used to parametrize an Alexander-Anderson model which takes into account the interaction of $d$ electrons with itinerant electrons. A number of adatom configurations containing one to seven H atoms were analyzed. The sequential addition of hydrogen atoms is found to monotonically reduce the total magnetic moment of the cluster with the effect being strongest when the H atoms sit at low-coordinated sites. Decomposition of the charge density indicates a transfer of 0.3 electrons to each of the H atoms from both the Fe atoms and from the copper substrate, irrespective of adsorption site and coverage. The magnetic moment of only the nearest neighbor Fe atoms is reduced, mainly due to increased population of minority spin $d$ states. This can be modeled by increased indirect coupling of $d$ states via the conduction $s$ band in the Alexander-Anderson model.

Electronic origin of the anomalous segregation behavior of Cr in Fe-rich Fe-Cr alloys

The energy of segregation of Cr from the bulk to the topmost surface layer in dilute Fe-Cr alloys is endothermic but small. On the other hand, the segregation energy from the bulk to the subsurface layer is not only endothermic but relatively large. Thus, the subsurface layer presents a huge barrier, not only for the segregation of Cr from the bulk to the surface, but also for its diffusion from the surface into the bulk. This means that the topmost layer alone cannot be used to determine the barrier for Cr segregation (the segregation energy), which is determined in these alloys by the subsurface layer. We present the results of our ab initio electronic structure calculations on the segregation of Cr as an impurity in Fe, which shed some light on the origin of this anomalous behavior. We find that the interaction of a Cr atom situated in the subsurface layer, in contrast to those in the other layers, is quite complex with its nearest neighbor Fe atoms due to the magnetism of both Fe and Cr, leading to a highly unfavorable electronic structure. This in turn leads to a large endothermic energy of solution of Cr in the subsurface layer. These results are important for a proper understanding of the properties of the technologically important class of Fe-Cr steels with low concentrations of chromium.

Crystal structure, phase transition, and magnetic ordering in perovskitelikePb2−xBaxFe2O5solid solutions

The crystal and magnetic structures of the $x\ensuremath{\approx}1$ member of the ${\text{Pb}}_{2\ensuremath{-}x}{\text{Ba}}_{x}{\text{Fe}}_{2}{\text{O}}_{5}$ solid solution series have been studied using x-ray and neutron powder diffraction, electron diffraction, high-resolution electron microscopy, and M\"ossbauer spectroscopy. ${\text{Pb}}_{1.08}{\text{Ba}}_{0.92}{\text{Fe}}_{2}{\text{O}}_{5}$ has two polymorphic forms with the orthorhombic unit cell with $a\ensuremath{\approx}\ensuremath{\surd}2{a}_{p}$, $b\ensuremath{\approx}{a}_{p}$, and $c\ensuremath{\approx}4\ensuremath{\surd}2{a}_{p}$ (${a}_{p}$---the parameter of the perovskite subcell) with the $Pnma$ space group of the low-temperature (LT) phase and the $Imma$ space group of the high-temperature (HT) phase, which are related by a phase transition at ${T}_{c}\ensuremath{\approx}540\text{ }\text{K}$. The crystal structures of both polymorphs were refined from neutron powder-diffraction data at $T=14\text{ }\text{K}$ and $T=700\text{ }\text{K}$. The structure consists of parallel perovskite blocks with the thickness of two ${\text{FeO}}_{6}$ octahedra linked together by infinite chains of edge-sharing distorted ${\text{FeO}}_{5}$ trigonal bipyramids with two columns of the Pb cations in between characterized by the asymmetric coordination environment due to localized $6{s}^{2}$ lone electron pair. Two mirror-related configurations of the trigonal bipyramidal chains are ordered in the LT structure; their arrangement becomes disordered in the HT structure. Below ${T}_{N}=625\text{ }\text{K}$, ${\text{Pb}}_{1.08}{\text{Ba}}_{0.92}{\text{Fe}}_{2}{\text{O}}_{5}$ transforms into an antiferromagnetically ordered state. The antiferromagnetic (AFM) structure with a propagation vector $\mathbf{k}=[0,\frac{1}{2},\frac{1}{2}]$ is characterized by an antiparallel spin alignment for all nearest-neighbor Fe atoms in the perovskite blocks, which stack on to each other at the trigonal bipyramidal chains, resulting in alternating antiparallel and parallel arrangement of spins on going along the common edge of the ${\text{FeO}}_{5}$ trigonal bipyramids. An unusual spin flipping dynamic behavior was revealed by M\"ossbauer spectroscopy and related to a specific character of superexchange interactions inside the chains of the ${\text{FeO}}_{5}$ trigonal bipyramids.

Poly[chlorido-μ3-1,2,4-triazolato-iron(II)

The title compound, [Fe(C2H2N3)Cl]n, was prepared from a hydro­thermal reaction of iron(II) chloride and 1,2,4-triazole. It is isostructural with its MnII, CoII, NiII and ZnII analogues. The FeII cation shows a slightly distorted square-based pyramidal coordination environment, being surrounded by three crystallographically independent N atoms of three different triazolate ligands and a chloride ligand. A polymeric layer is formed by the triply bridging nature of the 1,2,4-triazolate ligand, which is bonded to three different Fe atoms through its three N atoms. The layer contains both binuclear units and tetra­nuclear units. In the binuclear units, two Fe atoms are bridged by two nearly coplanar triazolate groups through the 1,2-positions, affording a six-membered ring around an inversion center. Each binuclear unit is further connected to four parallel units through the coordination of the N atoms of the triazolate groups. Four adjacent units, which are pairwise parallel, afford 16-membered tetra­nuclear units, in each of which the two nearest-neighbor Fe atoms are bridged by a single triazolate group through the 1,4-positions.

Charge transfer in FeO: A combined molecular-dynamics and ab initio study

Molecular-dynamics simulations and ab initio electronic structure calculations were carried out to determine the rate of charge transfer in stoichiometric wustite (FeO). The charge transfer of interest occurs by II/III valence interchange between nearest-neighbor Fe atoms, with the Fe(III) constituting a "hole" electronic defect. There are two possible nearest-neighbor charge transfers in the FeO lattice, which occur between edge-sharing or corner-sharing FeO(6) octahedra. Molecular-dynamics simulations predict charge-transfer rates of 3.7 x 10(11) and 1.9 x 10(9) s(-1) for the edge and corner transfers, respectively, in good agreement with those calculated using an ab initio cluster approach (1.6 x 10(11) and 8.0 x 10(8) s(-1), respectively). The calculated rates are also similar to those along the basal and c-axis directions in hematite (alpha-Fe(2)O(3)) determined previously. Therefore, as is the case for hematite, wustite is predicted to show anisotropic electrical conductivity. Our findings indicate that a rigid-ion model does not give acceptable results, thus showing the need to account for the change in polarizability of the system upon charge transfer. Our model achieves this by using a simple mechanical shell model. By calculating the electronic coupling matrix elements for many transition state configurations obtained from the molecular-dynamics simulations, we found evidence that the position of the bridging oxygen atoms can greatly affect the amount of electronic coupling between the donor and acceptor states. Finally, we address the effect of oxygen vacancies on the charge transfer. It was found that an oxygen vacancy not only creates a driving force for holes to transport away from the vacancy (or equivalently for electrons to diffuse toward the vacancy) but also lowers the free-energy barriers for charge transfer. In addition, the reorganization energy significantly differed from the nondefective case in a small radius around the defect.

First-principles study on the effect of impurities at the front of cracks inα−Fe

By using two first-principles methods, discrete variational method (DVM) and DMol, within the framework of density-functional theory, the effects of nonmetal light impurities (C, N, and O) on a fracture in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}$ are studied. We calculate the curve of binding energy $({E}_{b})$ vs the displacements of the nearest-neighbor Fe atoms of the impurity, and find that C and N tend to hold onto these two Fe atoms. Calculated PDOS and deformation charge density reveal that C and N prefer to form polarized bonds vertical to the propagation direction of the crack with the nearest-neighbor Fe atoms. Furthermore, interaction of the C $(\mathrm{N})\ensuremath{-}\mathrm{Fe}$ pair which is vertical to the crack plane is much larger than that of the C $(\mathrm{N})\ensuremath{-}\mathrm{Fe}$ pair which is parallel to the plane. This anisotropic bonding character indicates that doping C or N impedes the propagation of the crack, thus can improve the mechanical property of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}$. On the contrary, O tends to push its nearest-neighbor Fe atoms away, and exhibits weak interaction with its nearest-neighbor atoms. O also weakens the interaction of $\mathrm{Fe}\ensuremath{-}\mathrm{Fe}$ pair in bulk. Thus O has detrimental effects on the ductility of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}$.

Properties of incommensurate spin density waves in iron aluminides (invited)

Neutron diffraction in Fe(Al) reveals incommensurate spin density waves (SDWs) in alloys known to be spin glasses. The wave vectors for crystals of Fe(34Al), Fe(40Al) and Fe(43Al) show n varying from 11 to 6 for q=2π(h±1/n,k±1/n,l±1/n)/ao, where (h,k,l) and ao characterize the parent bcc lattice of the CsCl structure. The magnetic reflections are present far above the spin-glass freezing temperatures. These SDWs keep the spins on nearest-neighbor Fe atoms close to parallel, in contrast with SDWs in Cr, which keep nearest-neighbor spins close to antiparallel. The competition between near-neighbor Fe–Fe ferromagnetism and 180° superexchange through the Al site has been used to explain the spin-glass behavior, but the appearance of the SDWs calls for a more fundamental source of the periodicity. The phase shift mechanism for SDW interactions with magnetic moments is invoked to explain the breadth of the peaks, which resemble the results for Cu(Mn), Pd(Mn), and Pd(Cr). The data are interpreted using cubic symmetry, but it has yet to be established whether the wave vectors all occur in a single domain or whether there are multiple domains. There are 48 wave vectors of magnitude almost equal to the {110} wave vectors of the bcc lattice, which could stabilize the SDWs by spanning the Fermi surface. These unanticipated results should have pervasive ramifications for the theory of metallic magnetism.

Incommensurate Spin Density Waves in Iron Aluminides

Neutron diffraction in Fe(Al) reveals incommensurate spin density waves (SDWs) in alloys known to be spin glasses. The wave vectors for crystals of Fe(34Al), Fe(40Al), and Fe(43Al) show n varying from 11 to 6 for q-->=2pi(h+/-1/n,k+/-1/n,l+/-1/n)/a(0), where (h,k,l) and a(0) characterize the parent bcc lattice of the CsCl structure. The magnetic reflections are present far above the spin-glass freezing temperatures. These SDWs keep the spins on nearest-neighbor Fe atoms close to parallel, in contrast with SDWs in Cr, which keep nearest-neighbor spins close to antiparallel.

Ordering transformation and magnetic properties of Fe 59.75Pt 39.5Nb 0.75

We have investigated the effect of various heat treatments on the face-centered-cubic–face-centered-tetragonal transformation rate of Fe 59.75Pt 39.5Nb 0.75 bulk alloys. The degree of atomic ordering accompanying this transformation leads to substantial changes in the Curie temperature, which we discuss in terms of a changing number of nearest-neighbor Fe atoms. Additional information on the phase transformation and on the degree of atomic ordering was obtained from the occurrence of the Hopkinson effect and from the c/ a ratio and the line width derived from X-ray diffraction. We show that the initial homogenization temperature has a strong influence on the ultimate magnetic properties obtained in a subsequent low-temperature annealing treatment.

A Theoretical Study of a H–H Pair on the BCC Fe(100) Surface

Hydrogen adsorption on Fe(100) was analyzed using a semiempirical theoretical method. Calculations were performed using a Fe 130 cluster. Adsorption sites for one and two hydrogen atoms on the surface correspond to local energy minima configurations. Changes in the electronic structure of surface Fe atoms were analyzed for the system without hydrogen and with one and two adsorbed hydrogen atoms. Fe atoms close to H weaken their metallic bond. This is due to the formation of H–Fe bonds. Hydrogen influences only its nearest neighbor Fe atoms. The H–H interaction was also analyzed and our results show that H–Fe interaction is much stronger than any possible H–H interaction. No additional decohesion is observed in the Fe–Fe bonds; however, more Fe–Fe bonds are affected.