Fe Magnetic Moments Research Articles

Effect of local environment on moment formation in iron silicides

The effect of local environment on the formation of magnetic moments on Fe atoms in iron silicides is studied by combination of ab initio and model calculations. The suggested model includes all Fe d- and Si p-orbitals, intra-atomic Coulomb interactions, inter-atomic Fe-Fe exchange and hopping of electrons to nearest and next nearest neighboring atoms. The parameters of the model are found from the requirement that self-consistent moments on atoms and density of states found from ab initio and model calculations within the Hartree-Fock approximation are close to each other as much as possible. Contrary to the commonly accepted statement that in the ordered Fe3Si and FexSi1−x alloys an increase of the Si concentration within nearest environment of Fe atoms results in a decrease of Fe magnetic moment we find that a crucial role in the formation of magnetic moments is played by the second coordination sphere of Fe atoms. Particularly, the Fe atoms have higher magnetic moments in amorphous films compared to the epitaxial ones due to decreasing the number of iron-atoms in the next nearest environment. Both our model and ab initio calculations confirm existence of known spin crossover with pressure and predict second crossover at higher pressure.

Magnetic properties of sigma-phase FeCrX (X=Co, Ni) alloys: Experimental and theoretical study

Experimental (vibrating sample magnetometry) and theoretical (electronic structure calculations using self-consistent Korringa-Kohn-Rostoker Green's function method, also including the coherent potential approximation) studies were performed on a series of intermetallic σ-phase FeCrX (X = Ni, Co) compounds in systematic way. In σ-FeCrNi (Ni concentration is limited to ∼ 10 at%) a slight enhancement of magnetic properties when replacing Fe with Ni atoms and a slight attenuation of these properties when replacing Fe with Cr was experimentally observed. In the case of σ-FeCrCo, the reason for disappearance of magnetic properties was explained, since the substitution of Fe with Co leads to systematic attenuation of TC and μ¯. Consequently, σ-CoCr was established to be non-magnetic from accurate measurements of magnetisation for series of σ-FexCrCo samples with x tending to zero. The decrease of average magnetic moment in σ-FeCrCo well corroborates with the results of electronic structure calculations assuming different models for chemical disorder. In order to enlighten reasons for magnetism onset in σ-FeCrX, the KKR-CPA spin-polarised density of states and magnetic moments of Fe impurities diluted on all sublattices were computed in σ-CoCr and hypothetical σ-NiCr. In both cases, the largest Fe magnetic moment was calculated on B (∼2μB) and next C (∼1.3μB and D (∼0.5μB) sites, while almost vanishing values were found on A and E sites.

Exchange interactions and Curie temperatures in Fe2 NiZ compounds

Using the spin-polarized relativistic Korringa-Kohn-Rostoker method, we study the exchange interactions and Curie temperatures of Fe2 NiZ (Z= Al, Ga, Si, Ge) with the Hg2 CuTi structure. The Heisenberg exchange coupling parameters and Curie temperatures are obtained by calculating the exchange interactions between the constituents. It is found the nearest neighbor exchanges play a leading role in interactions, the calculated Curie temperatures, which are higher than the room temperature, decrease when changing Z from III to IV group. In addition, we study the influence of lattice constant on magnetic moments and Curie temperatures. It is found the Fe magnetic moments become more larger due to the reduction of atomic orbital overlap as the expansion of lattice parameters. Only small changes are observed for the moment of Ni atom, while for the Z atom, the magnetic moment is nearly constant. Finally, the Curie temperatures of Fe2 NiZ compounds under different lattice constants are acquired by using the mean-field approximation.

When Density Functional Approximations Meet Iron Oxides.

Three density functional approximations (DFAs), PBE, PBE+U, and Heyd-Scuseria-Ernzerhof screened hybrid functional (HSE), were employed to investigate the geometric, electronic, magnetic, and thermodynamic properties of four iron oxides, namely, α-FeOOH, α-Fe2O3, Fe3O4, and FeO. Comparing our calculated results with available experimental data, we found that HSE (a = 0.15) (containing 15% "screened" Hartree-Fock exchange) can provide reliable values of lattice constants, Fe magnetic moments, band gaps, and formation energies of all four iron oxides, while standard HSE (a = 0.25) seriously overestimates the band gaps and formation energies. For PBE+U, a suitable U value can give quite good results for the electronic properties of each iron oxide, but it is challenging to accurately get other properties of the four iron oxides using the same U value. Subsequently, we calculated the Gibbs free energies of transformation reactions among iron oxides using the HSE (a = 0.15) functional and plotted the equilibrium phase diagrams of the iron oxide system under various conditions, which provide reliable theoretical insight into the phase transformations of iron oxides.

Magnetic properties and Curie temperatures of disordered Heusler compounds:Co1+xFe2−xSi

The local atomic environments and magnetic properties were investigated for a series of Co(1+x)Fe(2-x)Si (0<x<1) Heusler compounds. While the total magnetic moment in these compounds increases with the number of valance electrons, the highest Curie temperature (Tc) in this series was found for Co1.5Fe1.5Si, with a Tc of 1069 K (24 K higher than the well known Co2FeSi). 57Fe M\"ossbauer spectroscopy was used to characterize the local atomic order and to estimate the Co and Fe magnetic moments. Consideration of the local magnetic moments and the exchange integrals is necessary to understand the trend in Tc.

Efficient Room-Temperature Cooling with Magnets

Magnetic cooling is a highly efficient refrigeration technique with the potential to replace the traditional vapor compression cycle. It is based on the magnetocaloric effect, which is associated with the temperature change of a material when placed in a magnetic field. We present experimental evidence for the origin of the giant entropy change found in the most promising materials, in the form of an electronic reconstruction caused by the competition between magnetism and bonding. The effect manifests itself as a redistribution of the electron density, which was measured by X-ray absorption and diffraction on MnFe(P,Si,B). The electronic redistribution is consistent with the formation of a covalent bond, resulting in a large drop in the Fe magnetic moments. The simultaneous change in bond length and strength, magnetism, and electron density provides the basis of the giant magnetocaloric effect. This new understanding of the mechanism of first order magneto-elastic phase transitions provides an essential ...

Peculiarities of magnetic properties of Nd3+ ions in the Nd0.5Gd0.5Fe3(BO3)4 crystal in the optically excited states 4(F7/2 + S3/2) and (4G9/2 + 2K13/2 + 4G7/2)

Polarized absorption spectra of f-f absorption bands 4I9/2 → 4(F7/2 + S3/2) and (4G9/2 + 2K13/2 + 4G7/2) in the Nd3+ ion in the Nd0.5Gd0.5Fe3(BO3)4 single crystal were studied as a function of temperature in the range of 2–40 K and as a function of magnetic field in the range of 0–65 kOe at 2 K. It was found out that the selection rules for f-f electron transitions substantially changed in the magnetically ordered state of the crystal, and they strongly depended on the orientation of the Fe and Nd ions magnetic moments relative to the light polarization. The splitting of the excited states of the Nd3+ ion in the exchange field of the Fe sublattice were determined. It was revealed that the value of the exchange splitting (the exchange interaction) did not correlate with the theoretical Landé factor. The Landé factors of the excited states were experimentally found. The local reorientation field induced metamagnetic transitions were observed in the excited states. In some excited states the energetically favorable orientation of the Nd3+ ion magnetic moment is opposite to that in the ground state.

Hydrostatic pressure driven spin, volume and band gap collapses in SmFeO3: a GGA + U study

The crystal structure, magnetic and electronic properties of SmFeO3 under hydrostatic pressure have been studied by first-principles calculations within the generalized gradient approximation plus Hubbard U (GGA + U). The iso-structural phase transition with spin, volume and band gap collapses can be induced by a large enough hydrostatic pressure. The high-spin (HS) state of Fe3+, with the magnetic moment of ~4 μB, is retained at low pressure. The spin crossover occurs at a transition pressure (~68 GPa) with the magnetic moment of Fe3+ decreasing to ~1 μB in low-spin (LS) state. Meanwhile, the reductions of cell volume (by ~−5.43%) and band gap (from >2 eV to ~1.6 eV) of SmFeO3 are obtained when the HS–LS transition happens. Finally, the critical pressure of HS–LS transition, magnetic and electronic properties are found to be Hubbard U dependent.

Anomalous Hall effect studies on Tb–Fe thin films

TbxFe100−x (with x=11, 25, 31 and 44) thin films were prepared with the substrates kept at a temperature of 300°C and the Hall resistivities and electrical resistivities were investigated in the temperature range 25–300K. The sign of Hall resistivity is found to change from positive for x=31 to negative for x=44 film at temperatures 25K and 300K, reflecting the compensation of Tb and Fe magnetic moments between these two compositions. Perpendicular magnetic anisotropy was observed in the films of x=25 and 31 at 25K as well as at 300K. The Hall resistivity is seen to increase for the films of x=11 and 31 with increasing temperature, while it decreases for the films of x=25 and 44 with increasing temperature. The temperature coefficients of electrical resistivities of these films are seen to be positive. The presence of perpendicular magnetic anisotropy (refers to magnetic anisotropy, in this paper) in the temperature range 25–300K in Tb25Fe75 and Tb31Fe69 and their metallic nature are indicators that the Tb–Fe films deposited at higher temperatures are more suitable for magneto optic data storage applications than their amorphous counterparts, due to the stability of the former.

Investigation of Magnetic and Electronic Properties of Sputtered Fe/Au Multilayers

We present an experimental and theoretical investigation of magnetic and electronic properties of sputtered Fe/Au multilayers with an emphasis on interfacial effects. Spin-wave theory was used to simulate the change of magnetization as a function of temperature for multilayers with different Fe thicknesses, in which several fundamental magnetic parameters were extracted. The Fe thickness dependence of magnetization shows a significant increase of Fe magnetic moment at small Fe thicknesses. Close to the interface, the Fe moment is found to be 2.9 μB, which is higher than the value reported in bulk Fe (2.2 μB). It is shown that the increase in Fe moment is mainly associated with the increase of the Fe–Fe distance close to the interface due to the lattice mismatch between Fe and Au. First-principle calculations were performed as well and have supported the proposed scenario.

Ultrasharp magnetization steps in the antiferromagnetic itinerant-electron systemLaFe12B6

The remarkable intrinsic magnetic properties of the $\mathrm{LaF}{\mathrm{e}}_{12}{\mathrm{B}}_{6}$ compound have been studied by neutron powder diffraction (NPD) and magnetization measurements. The NPD measurement reveals that $\mathrm{LaF}{\mathrm{e}}_{12}{\mathrm{B}}_{6}$ exhibits an antiferromagnetic (AFM) structure that can be described with a magnetic propagation vector of (\textonequarter{}, \textonequarter{}, \textonequarter{}) below ${T}_{\mathrm{N}}$. In the amplitude-modulated model used for the refinement, the Fe magnetic moments are confined to the $ab$ plane with a maximum value of $0.43 {\ensuremath{\mu}}_{\mathrm{B}}$ at 1.5 K. It is shown that the AFM state can be transformed to a ferromagnetic (FM) state via a field-induced first-order transition accompanied with a huge magnetic hysteresis. The $\mathrm{LaF}{\mathrm{e}}_{12}{\mathrm{B}}_{6}$ compound is not only the unique stable $R\mathrm{F}{\mathrm{e}}_{12}{\mathrm{B}}_{6}$ phase, along the rare-earth $R$ series but also presents unique magnetic behavior for a purely $3d$ itinerant electron system, including particularly low ordering temperature ${T}_{\mathrm{N}}=36\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, remarkably small Fe moment, unusual amplitude-modulated magnetic arrangement, and a multicritical point in the magnetic phase diagram. In addition, we reveal that at 2 K, the AFM-FM transition is abrupt, leading to a large increase of the Fe magnetic moment up to $1.55\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$; the magnetization curve presents ultrasharp steps, giving rise to an unusual staircaselike behavior.

Site preference in isoelectronic Heusler alloy Fe2RuSi

The site preference, electronic structure, and magnetism of Heusler alloy Fe2RuSi are investigated theoretically and experimentally. The magnetic and electronic properties of Heusler alloys are strongly related to the atomic ordering and site preference in them. Usually, the site preference of the transition metal elements is determined by the number of their valence electrons. However, the recent results suggest that some new possible factors such as atomic radius should also be considered. Here we compare the phase stabilities of several different atomic orderings like XA, L21, DO3, L21B in Fe2RuSi, in which Fe and Ru atom have 8 valence electrons each, thus the influence of number of their valence electrons can be omitted. First-principles calculations suggest that Ru atom prefers entering sites A and C in the lattice. In ground state, the most stable structure is of XA type, in which Fe and Ru atoms occupy A and C sites, respectively and the second stable structure is L21B type, in which Fe and Ru atoms occupy A and C sites randomly. With Ru atom entering into the B site, the total energy increases rapidly. Thus there is still a strongly preferable occupation of Ru though Fe and Ru atom are isoelectronic. This confirms that the valence electrons rule may be not enough to determine the site preference of the transition metal element in Heusler alloy. The preferable occupation of Ru atom in Fe2RuSi can be explained from the electronic structure. It is found that in the XA DOS, there is strong hybridization between the electrons of the nearest Ru and Si or Fe (B) atom. However, in the high energy L21 structure the hybridization between Ru and the nearest Fe (A, C) is weak, which reduces its phase stability. This is confirmed further by the charge density difference calculation. Single phase Fe2RuSi with a lattice parameter of 5.79 is synthesized successfully. Comparing the superlattice reflections (111) and (200) in the experimental XRD pattern with those in the simulated patterns for different structures, we find that Fe2RuSi crystallizes in L21B structure rather than the most stable XA one at room temperature, which mainly originates from the contribution of mixed entropy to the free energy, and its caused atomic disorder at high temperatures. This disorder can be retained during the cooling procedure, while it does not influence the conclusion that Ru atom prefers the (A, C) sites in Fe2RuSi strongly. Finally, the saturation magnetization Ms at 5 K is 4.87 B/f.u., which agrees well with the theoretical result. The large total magnetic moment mainly comes from the contributions of Fe, especially Fe magnetic moments on B sites.

Changes of the local magnetic properties of the optically excited Nd3+ ions and their manifestation in the near IR spectra of the Nd0.5Gd0.5Fe3(BO3)4 crystal

Polarized absorption spectra of f–f transitions 4I9/2→4F3/2 and (2H9/2+4F5/2) in the Nd3+ ion in the Nd0.5Gd0.5Fe3(BO3)4 single crystal were studied as a function of temperature in the range of 2–40K and as a function of magnetic field in the range of 0–65kOe at 2K. It was found out that the selection rules for f–f electron transitions substantially changed in the magnetically ordered state of the crystal, and they strongly depended on the orientation of the Fe and Nd ions magnetic moments relative to the light polarization. The splitting of the ground and excited states of the Nd3+ ion in the exchange field of the Fe sublattice were determined. It was revealed that the value of the exchange splitting (the exchange interaction) in the excited states did not correlate with the theoretical Landé factors. The Landé factors of the excited states were experimentally found. In general, the local magnetic properties in the vicinity of the excited ion depend substantially on its electron state. In particular: (1) in one of the excited states a weak ferromagnetic moment appears, (2) the changes of type of the local magnetic anisotropy take place in some excited states, and (3) in some excited states the energetically favorable orientation of the Nd3+ ion magnetic moment is opposite to that in the ground state. In some excited states the nonequivalent Nd3+ centers were found out.

Investigation on structural and magnetocaloric properties of LaFe13−xSix(H,C)y compounds

The first part of this study is devoted to LaFe13−xSixHy hydrides. The parent intermetallic alloys were synthesized by high energy ball milling. The insertion of H atoms was carried out by solid/gas reaction using a Sievert apparatus. Their thermodynamic properties show a very fast kinetic absorption at 298K under 10 bars depending on Si content. The structural results combined with magnetic measurements allow to explain the magnetovolumic effect on the Curie temperature. The second part of this study is dedicated to LaFe13−xSixCy carbides. They were synthesized by solid/solid reaction between the intermetallic powder and the anthracene. Their structural as well as magnetocaloric properties were investigated. Further structural and magnetic results were carried out by neutron diffraction. A complete crystallographic characterization was made. The cell parameter and the Fe magnetic moments versus temperature were determined. Moreover, the misunderstanding on interstitial site was clear up.)

Phase formation, thermal stability and magnetic moment of cobalt nitride thin films

Cobalt nitride (Co-N) thin films prepared using a reactive magnetron sputtering process are studied in this work. During the thin film deposition process, the relative nitrogen gas flow (RN2) was varied. As RN2 increases, Co(N), Co4N, Co3N and CoN phases are formed. An incremental increase in RN2, after emergence of Co4N phase at RN2 = 10%, results in a linear increase of the lattice constant (a) of Co4N. For RN2 = 30%, a maximizes and becomes comparable to its theoretical value. An expansion in a of Co4N, results in an enhancement of the magnetic moment, to the extent that it becomes even larger than pure Co. Such larger than pure metal magnetic moment for tetra-metal nitrides (M4N) have been theoretically predicted. Incorporation of N atoms in M4N configuration results in an expansion of a (relative to pure metal) and enhances the itinerary of conduction band electrons leading to larger than pure metal magnetic moment for M4N compounds. Though a higher (than pure Fe) magnetic moment for Fe4N thin films has been evidenced experimentally, higher (than pure Co) magnetic moment is evidenced in this work.

Search for Fe magnetic ordering in the 40 K superconductor (Li0.8 Fe0.2)OHFeSe

We report the results of x-ray diffraction and 57 Fe Mössbauer spectroscopy study, complemented by ab-initio electronic structure and the hyperfine-interaction parameters calculations, of the 40 K superconductor (Li0.8 Fe0.2)OHFeSe. The superconductor crystallizes in the tetragonal space group P4/nmm with the lattice constants a = 3.7865(2) Å and c = 9.2802(6) Å. Evidence is provided for the presence of an anisotropic mixture of strong covalent and weak ionic chemical bonding and of metallic characteristics. The Mössbauer spectra consist of two quadrupole-doublet patterns corresponding to Fe2+ at the 2a and 2b sites. Contrary to the claims in the literature, we demonstrate unequivocally that there is no magnetic ordering of the 2a-site Fe magnetic moments down to 2.0 K. The calculated hyperfine-interaction parameters at the two Fe sites show general agreement with the experimental ones. The Debye temperatures for the 2a and 2b sites are found to be 186(21) and 397(4) K, respectively.

Interface Fe magnetic moment enhancement in MgO/Fe/MgO trilayers

We model room temperature soft x-ray resonant magnetic reflectivity to determine a 24% increase of the Fe magnetic moment of the 2–3 monolayers next to both MgO interfaces in a MgO(3 nm)/Fe(12 nm)/MgO(001) heterostructure. This direct measurement of such enhanced interface magnetic moments for buried interfaces confirms theoretical predictions and highlights the importance of considering inhomogeneous in-depth magnetic profile in Fe/MgO based magnetic tunnel junctions.

Mössbauer spectroscopy, magnetic, and ab initio study of the Heusler compound Fe2NiGa

The structural, electronic, magnetic, elastic, and hyperfine-interaction properties of Fe2NiGa have been determined by means of X-ray diffraction, 57Fe Mössbauer spectroscopy, and magnetic measurements and ab initio calculations. The compound studied crystallizes in the cubic space group F4¯3m with lattice constant a=5.7961(4)A˚. Evidence is provided for the presence of significant structural disorder in the compound. Fe2NiGa is predicted to be half-metallic with covalent chemical bonding. It orders ferromagnetically with the Curie temperature TC=586.0(7)K. The saturation magnetization per formula unit and the estimated Fe magnetic moments at the A and B sites are 3.00, 1.87(2), and 2.25(2) μB, respectively. The ab initio calculations overestimate the values of the A- and B-site Fe magnetic moments. It is observed that the magnetic properties of Fe2NiGa are very strongly dependent on its heat treatment. The calculated hyperfine-interaction parameters show general agreement with the experimental ones. It is demonstrated that the compound studied decomposes when heated and kept at temperatures above around 500K. The Debye temperature of Fe2NiGa is found to be 378(5)K.

X-ray magnetic circular dichroism on La2/3Ca1/3Mn0.97Fe0.03O3 thin films

The element-selective technique of X-ray magnetic circular dichroism (XMCD) has been used to study the magnetic properties of La2/3Ca1/3Mn0.97Fe0.03O3 (LCMFO) thin films. XMCD measurements below the ferromagnetic ordering temperature at the Mn and Fe L2,3 absorption edges allow the determination of the contributions and relative orientations of the Mn and Fe magnetic moments. A reduction in the Mn L2,3 XMCD signal of the LCMFO sample compared to that for the parent La2/3Ca1/3MnO3 (LCMO) system reveals important modifications in the electronic and magnetic properties with the presence of Fe. The Fe L2,3 X-ray absorption (XAS) for the LCMFO film is characteristic of Fe3+, and the comparison with multiplet calculations shows that the Fe dopants occupy octahedral sites in the crystal, which is consistent with Fe3+ substituting Mn3+ in LCMO. The magnetic moments of Mn and Fe are found to align antiparallel, which suggests the presence of Mn–O–Fe superexchange coupling. This result is consistent with macroscopic magnetometry measurements on the LCMFO system, which show a decrease in saturation magnetization of LCMO with Fe doping.

Reversible Fe Magnetic Moment Switching in Catalytic Oxygen Reduction Reaction of Fe-Phthalocyanine Adsorbed on Ag(110)

The consequences of the evolution of the active site in the oxygen reduction reaction catalyzed by iron phthalocyanine (FePc) submonolayers deposited on the Ag(110) surface are explored. The capacity of reversibly and sharply switching the magnetic state of FePc molecules by chemical means is conclusively evidenced by X-ray magnetic circular dichroism (XMCD) at the Fe L2,3 absorption edge. In the molecular oxygen-dosed phase, oxygen intercalates between the molecule and the surface, thereby switching the Fe magnetic moment from a nearly negligible (mTotexp ≈ 0.26(1) μB) to an order of magnitude larger value (mTotexp ≈ 2.1(2) μB). Moreover, the characteristic XMCD spectrum undergoes a crossover from metallic to oxidized Fe(III)-like line shape, in accordance with an oxygen-induced decoupling of the molecular electronic states from the underlying metal surface. The FePc acts as a reversible bifunctional chemical–magnetic switch at the atomic monolayer scale.