Local spin configurations of Fe atoms in the Rh1−x Fex (x=0.1, 0.2, and 0.3) system with competing exchange interactions

The local spin configurations of Fe atoms in the magnetically ordered alloys Rh_1?x Fe_x (x=0.1, 0.2, and 0.3) have been investigated by Mössbauer spectroscopy. The Mössbauer absorption spectra are measured in the range from 5 K to temperatures of the transition to the paramagnetic state. The measurements in magnetic fields with a strength up to 5 T are carried out at a temperature of 4.2 K. Analysis of the magnetic-hyperfinefield distribution functions demonstrates that Fe atoms form discrete sets of collinear spin configurations corresponding to different net moments of the nearest coordination sphere. The spin structure of the alloys is governed by a random distribution of Fe atoms over the lattice sites and the competition between the Fe-Rh ferromagnetic exchange interaction and the antiferromagnetic interaction of the neighboring Fe atoms. No spin frustration and spin “melting” effects characteristic of spin glasses are revealed in the Rh-Fe alloys.     

On the origin of the giant magnetic moments of Fe and Co in Cs Films

To unravel the mystery of the recently observed giant magnetic moments of Fe and Co in Cs films, orbital-polarization corrected relativistic spin density functional calculations have been performed. Unlike other transition–metal systems where the orbital magnetic moments are quenched, Fe and Co in Cs as well as in other alkali metals are found to possess a giant orbital moment of 2–3 μ_B along with a large spin moment. Also, these free atom-like spin and orbital magnetic moments in Cs would not be squashed under large lattice contractions up to 23% around the impurity atoms. The induced moments on the host atoms are small. The results offer an explanation for the origin of the giant magnetic moments of Fe and Co in Cs films.     

A DFT study of the electronic and magnetic properties of Fe2MnSi1−xGex alloys

We performed density functional theory (DFT) calculations to study the structural, electronic and magnetic properties of Fe₂MnSi_1−xGe_x alloys (x=0, 0.25, 0.50, 0.75, and 1.00). The lattice constant is found to increase linearly as a function of Ge concentration with a decrease in the formation energy. The total magnetic moment is found to be 3 μ_B for all alloys with the most contribution from Mn local magnetic moments. Iron atoms, however, exhibit much smaller spin moments about 10% of the bulk value. It seems that due to the proximity of Fe, magnetic moments have been induced on the sp atoms, which couple antiferromagnetically with Fe and Mn spin moments. Although, the band gap remains almost constant (0.5 eV), the spin–flip gap decreases as a function of x.     

Effect of Fe substitution on the magnetic properties of half-Heusler alloy CoCrAl

The effect of Fe substitution for the vacant site in half-Heusler alloy CoCrAl is studied. A series of single phase CoFe_xCrAl (x=0.0, 0.25, 0.5, 0.75 and 1.0) alloys has been successfully synthesized. The lattice constant is found to increase almost linearly with increasing Fe content, indicating Fe atoms enter the lattice of CoCrAl instead of existing as a secondary phase. When Fe entering the vacant site, spin polarization occurs and the alloy turns from a semimetal in CoCrAl to a half-metallic ferromagnet (HMF) in CoFeCrAl. This is due to the reconstruction of the energy band with Fe substitution. The Curie temperature and saturation magnetic moments are enhanced and increase monotonically with increasing Fe content. The variation of the spin moment follows the Slater-Pauling curve and agrees with the theoretical calculation as well.     

Magnetic properties of R4Co3 (R = Gd, Y) compounds as observed by NMR

The zero field spin echo nmr spectrum of Gd₄Co₃ taken at 4.2 K is analysed and discussed on the basis of having magnetic moments of Co atoms at different structural lattice sites. For Y₄Co₃ the spin echo nmr spectrum taken as a function of the external field is discussed and explained on the basis of the coexistence of Co atoms carrying localized magnetic moments and paramagnetic Co atoms in this compound at determined structural sites.     

Mössbauer study of the intermetallic compound Nd2Fe14B. II. Temperature dependence and spin reorientation

The ⁵⁷Fe Mössbauer effects of Nd₂Fe₁₄B were measured in a temperature range of 4.2−300 K. Below the spin reorientation transition temperature T_sc = 148 K, the spectra were satisfactorily analyzed with twelve Zeeman sextuplets due to splitting of six crystallographic Fe-sites into twelve non-equivalent sites. It was shown that the magnetic moments of the Fe and the Nd atoms are non-collinearly coupled in the magnetic structure with canted moments below T_sc. The directions of the moments at 4.2 K are inclined at 27° for Fe and at 58° for Nd from the c-axis to the [110] direction. The average moments are 2.27μ_B for Fe and 3.3μ_B for Nd at 4.2 K. The increase of the average hyperfine field with decreasing temperature is suppressed below T_sc, and its value at 4.2 K is reduced by 1% from the value of 337 kOe which is observed in Y₂Fe₁₄B and also estimated for Nd₂Fe₁₄B by extrapolating the values above T_sc. On the other hand, the Nd moment increases abruptly around T_sc as the temperature decreases. The directions of the principal axes of electric field gradients on the six distinct Fe-sites were also obtained. The anomalous temperature dependence of quadrupole splittings and isomer shifts was observed around T_sc. They were discussed in a framework of the changes in the band structure and the lattice parameters incidental to the spin reorientation transition.     

Magnetic structure of the GdAl3 intermetallic compound as seen via Mössbauer spectroscopy data for 119Sn impurity atoms

The magnetic hyperfine interaction for ¹¹⁹Sn impurity atoms in GdAl₃ frustrated antiferromagnetic compound has been investigated by Mössbauer spectroscopy technique. Two magnetic subspectra with the ratio of the intensities 2:1 were observed. At 4.5 K, the values of the magnetic hyperfine field are 4.00(2) and 1.35(2) T. The peculiarities of the Mössbauer spectrum provide an opportunity to propose a plausible model of the spin arrangement in the GdAl₃ lattice. In each GdAl₃ basal layer, the magnetic moments of the Gd atoms form three-sublattice 120° spin structure that is peculiar to magnetically frustrated compounds. The appearance of two magnetically nonequivalent Sn sites is a result of vector summation of the transferred polarizations from Gd moments located in two adjacent GdAl₃ planes. The anomalous temperature behavior of the hyperfine field is characteristic of frustrated systems with competing exchange interactions.     

NEUTRON-POWDER-DIFFRACTION STUDY OF ErFe11.35Nb0.65 AND ErFe11.35Nb0.65Ny

ErFe_11.35Nb_0.65 AND ErFe_11.35Nb_0.65N_y have been synthesized and neutron-powder-diffraction experiments at room temperature performed. The ErFe_11.35Nb_0.65N_y nitride, obtained by gas-solid reaction, retains the ThMn₁₂-type structure of its parent compound. The Nb atoms occupy 8i sites and the nitrogen atoms are located at 2b sites. The atomic magnetic moments of the Er ions are antiparallel to those of the Fe atoms. Upon nitrogenation, the lattice cell expands mainly along the a-axis and the atomic magnetic moments of Fe are enhanced.     

Magnetic structure and preferential occupation of Fe in the composite compound Nd2Co6Fe

The crystal and magnetic structures of the composite compound Nd₂Co₆Fe have been investigated by high-resolution neutron powder diffraction and X-ray powder diffraction. The compound crystallizes in the hexagonal Ce₂Ni₇-type structure consisting of Nd(Co,Fe)₂ and Nd(Co,Fe)₅ structural blocks alternately stacked along the c-axis. Multi-pattern Rietveld refinement of neutron diffraction and X-ray diffraction data at room temperature reveal that substitution of Fe for Co occurs exclusively in the Nd(Co,Fe)₅ structural blocks. The preferential occupation of the Fe atoms in the structure is discussed based on the mixing enthalpy between Nd and Fe atoms and on the lattice distortions. In agreement with the reported magnetic phase diagram of the Nd₂Co_7−xFe_x compounds, magnetic structure models with the moments of all atoms in the ab plane at 300 K and along the c-axis at 450 K provide a satisfactory fitting to the experimental neutron diffraction data. The refinement results show that the atomic moments of (Co,Fe) atoms within the Nd(Co,Fe)₅ blocks decrease slightly with temperature, whereas the atomic moments of Nd in the compound and of (Co,Fe) atoms at the interface between the Nd(Co,Fe)₂ and Nd(Co,Fe)₅ blocks are reduced significantly.     

Electronic structure and magnetism of R(Fe,Si)12 (R = Y, Nd)

The newly developed full-potential linearized augmented plane wave (LAPW) and local orbitals (lo) based on standard APW methods are briefly introduced, and the structure and magnetic properties of R(Fe, Si)₁₂ compounds (R = Y, Nd) are calculated using the method. The distribution of Si at different sites is analyzed based on total energy of one crystal unit with structure having been optimized. The characters of magnetic moments, total density of states (TDOS) and partial density of states (PDOS) for different crystal sites Si occupies are obtained and analyzed. The results show that the total magnetic moments of RFe₁₀Si₂ (R = Y, Nd) are larger than those of RFe₁₀ M ₂ (M = Ti, V, Cr, Mn, Mo and W) and the hybridization mechanism is seen as follows. Si(8j) reduce the magnetic moments of Fe at three sites, however, Si(8f) mainly reduce the magnetic moments of Fe(8i) and Fe(8j) atoms. The Curie temperature is markedly enhanced by the introduction of Si atoms according to spin fluctuation of DOS at Fermi level.     

A first principles study on the full-Heusler compound Mn2CuSi

Using a state-of-the-art full-potential electronic structure method within the generalized gradient approximation (GGA), we study the electronic structure and magnetic properties of the Mn₂CuSi full-Heusler alloy. Calculations show that CuHg₂Ti-type structure alloy is a half-metallic ferrimagnet with the Fermi level (ε_F) being located within a tiny gap of the minority-spin density of states. The conduction electron at ε_F keeps a 100% spin polarization. A total spin moment, which is mainly due to the antiparallel configurations of the Mn partial moments, is −1.00μ_B for a wide range of equilibrium lattice parameters. Simultaneously, the small spin magnetic moments of Cu and Si atoms are antiparallel. The gap mainly originates from the hybridization of the d states of the two Mn atoms. Thus, Mn₂CuSi may be the compound of choice for further experimental investigations.     

Magnetic state and properties of the Fe0.5TiSe2 intercalation compound

The Fe_0.5TiSe₂ compound with a monoclinic crystal structure has been prepared by intercalation of Fe atoms between Se-Ti-Se sandwiches in the layered structure of TiSe₂. The crystal and magnetic structures, electrical resistivity, and magnetization of the Fe_0.5TiSe₂ compound have been investigated. According to the neutron diffraction data, the Fe_0.5TiSe₂ compound has a tilted antiferromagnetic structure at temperatures below the Néel temperature of 135 K, in which the magnetic moments of Fe atoms are antiferromagnetically ordered inside layers and located at an angle of approximately 74.4° with respect to the layer plane. The magnetic moment of Fe atoms is equal to (2.98 ± 0.05)μ_B. The antiferromagnetic ordering is accompanied by anisotropic spontaneous magnetostrictive distortions of the crystal lattice, which is associated with the spin-orbit interaction and the effect of the crystal field.     

Pressure-Induced Decrease in the Curie Temperature of Gd<Subscript>2</Subscript>Fe<Subscript>17</Subscript>: LSDA+U Calculations

To explain the magnetic properties of advanced ferromagnetic intermetallic compounds of the R₂Fe₁₇ (R is a rare-earth element) class, experimentalists often use the hypothesis of competition between ferromagnetic exchange and antiferromagnetic exchange between four types of the nearest iron atoms in nonequivalent lattice sites. For the rhombohedral Gd₂Fe₁₇ ferromagnet, we calculate the magnetic moments of iron and gadolinium ions, the parameters of exchange between Fe atoms, and Curie temperature T_C at a zero pressure and during hydrostatic lattice compression. The magnetic moment of the unit cell of Gd₂Fe₁₇ is shown to decrease under pressure, and this decrease is almost completely associated with a decrease in the magnetic moments of Fe rather than Gd ions, the pressure dependence of the magnetic moments of which is weaker by an order of magnitude. In contrast to the hypothesis regarding the competition of exchange interactions between different kinds of Fe atoms, the parameters of exchange between the nearest iron atoms in different crystallographic sites are found to be positive ferromagnetic (at a zero pressure and during compression), and a ferromagnetic character of interaction is shown to remain unchanged under pressure even for Fe atoms in the so-called dumbbell sites with the nearest interatomic distances. The Curie temperature T_C of Gd₂Fe₁₇ is shown to decrease with increasing pressure. The changes in the exchange parameters and the magnetic moments of Gd₂Fe₁₇ during compression are found to be mainly related to a change in the position of energy spectrum branches with respect to each other and the Fermi level ?_F rather than to a change in the overlapping of wavefunctions, which play a minor role.     

A First-Principles Investigation of the Structural and Magnetic Stability of Fe/GaAs(001)

The electronic properties of the interface of Fe/GaAs(001) have been investigated by using first-principles and molecular-dynamics techniques. While the ground state is ferromagnetic for all structures considered, a ferrimagnetic spin structure is found to be very close in energy (<1 meV). The observed lowering of the magnetic moments when relaxing the atomic positions is believed to be connected to this close in energy lying metamagnetic state. On the other hand, the magnetic moments of the Fe atoms at the interface are large, which can be explained by the bulk-like behavior of the density ot states of interface atoms.     

Magnetism of mass-filtered nanoparticles on ferromagnetic supports

The magnetic properties of 3d-metal clusters significantly differ from bulk behavior and, for small clusters, strongly depend on the number of atoms within each cluster. Such phenomena are caused by a narrowing of electronic states and the high ratio of surface to volume atoms giving rise to enhanced magnetic orbital moments. However, even large Fe nanoparticles (6–12 nm) deposited onto ferromagnetic surfaces show enhanced orbital moments. At a low coverage large iron clusters on a cobalt film exhibit a nearly doubled value for the orbital moments when compared to bulk behaviour. With increasing coverage, the orbital moment is clearly reduced. Additionally, the spin and orbital moments of iron and cobalt in Fe₅₀Co₅₀ alloy clusters with a size of 7.5 nm on a nickel substrate have been investigated. FeCo alloys are known to exhibit very high magnetic moments for soft magnetic materials. PACS 73.22.-f; 75.75.+a; 81.07.-b     

Magnetic properties of stable structures of small binary clusters

We have studied the optimum geometries and the magnetic behavior of small binary Fe_nGe_m (n+m≤4) clusters usingab initio spin-polarized density functional calculations. Our results reveal that the optimized clusters present high values in the HOMO–LUMO gap and generally prefer structures with high geometries, the local magnetic moments at Fe atoms present an enhancement with respect to Fe bulk magnetization, whereas the Ge atoms present local magnetic moments whose modulus take significative values. The magnetic coupling between Fe and Ge atoms is mainly antiferromagnetic-like. All the clusters studied here present a charge transference from Fe atoms to Ge atoms.     

Enlightening the stable ferromagnetic phase of SrAO3(A= Cr,Fe and Co) compounds using spin polarized quantum mechanical approach

This is an investigation of the atomic structure, opto-electronic and magnetic properties of SrAO₃ (A = Cr, Fe and Co) compounds using first principles method. The ferromagnetic behavior is found the most stable phase for SAO. The calculated Goldschmidt's tolerance factor values predict that the studied compounds have a stable structure. Moreover, the calculated formation energy shows that SrAO₃compounds are thermodynamically stable. The calculated density of states shows that the present compounds are metal and the direction of the magnetic moments of SrCrO₃ is anti parallel to its spin. The charge density contours display a mixture of the covalent and ionic bonds between the content atoms of SrAO₃ compounds. The optical parameters are calculated using the dielectric function real and imaginary parts. From the electronic and optical properties results, beneficial industrial applications can be expected for the present compounds.     

Theoretical study of magnetism in RM12B6 compounds (R=Y,La or Ce; M=Fe,Co)

Electronic structure study for RM₁₂B₆ intermetallics (R=Y, La or Ce; M=Fe, Co) was performed. Fixed spin moment calculations for different volumes of unit cell were used to find low and high moment states in LaFe₁₂B₆. Obtained results are in agreement with previously obtained experimental and theoretical results. Total magnetic moments obtained for YCo₁₂B₆ and LaCo₁₂B₆ are in fair agreement with experimental values. In CeCo₁₂B₆ discrepancy between theory and experiment seems to be more pronounced. It seems that the calculated Co magnetic moments could be overestimated in the studied RCo₁₂B₆ compounds. Present calculations indicate that Fe (Co) atoms prefer 18(h) (18(g)) atomic positions what is in agreement with experiment.     

MAGNETIC PROPERTIES OF BALL MILLED FexCu1-x (x=0.4, 0.5) ALLOY

The samples Fe_0.4Cu_0.6 and Fe_0.5Cu_0.5 ball milled for 50 h are investigated by X-ray diffraction, M?ssbauer spectra, as well as magnetic measurement. The experiments show that the structure of the samples is fcc, with lattice constant 0.361 nm and there are fcc Fe-rich phase and fcc Cu-rich phase in the samples. Most of Fe atoms (91%) are in the fcc Fe-rich phase, which is a ferromagnetic phase. The M-H curve at 1.5 K shows the saturation magnetization of the samples are 80.5 emu/g and 101.6 emu/g for Fe_0.4Cu_0.6 and Fe_0.5Cu_0.5 respectively. The average magnetic moment of Fe atoms is deduced to be 2.40 μ_B . Compared with theoretical predication, the Fe atoms in the fcc phase are in high spin state.     

Theory of localized magnetic moments in metals I finite cluster approach

The origin of localized magnetic moments formation in metals is investigated theoretically using a self-consistent local spin density molecular cluster approach. Clusters with up to 55 atoms are employed to describe isolated impurity local moment behavior in the cases of Fe$\text{Ag}$ and Fe$\text{Pd}$. Densities of states and spin magnetic moments were determined and compared with results of spectroscopic (notably photoemission) and magnetization measurements, respectively. In the case of a noble metal host, the spin magnetization density is found to be highly localized around the Fe site; the iron moment is ≈ 3.9μ_B and the polarization of the host Ag atoms is small. In the case of a transition metal host, the iron moment is ≈ 3.2 μ_B but here the strong hybridization of the Fe-3d and Pd-4d states results in a large induced magnetic moment in the host PD metal — in essential agreement with experiment for this giant moment system.