Mechanical properties of bcc Fe-Cr alloys by first-principles simulations

The effect of chromium content on the fundamental mechanical properties of Fe-Cr alloys has been studied by first-principles calculations. Within a random solid solution model, the lattice constants and the elastic constants of ferromagnetic bcc Fe_1?x Cr _x (0? · ?0.156) alloys were calculated for different compositions. With addition of Cr content, the lattice parameters of Fe-Cr alloys are larger than that of pure Fe solid, and the corresponding Young??s modulus and shear modulus rise nonmonotonically with the increasing Cr content. All alloys (except 9.4 at% Cr) exhibit less ductile behavior compared with pure bcc Fe. For the Fe_1?x Cr _x (0? · ?0.156) alloys, the average magnetic moment per atom decreases linearly with the increasing Cr concentration.     

Magnetic moment distributions and form factors in ferromagnetic nickel-ruthenium alloys

Using polarised neutrons, the full three-dimensional magnetic structure amplitudes in the Ni_1?c Ru _c single crystals forc = 0·027, 0·033 and 0·046 were measured. Moment density maps in various portions of the Wigner-Seitz cell were obtained. It is seen from these maps that unlike Ni-based alloys with 3d impurities, the introduction of Ru to the Ni matrix produces extensive perturbations in the diffuse moment density, giving rise to a netpositive diffuse moment which tends to increase with Ru concentration. The asphericity of the host moment at first increases and then decreases with increasing Ru content. Another significant outcome of the present study is the evidence for the reversal of the sign of the Ru moment, from negative to positive, obtained by comparing the shape of the spherical site form factors of the three-alloy concentrations with the Ni spherical form factor itself. The sign reversal of the impurity moment is confirmed by the form factor analyses. Strong local environmental effects seem to play a major role in this alloy system.     

X-ray magnetic circular dichroism at IrL2,3 edges in Fe100−x Ir x and Co100−x Ir x alloys: Magnetism of 5d electronic states

The formation of induced 5d magnetic moment on Ir in Fe_100−x Ir _x (x=3, 10 and 17) and Co_100−x Ir _x (x=5, 17, 25 and 32) alloys has been investigated by X-ray magnetic circular dichroism (XMCD) at Ir L_2,3 absorption edges. Sum rule analysis of the XMCD data show that the orbital moment of Ir is in the range of −0.071(2)μB to −0.030(1)μB in Fe-Ir alloys and −0.067(2)μB to 0.024(1)μB in Co-Ir alloys. We find that the total moment of Ir in Fe-Ir alloys is approximately 1/5 of the total 3d moment on Fe at all the three compositions. In contrast, the total moment on Ir in Co-Ir alloys varies between 1/6 to 1/16 of the 3d moment on cobalt. The observed trends of Ir moments and the role of interatomic exchange interactions in 5d moment formation are discussed.     

Preparation and properties of the system CoPxS2−x

Lattice parameters were determined for members of the system CoP_xS_2?x (0 ≤ x ≥ 1). A cubic region was observed for 0 ≤ x ≤ 0·5 and a tetragonal region for 0·5 < x ≤ 1. Magnetization and susceptibility measurements were made on the sample CoP_xS_2?x (0 ≤ x ≤ 0·5) from 4·2 to 500°K and to 9·5 kOe. The ferromagnetic Curie temperature decreased almost linearly with composition from a value of 122(1)°K for x = 0 to 23(1)°K at x = 0·3. The saturation magnetization passed through a maximum at 41·5(2) e.m.u./g at x = 0·05, increasing from 40·0(2) e.m.u./g for x = 0, and then decreased to below 15 e.m.u./g for x = 0·3. The paramagnetic data gave a P²_eff/CO²⁺ that increased linearly with phosphorus substitution from 4·4(2)μ_B at x = 0 to 5·5(2)μ_B² at x = 0·5. The Weiss constant decreased almost linearly from 150(3)°K for x = 0 to 85(2)°K for x = 0·3. The magnetic properties of this system are compared with those of the system CoAs_xS_2?x and CoSe_xS_2?x. Changes, with composition, in the Curie and Weiss temperatures are almost identical in the three systems for x ≤ 0·25. As with CoAs_xS_2?x the ferromagnetic moment reaches a maximum at x = 0·05 and then decreases with increasing x, although the magnetic interactions remain ferromagnetic for all x ≤ 0·5.     

Structural and magnetic studies of the alloy system GdInxAg1-x

The structural and magnetic studies below room temperature of the alloy system GdIn_xAg_1-x(0?x?1) are reported. It has been found that alloys with x?0.5 crystallize in CsCl type cubic structure, but the distortion of the unit cell starts for alloys with x#62;0.5 and for these alloys the unit cell becomes tetragonal. No phase change is evident from the low temperature (295-8 K) structural studies. However, a break appears in the X^-1_m vs. T linear plot of each alloy of this system at a specific temperature (designated as break temperature T_B). The variation of T_B with x is similar to the variation of phase transition temperature with x reported for LaIn_xAg_1-x. Close agreement has been found in the values of effective magneton number (p), magnetic ordering [Néel (T_N) or Curie (T_C)] and paramagnetic Curie (θ_p) temperature for materials studied by us and earlier workers. The variation of magnetization with applied field strength (2.5-65)×10⁵ Am^-1) at 4.2 K has also been reported for ferromagnetic of this system. It has been concluded that alloys with 0.4?x?0.6 are simple ferromagnets with parallel alignment of magnetic moment in the ground state. The angular arrangement of the magnetic moment starts appearing in the ground state for alloys with x?0.4 for x#62;0.6 and continues till x becomes closure to 0.17 or 0.84. The alloys with x=0.17 or x=0.84 have θ_p and T_C equal to zero and appear paramagnetic. Angular arrangement in spins again appear for alloys with x?0.17 or x#62;0.84, however all materials with 0?x?0.17 or 0.84?x?1 remain antiferromagnetic.     

Lattice vibrations of AsxP1−xIn solid solutions

On the basis of investigations of reflection spectra from As_xP_1?xIn solid solutions, new data have been obtained on the character of the vibrational spectrum for compositions in which x is, 0, 0·2, 0·3, 0·4, 0·5, 0·6, 0·8, 0·9 and 1. The variation of the frequencies and oscillator strengths that characterize As and P atom vibrations has been determined as a function of composition. From the comparison of calculated with experimental reflection spectra, the dispersion of the real part (∈₁) and the imaginary part (∈₂) of the dielectric constants of the substance as well as |∈| have been estimated. The values of the nonlocal and local parts of the effective ion charge of InAs and InP were also determined.     

A Mössbauer study of hyperfine interaction in the systems Co x Mn3−x−y Fe y O4 and Ni x Mn3−x−y Fe y O4

A Mössbauer study of systems Co _x Mn_3?x?y Fe _y O₄ and Ni _x Mn_3?x?y Fe _y O₄ for values ofx=0·1, 0·5, 1·0 andy ranging from 0·1 to 2·0 in steps of 0·2 have been made. At room temperature samples fory values ranging in between 0·1 to 0·5 exhibit paramagnetic behaviour while all spectra for values ofy between 0·6 to 0·8 show relaxation effects. Well-defined hyperfine Zeeman spectra are observed for all the samples withy>0·8 and resolved in two sextets corresponding to octahedral and tetrahedral site symmetries and a central doublet probably due to the presence of super-paramagnetic particles in the system. The hyperfine field at⁵⁷Fe nucleus reduces with decreasing iron cobalt and nickel concentration. These observations have been explained in terms of site preference of cations and exchange interactions.     

New phases and chemical short range order in co-deposited CoFe thin films with bcc structure: an NMR study

A ⁵⁹Co NMR study has been carried out on several series of co-evaporated Co₁-_xFe_x thin-film alloys prepared on MgO (001), GaAs (100), and GaAs (110) substrates at deposition temperatures between 175°C and 500°C. The sample thicknesses varied between 100 Å and 1000 Å and the alloy concentrations were in the range 0:1 < x < 0:3. X-ray diffraction and NMR show that the stability limits of the bcc phase in CoFe alloys is shifted from the x = 0:25 observed in the bulk alloys down to about x = 0:11 in thin films. For x = 0:27 and at the deposition temperature of 500°C, a new ordered phase has been stabilised where Co has two Fe atoms only in its first coordination shell. Other samples, grown at lower temperatures, also exhibit an exotic chemical short range order (CSRO) where Co coordinations with zero and two Fe neighbours dominate. A mixture of bcc Co (and not fcc Co as in the bulk alloys) and unknown ordered bcc intermetallics can account for the observed CSRO. Theoretical ground-state phases for the bcc lattice are considered in order to explain the observations.     

Mossbauer study of ferrite systems Co x Mn1−x Fe2O4 and Ni x Mn1−x Fe2O4

Mössbauer spectra of Co _x Mn_1?x Fe₂O₄ and Ni _x Mn_1?x Fe₂O₄ ferrites withx values ranging from 0·1 to 0·8 in steps of 0·1 have been recorded at room temperature. All spectra exhibit well-defined Zeeman hyperfine patterns. It has been observed that hyperfine field at Fe³⁺ nucleus increases more rapidly by nickel substitution than by cobalt substitution. This has been explained in terms of exchange interactions and cation distribution in the spinels. Hyperfine fields, isomer shifts and quadrupole splittings have been determined.     

A neutron small angle scattering study of the onset of ferromagnetism in TiBe2-xCux alloys

The onset of ferromagnetic order in TiBe_2-xCu_x alloys, with x = 0.5, 0.4, 0.3, 0.2 and 0.15, has been examined using the technique of neutron small-angle scattering. The Curie temperature, T_c, for these alloys has been determined from the temperature dependence of the magnetic critical scattering. A linear extrapolation of T_c versus copper concentration yields a critical concentration for ferromagnetic order of x_c = 0.05 ± 0.02. For the alloys with x = 0.4, 0.5 the lineshape of the magnetic critical scattering, at and above T_c, is well explained by the Ornstein-Zernicke form of the spin correlation function. For the lower concentration alloys the exact form of the spin correlation function is still unclear.     

Möβbauereffekt am61Ni

With the Möβbauer technique the hyperfine splittings of the 67.4 keV transition of⁶¹Ni in compounds and alloys have been studied. A magnetic momentΜ _61,4=+ (0,477 ±0.031)Μ _n of the first excited state has been determined. From isomer shifts an order of magnitude estimate of the change in radius ofδ〈r ²〉/〈r ²〉=?6· 10^?4 can be inferred. The electric quadrupole interaction of the first excited level has been observed in Ni-J-boracite. In addition the hyperfine fields at the nickel sites were determined in the alloy series Ni _x Fe_1-x (0≦x≦1). In one case an effect of polarization in a high external field is measured. The fields in some rare earth (RE) intermetallic compounds of the form RENi₂ were deduced from linewidth to be less than 15 kOe.     

Influence of a small lattice deformation on the superconductive critical temperature of alloys with the Cu3Au-type structure

Lattice dimensions and superconductive critical temperature, T_c, of Ca_1?xSr_xPb₃ alloys have been measured. The Cu₃Au-type cubic structure of CaPb₃ becomes tetragonally distorted near x=0·50. At the same composition a marked increase in T_c is found, which is explained in terms of Brillouin zone effects.     

Characterization of pure and copper-doped iron tartrate crystals grown in silica gel

Single crystal growth of pure and copper-doped iron tartrate crystals bearing composition Cu _x Fe_(1???x)C₄H₄O₆·nH₂O, where x = 0, 0.07, 0.06, 0.05, 0.04, 0.03, is achieved using gel technique. The elemental analysis has been done using energy-dispersive X-ray analysis (EDAX) spectrum. The characterization studies such as Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (XRD), magnetic analysis and thermal analysis have been done for crystals with x = 0 for pure iron tartrate and with x = 0.05 for copper-mixed iron tartrate crystals. A detailed comparison has been made between pure and doped crystals.     

Phase transformation and magnetic properties of SmCo7−xBx alloys prepared by mechanical alloying

The phase transformation and magnetic properties of SmCo_7−xB_x (x=0, 0.2, 0.5 and 1) alloys prepared by mechanical alloying have been investigated systematically. The coercivities of the alloys without B increase with increasing annealing temperature, as a consequence of complete crystallization of TbCu₇-type phase. The substitution of B for Co is favorable to the formation of Th₂Zn₁₇- and CaCu₅-type phases when annealed at 650°C, accompanied by the enhancement of the coercivities. Increasing the annealing temperature causes the formation of soft magnetic phase Sm₂Co₁₄B in the B-substituted alloys. In the alloys with x=0.5 and 1 annealed at 850°C, the major phase is Sm₂Co₁₄B, which degrades the magnetic properties sharply. A remanence enhancement has been observed in the SmCo_7−xB_x alloys due to the exchange coupling of the nanoscale structure.     

X-ray spectroscopic investigation of some spinel structures

K-absorption edges of cations in the manganites of magnesium, nickel, copper, zinc and cadmium and ferrite samples of composition, Mg_1?x Mn_x Fe₂O₄ (x=0, 0·25, 0·50, 0·75, 1·0), have been recorded employing a 40 cm curved mica crystal spectrograph of transmission type. It is observed that the absorption edges for the specimen shift towards the shorter wavelength side of the metal edge position. Shifts of both the absorption edges and the main absorptoin peaks for ferrites and manganites have been compared with compounds in which the oxidation state of the cation is well known. It is found that the manganese ions in these ferrites and manganites exist in valence states two and three respectively while iron in the ferrite samples is present in oxidation state three. An attempt has been made to interpret the observed absorption edge features in the light of neutron and X-ray diffraction studies on the same ferrite and manganite samples.     

Low temperature conversion electron Mössbauer spectroscopy on FexGe1−x amorphous thin films

Conversion Electron Mössbauer Spectroscopy has been performed between room temperature and 4.2 K on a series of thin films 1000 Å thick of amorphous Fe_xGe_1?x alloys, 0.4 ? x ? 0.60. It shows that the onset of ferromagnetism for x > 0.4 is associated with the apparition of a small magnetic moment in Fe atoms, increasing rapidly with x. For a given composition, the distribution in the values of moments born by the Fe atoms is large, reflecting the statistical fluctuations of the nearest neighbour environment of the iron atoms.     

Structure of α-FeSi alloys with 8 and 10 at % silicon

The atomic structure of single-crystal samples of Fe_{1 ? x} Si _x (x = 0.08, 0.10) alloys has been investigated using X-ray diffraction. It has been shown that the body-centered cubic (BCC) lattice of these alloys contain clusters with local ordering of the B2 type, which are characteristic of alloys with depleted composition (x = 0.05?0.06). The Fe₃Si phase with the D0₃ structure has been revealed at silicon concentrations x = 0.08 and 0.10. The volume fraction of Fe₃Si-phase regions increases both with an increase in the silicon concentration in the Fe_{1 ? x} Si _x alloy and during annealing of the samples with this silicon concentration at a temperature of 450°C. Based on the results obtained, it has been concluded that the anisotropic distribution of the B2 clusters, which arises as a result of thermomagnetic or thermomechanical treatment, is responsible for the induction and stability of the uniaxial magnetic anisotropy in the Fe_{1 ? x} Si _x (x = 0.05?0.10) alloys.     

Structural and magnetic properties of the Dy1−xThxFe3 system and some compounds of the form M0·1Th0·9Fe3

The structural and magnetic properties of the pseudobinary Dy_1?xTh_xFe₃ system, crystallizing in PuNi₃ type structure, have been investigated. For x<0·4 the magnetization vs temperature behavior can be understood on a ferrimagnetic coupling between the dysprosium and iron sublattices. For X=0·6 to 0·9 the magnetization vs temperature data show a marked transition at temperatures above 100°K. Lattice constants indicate a discontinuity in the lattice parameter c between X=0·6 and 0·5. Similar anomalies in the M vs T behavior were observed in M_0·1Th_0·9Fe₃ (where M=Sc, Y, Pr, Lu) compounds. These results can be explained on the basis of Friedel's model of transition metal d-bands.     

Magnetic properties of metallic glasses of the type Fe80−x Pd x B20 and Fe80−x Pt x B20

Amorphous alloys of the type Fe_80???xPd_xB₂₀ and type Fe_80???xPt_xB₂₀ for 0?≤?x?≤?50 have been investigated by means of ⁵⁷Fe Mössbauer spectroscopy and magnetisation measurements in temperatures from 4.2 up to 300 K. Curie temperatures and crystallisation temperatures are found by DTMG-DTA method. Mössbauer spectroscopy magnetic field is observed to visible increase for x?=?1 and 1.5% at room temperature for Pd, while a decrease is observed for higher x values. Curie temperature for Pd alloys has a maximum at x?=?4 with T _C?=?753 K, which supports enforcing influence of Pd at low concentrations of Pd for magnetic interactions. We discuss different explanations for these measurements and compare with other findings for high Pd concentrations and alloys with Pt instead of Pd.     

Magnetic properties of Mn-rich Rh2Mn1+xSn1−x Heusler alloys

X-ray powder diffraction and magnetization measurements have been carried out on Rh₂Mn_1+xSn_1−x (0≤x≤0.3) alloys. The alloys, which crystallize in the L2₁ structure, were found to exhibit ferromagnetic behavior. The lattice constant a at room temperature decreases with increasing x, whereas the Curie temperature T_C decreases linearly. At 5 K the magnetic moment per formula unit first increases with increasing x and then saturates for x≥0.2. The experimental results are discussed in terms of the influence of the Mn-Mn exchange interactions between the Mn atoms on the Sn and Mn sites.