Magnetic properties and magnetic interactions in chromium-rich Cr-Fe alloys

The results of magnetization measurements on several disordered b.c.c. Cr-Fe alloys with 1, 1.5, 2.4, 5.3, 12 and 14.2 at % of Fe are reported. The measurements were done in pulsed magnetic fields up to 330 kOe and for the two last alloys also in static magnetic fields up to 44 kOe as in function of temperature. The data support the recently proposed model of magnetic interactions in these alloys by Friedel and Hedman. For the alloys with iron concentration equal to and greater than 2.4 at % we observe, at low temperature, the occurence of a ferromagnetic component in the magnetization curves saturating at an external field of about 140 kOe arising from ferromagnetic iron-rich cluster. At liquid helium temperature the localized iron moments within such iron clusters increase from 1.4μ_B to 1.8μ_b when the iron concentration changes from 2.4 at % to 14.2 at %. At higher iron concentrations we observe a spin glass like transition connected with a freezing of ferromagnetic clusters at very low temperature. Both phase boundaries connected with supermagnetic-paramagnetic and superamagnetis-spin glass like transitions are given.     

Magnetic moment distribution in bcc FeTi alloys

The elastic diffuse scattering of polarized neutrons with 4.25 Å wavelength from polycrystalline disordered bcc Fe-Ti alloys with 2.88 at % and 4.92 at % of Ti has been measured at room temperature in the scattering vector range 0.1 Å^?1 ? K ? 2.4 Å^?1. Analysis of the results shows the existence of a large negative moment of Ti decreasing in absolute value with alloy concentration. The Fe moment is nearly constant up to 2.88 at % of Ti and thereafter decreases. The magnetic moment disturbance is more pronounced at lower Ti concentration whereas the nuclear short range is more important at higher impurity content. Both effects are limited up to the second nearest neighbours.     

Study of the heavy-fermion compound CeRu<Subscript>2</Subscript>Si<Subscript>2</Subscript> by the small-angle neutron scattering method

In order to directly observe neutron scattering by heavy fermion quasiparticles at low temperatures, a CeRu₂Si₂ single crystal has been studied by the small-angle neutron scattering method. In the experiment, neutron scattering is observed at T = 0.85 K for momentum transfers q ≤ 0.04 Å^?1, which is treated as the orbital component of magnetic scattering by heavy fermion quasiparticles. It has been found that the application of a magnetic field H = 1 T leads to both an increase in the observed scattering and its anisotropy with respect to the field direction. Moreover, measurements in the magnetic field reveal additional scattering for q > 0.04 Å^?1, which is well described by a Lorentzian and is interpreted as neutron magnetic scattering by spin-density fluctuations with a correlation radius R_c ≈ 30 Å.     

The spin wave dispersion relation of disordered Pd0.5Fe0.5

The spin wave spectrum of chemically disordered Pd_0.5Fe_0.5 has been determined by neutron inelastic scattering. At small wavevectors, the dispersion curve is quadratic and isotropic with a spin wave stiffness constant of 47.9 ± 3.3 THz Ų, at room temperature. There is little sign of the anisotropic behaviour observed in the ordered alloy Pd₃Fe.     

Spin waves in Fe3−xMnxSi ordered alloys

Spin wave dispersion relations in Fe_3?xMn_xSi ordered alloys have been measured by neutron scattering for alloys with x = 0.04 and x = 0.18. The dispersion relations have been found to be quadratic in the magnon wavevector q? up to about half of the Brillouin zone along [111] direction. The measured values of the spin wave stiffness constant are 169±7 meV Å⁰² and 207±7 meV Å⁰² for samples with x = 0.18 and x = 0.04, respectively. The energy gap found from the rough data turned out to be almost totally due to the triple-axis spectrometer resolution properties. The nearest neighbour exchange integral for FeFe interaction, calculated from the above data, equals to 22.5±1.0 meV.     

Low temperature specific heat and magnetic properties of V/FE and V/FE/H alloys

Low temperature specific heats of V_1?xFe_x (0?x?0.34) and V_1?xFe_xH_n (x = 0.02; 0.05; 0.1; 0.15 a 0 ? n ? 0.98) were investigated between 1.5 and 16 K.In V/Fe alloys temperature independent band paramagnetism was observed at iron concentrations below 20at%; at higher iron contents local magnetic moments occur and contribute magnetic terms to the low temperature specific heat. According to Schröder [14] these contributions can be explained by thermal agitation of superparamagnetic clusters of the Fe atoms with local moments. In the temperature range of the specific heat measurements the magnetic terms are dependent on temperature (contrary to Schröder) and can be described by Planck-Einstein functions. A separation of the magnetic contributions from the lattice and electronic term of heat capacity could be obtained in a satisfying way by least squares fitting with three Planck-Einstein functions.In “nonmagnetic” V/Fe alloys with less than 20at% Fe absorption of hydrogen generates local magnetic moments and superparamagnetic contributions. These are, therefore, connected primarily more with the band electron concentration n_e, than with the iron content X_Fe At the highest n_e, values, n_e >0, large negative deviations from the regular course of the heat capacity plot are observed; these can be related to mictomagnetic behavior (freezing temperature of spin-glass state above the range of measurements, T_F > 16 K).After correction for the superparamagnetic contributions there remains in the Fe containing alloys an appreciable enhancement of the electronic heat coefficient γ, up to a factor of about 2 when compared with the iron-free V/H samples. This effect is interpreted as resulting from mass enhancements of the itinerant electrons by electron-magnon (and-paramagnon) interactions.     

The special features of the ground state in CeAl3

The temperature (3–60 K) and transferred momentum (0.3–2.3 Å^?1) dependences of the intensity of quasi-elastic magnetic neutron scattering were studied for the polycrystalline heavy-fermion CeAl₃ compound to elucidate the special features of its ground state. Transferred momentum variations caused oscillations of the intensity of quasi-elastic magnetic neutron scattering, which was evidence of magnetic correlations in the f-electron subsystem occurring in a fairly wide temperature range.     

The diffusive motion of silver ions in α-AgI: Results from quasielastic neutron scattering

The diffusive motion of silver ions in σ-AgI at 250°C has been studied by quasielastic cold neutron scattering. Spectra were taken in the range of wavevector transfer 0.5 < Q < 2.2Å^?1 for elastic scattering. The quasielastic line shapes contain a narrow and a broad component. They are compared to model calculations allowing for the superposition of two kinds of motion on two different time scales, a local random motion and a translational motion of the jump-diffusion type. The model closely fits the data. The local random motion takes place on a time scale of the order of 10^?12 s, with amplitudes of the order of 1 Å. It is probably caused by rapid fluctuations of the local potentials due to the diffusive motion of the other cations. The translational motion results in a mean displacement of the silver ion over a distance of the order of a lattice constant (5 Å) with a correlation time of the order of 10^?11s. This correlation time is composed of a residence time and a time-of-flight, which are both of comparable magnitude.     

In the wonderland of ultra-parallel neutron beams

Bragg reflections from single crystals yield angular widths of a few arcsec for thermal neutron beams. The Bonse-Hart proposal to attain a sharp, nearly rectangular profile by Bragg reflecting neutrons multiply from a channel-cut single crystal, was realized in its totality three and a half decades later by achieving the corresponding Darwin reflection curves for 5.23 Å neutrons. This facilitated SUSANS (Super USANS) measurements in the Q ～ 10^?5 Å^?1 range. The polarized neutron option was introduced into the SUSANS set-up by separating the up- and down-spin neutron beams by ～10 arcsec with a magnetic (air) prism. The neutron angular width has recently been reduced further by an order of magnitude to ～0.6 arcsec by diffracting 5.3 Å neutrons from a judiciously optimized Bragg prism. This constitutes the most parallel monochromatic neutron beam produced to date. I present the first SUSANS spectra probing the Q ～ 10^?6 Å^?1 domain, recorded with this beam.     

Precision neutron total cross section measurements on gold and cobalt in the 40 μeV-5 meV range

The slow neutron absorption cross sections of gold and cobalt have been accurately redetermined by transmission measurements in the neutron wavelength range 4 to 47 Å. For the range 4 to 7.6 Å a new time-of-flight spectrometer at the FRM reactor was used which involves a system of three synchronized choppers and a 150 m long guide tube as flight path. Utilizing the high wavelength resolution of the spectrometer, the time-of-flight wavelength scale could be accurately calibrated (±2 · 10^?3 Å) by means of various Bragg cutoff breaks observed in transmission on polycrystalline samples. Supplementary transmission measurements on gold and cobalt were performed in the range 11–47 Å using the time-of-flight spectrometer for ultracold neutrons at the FRM reactor. The absorption cross sections were evaluated considering corrections for incoherent and inelastic scattering and for the slight deviation from 1/v of the absorption of gold due to the 4.9 eV resonance level. For the absorption at subthermal energies we obtainσ _a /gl=(54.35±0.06) b/Å for Au, (20.66±0.04) b/Å for Co. Evaluation of the absorption cross sections at 2200 m/s neutron velocity gives (98.68±0.12) b for Au, (37.15±0.08) b for Co.     

Korringa relaxation rates of Mn ion in noble metal hosts

Neutron scattering measurements of the paramagnetic spectral function in dilute alloys of Mn in the noble metal hosts Ag and Au are reported. The spectral width is found to have the characteristic q-dependence due to Mn-Mn couplings for moderately concentrated alloys (?3 at% Mn) but is q-independent in the measured q-range (>0.3Å^-1) for Mn concentrations around 1 at% as found earlier in $\text{Cu}$-Mn alloys also. The dilute limit spectral width which is proportional to the Korringa relaxation rate decreases down the column of the periodic table following roughly the ration $\text{3: 1}\text{1}\text{2}\text{: 1}$ for Cu, Ag and Au, respectively. Assuming that the direct exchange constants are roughly the same in the three hosts, it would appear that the s-d mixing term decreases progressively with increasing “nobility” of the host.     

Impurity lattice dynamics of Fe in Cr from precision Mössbauer fraction measurements

Precision 〈x²〉-values and their anharmonicity been measured for ⁵⁷Fe in Cr between 80 K and 600 K. At 294 K. $\text{? = 0.798 ± 0.005}$. A theoretical analysis based upon Cr neutron scattering data yields 0.8 for the impurity/host force constant ratio.     

Structure of the Ag on Si(111) 7 × 7 interface by means of surface exafs

Polarization dependent surface extended X-ray absorption fine structure (SEXAFS) measurements are used to determine the structure of the Ag on Si(111)7 × 7 system at the early stages (< 3 monolayers (ML)) of interface formation. At room temperature (RT) Ag is found to initially (< 0.5 ML) chemisorb in the threefold hollow site, approximately 0.7 Å above the outermost Si layer with an average Ag-Si distance of 2.48±0.05 Å. Above monolayer coverage the SEXAFS spectrum is dominated by the Ag-Ag distance indicating Ag island formation on the surface. Upon heating (200 ?T? 600°C) a (√3 ×√3)R30° LEED pattern is observed. At the lowest coverage ( < 0.7 ML) this pattern is determined to arise from Ag atoms which are embedded in the threefold hollows, ~ 0.7 Å below the first and above the second Si layer, with a Ag-Si distance of 2.48 ± 0.04 Å. At higher coverage ($\text{\$}\text{?}$1 ML) Ag clusters are found to grow on this interface with the same Ag-Ag distance as in Ag metal. Our results are discussed in the context of previous experimental and theoretical results.     

Mössbauer and XRD study of novel quaternary Sn-Fe-Co-Ni electroplated alloy

Constant current electrochemical deposition technique was used to obtain quaternary alloys of Sn-Fe-Co-Ni from a gluconate electrolyte, which to date have not been reported in the literature. For the characterization of electroplated alloys, ⁵⁷Fe and ¹¹⁹Sn Conversion Electron Mössbauer Spectroscopy (CEMS), XRD and SEM/EDAX were used. XRD revealed the amorphous character of the novel Sn-Fe-Co-Ni electrodeposited alloys. ⁵⁷Fe Mössbauer spectrum of quaternary deposit with composition of 37.0 at% Sn, 38.8 at% Fe, 16.8 at% Co and 7.4 at% Ni displayed a magnetically split sextet (B = 28.9T) with broad lines typical of iron bearing ferromagnetic amorphous alloys. Magnetically split ¹¹⁹Sn spectra reflecting a transferred hyperfine field (B = 2.3T) were also observed. New quaternary Sn-Fe-Co-Ni alloys were successfully prepared.     

Coexistence of superconductivity and ferromagnetism in the Nb(500 Å)/Fe(39 Å)/[Si(34 Å)/Mo(34 Å)]40/Si nanostructure

The results of polarized neutron reflectometry investigations of the magnetic state of the Nb(500 Å) superconductor-⁵⁷Fe(39 Å) ferromagnet-Si(34 Å)/Mo(34 Å)]₄₀ superconductor nanostructure in the temperature range 2–60 K in the magnetic field with a strength of 0.5 kOe are presented. A decrease in magnetization in an iron layer for the transition of Nb(500 Å) and [Si(34 Å)/Mo(34 A)]₄₀ layers to the superconducting state has been found.     

Small angle diffuse neutron scattering: Measurements on V2O3 in the temperature range 400–600 K

Cold neutron small-angle scattering measurements on pure V₂O₃ in the high-temperature transition domain shows an anomalous feature namely a peak and a bump in the range 500–570 K. It is suggested that a plausible origin of this anomaly is the formation of giant “correlation” clusters of size ～ 600 Å and a magnetic moment density ～ 10^-7 μ/ų as the system suffers a broad M-I transition.     

Phase transitions in the atomic, electronic, and magnetic subsystems of LaSrMnO films versus the synthesis temperature

The evolution of the cluster structure in amorphous LaSrMnO films as synthesis temperature T _s increases from 20 to 300°C is considered. Two order-disorder phase transitions with different scale parameters are observed. One of them, the aggregation of disordered atoms into small (～20 Å) amorphous clusters at T _s = 100°C, shows up as a sharp increase in the intensity of diffuse X-ray scattering (diffuse halo 1) with a simultaneous suppression of incoherent (background) scattering. At T _s > 150°C, disordering dominates (I _incoh = I _max) until the next stage of ordering sets in at T _s = 250?300°C. At this stage, the crystalline phase forms from large (>100 Å) crystalline clusters. This amorphous-crystalline phase transition is characterized by the appearance of Debye lines and a reduction of the halo intensity. The structural phase transition to long-range order is accompanied by a decrease in the LaSrMnO resistivity from 10¹⁰ to 10 Ω cm and a change from the tunneling mechanism of conductivity involving metallic clusters (which is typical of granulated systems) to the hopping mechanism with a hop variable length following the Mott law ρ ～ exp(T ^?1/4). In the magnetic subsystem, the paramagnetic-ferromagnetic phase transition occurs.     

Cross Sections for Neutron–Deuteron Elastic Scattering in the Energy Range 135–250 MeV

We report new measurements of the neutron–deuteron elastic scattering cross section at energies from 135 to 250 MeV and center-of-mass angles from 80^? to 130^?. Cross sections for neutron-proton elastic scattering were also measured with the same experimental setup for normalization purposes. Our nd cross section results are compared with predictions based on Faddeev calculations including three-nucleon forces, and with cross sections measured with charged particle and neutron beams at comparable energies.     

Dispersion relation of low-energy excitations in Zr<Subscript>50</Subscript>Be<Subscript>50</Subscript> metallic glass

A high-resolution (1.5 meV) inelastic neutron scattering experiment is carried out to investigate in detail the low momentum (0.6–1.5 Å^?1) dynamic response in Zr₅₀Be₅₀ metallic glass. The results are obtained on the hot neutron three-axis spectrometer IN1 in the Laue-Langevin Institute (Grenoble, France). A comparison with recent neutron scattering results for the momentum transfer region above 1.4 Å^?1 shows a minimum in the dispersion relation of low energy excitations near the prepeak of the static structure factor. These excitations appear to be due to the short-range order in this amorphous system.     

Mössbauer study of incompletely alloyed nanocrystalline Fe100 − x Al x prepared by high energy ball milling

Mechanically alloyed Fe_100???x Al _x powders, with 20≤?x?≤90, have been studied by X-ray diffraction and room temperature ⁵⁷Fe Mössbauer spectroscopy. The milling time was chosen such that complete alloying does not take place. For a fixed milling time of 10 h, the rate of alloying was seen to increase exponentially with increase in Fe content. Mössbauer spectra of all the samples consist of a broad magnetic sextet and a quadrupole doublet. The isomer shifts and quadrupole splitting of the doublets are typical of Al-rich, Fe–Al alloys. The area under the quadrupole doublet is a maximum for x?=?66. Analysis of the Mössbauer spectra indicates the formation off- stoichiometric Fe₃Al phase for x?<?66, while the formation of Fe clusters is largely responsible for the magnetic hyperfine component in x?≥?66 compositions.