[en] The variation with composition of the magnetic properties of amorphous Fe--P--B alloys obtained by splat-cooling is discussed. For a given Fe concentration, the magnetic moment per atom anti μ and the Curie temperature T/sub c/ both increase with the substitution of B for P. The variation of anti μ and T/sub c/ as a function of the B content had the same discontinuity as that occurring between the two phases epsilon and psilon₁ of the crystalline Fe₃P/sub 1-x/B/sub x/ compounds. The variation of anti μ and T/sub c/ was also studied when one metalloid (P or B) substitutes for Fe, the other metalloid remaining constant. The value of anti μ was found to decrease when the P or B content is increased, as expected from a rigid band model. In the same concentration range, T/sub c/ increases with either P or B. The different behaviors of anti μ and T/sub c/ are tentatively explained by assuming that the amorphous Fe has two magnetic states (ferro and antiferromagnetic) as already suggested for crystalline Fe in a fcc environment

[en] A complete substitution of iron for nickel was obtained by splat-cooling in amorphous alloys of composition (Ni/sub 100-y/Fe/sub y/)₇₉P₁₃B₈. Results of high-field magnetization (up to 70 kOe), ac and dc low-field susceptibility, Curie temperature, and resistivity measurements over a temperature range of 1.7 to 300⁰K are reported. The Ni₇₉P₁₃B₈ alloy is not ferromagnetic, but the magnetization behavior as a function of field and temperature is typically that of alloys in the critical concentration range for ferromagnetism. The Fe₇₉P₁₃B₈ alloy is ferromagnetic with a Curie temperature T/sub c/ of 616⁰K. For y = 1 at. percent, the Fe atoms are magnetic. The variation of the moment per Fe atom as a function of y is discussed. When y is increased, the Ni atoms are likely to be polarized progressively and the moment per Ni atom would be roughly constant for y equal to or greater than 30 at. percent. Various magnetic behaviors were defined as a function of the Fe content. The value of T/sub c/ reaches a maximum for y similarly ordered 90 at. percent and extrapolates to zero for y similarly ordered 7 at. percent. Alloys within the range 1 equal to or less than y equal to or less than 10 at. percent did not exhibit well-defined Curie transition, but sharp maxima in low-field susceptibility measurements were observed at T/sub M/. The value of T/sub M/ is proportional to y for 1 equal to or less than y equal to or less than 4 at. percent, as in classical spin-glass regimes. For 4 less than y equal to or less than 10 at. percent, the variation of T/sub M/ as a function of y implies a more complicated type of magnetic ordering (micromagnetism or superparamagnetism). Homogeneous ferromagnetic ordering emerges only for y greater than 10 at. percent. Results of resistivity measurements are discussed in relation to the magnetic properties of different regimes in the magnetic phase diagram. 6 figures, 2 tables

[en] We have studied both pion- and photon-induced eta production in the nucleon resonance region. The πN → ηN reaction has been investigated with a dynamical coupled-channels formalism based on effective Lagrangians, including five meson-baryon states (πN, ηN, πΔ, σN, ρN). The γN → ηN process has been described by using a constituent quark formalism for the direct channel and by the same formalism as the one used for the πN → ηN reaction to derive the coupled-channels mechanisms. We have constructed models in the total energy range in the center-of-mass frame between the threshold of the considered reactions and 2 GeV approximately. These models embody known resonances in that energy range namely S₁₁(1535), S₁₁(1650), P₁₁(1440), P₁₁(1710), P₁₃(1720), P₁₃(1900), D₁₃(1520), D₁₃(1700), D₁₅(1675), F₁₅(1680), F₁₅(2000) and F₁₇(1990). Free parameters have been adjusted mainly with old experimental data for the π^-p → ηn reaction and more recent ones for γp → ηp. Knowing the heterogeneous character of the database for the reaction π^-p → ηn, we have obtained a satisfactory description of the observables. The most important ingredients of the model are S₁₁(1535), S₁₁(1650), P₁₃(1720), F₁₅(1680) et D₁₃(1520). For the photoproduction process, the database which includes differential cross sections and simple polarization observables is more copious and more consistent. The derived model is dominated by the following resonances: S₁₁(1535), S₁₁(1650), D₁₃(1520), P₁₃(1720) and F₁₅(1680). The sophisticated feature of the formalism and the amount of accurate data for the photoproduction process allow us to search for possible contributions from the so-called missing and/or new resonances. Our study shows an important role played by a new S₁₁, whose mass and width have been extracted (M = 1707 MeV, Γ = 222 MeV), and to a lesser extent by a new D₁₃ (M = 1950 MeV, Γ = 139 MeV). These results contribute to a better insight on the properties of known resonances. The new resonances underlined in this work are significant ingredients in understanding the baryon spectroscopy, especially within approaches based on QCD. (author)

[en] Splat-cooled alloys of composition (La/sub 100-x/Gd/sub x/)₈₀Au₂₀ have been obtained by complete substitution of gadolinium for lanthanum in amorphous La₈₀Au₂₀ matrix. Results of high-field magnetization (up to 70 kOe), ac and dc low-field susceptibility, and resistivity measurements over temperature range of 1.7 to 300⁰K for these alloys are reported. The La₉₀Au₂₀ alloys are superconducting at 3.5⁰K. For x approximately less than 1, a suppression of T/sub c/ described by the relation dT/sub c//dx approximately equal to -4.0⁰K per atomic per cent gadolinium is observed. For alloys within the concentration range 1 approximately less than x approximately less than 70, maxima in low field susceptibility measurements are observed. The ''ordering'' temperature, T/sub M/, are proportional to x for 1 less than or equal to x less than or equal to 16, similar to those observed in crystalline spin-glass alloys. For 16 approximately less than x approximately less than 70, T/sub M/ is increasing at a faster rate than in the low concentration region, and this intermediate type of ordering corresponds to a mictomagnetic regime. At x approximately greater than 70, a ferromagnetic regime emerges. The maximum Curie temperature is observed for Gd₈₀Au₂₀ at approximately 150⁰K. The moment per gadolinium atom is found to be constant and close to that of the crystalline value throughout the concentration range investigated. Results of resistivity measurements are correlated with the magnetic properties of different regimes in the magnetic phase diagram. 4 figures, 1 table

[en] Bulk magnetic properties of splat-cooled amorphous alloys of composition La/sub 80-x/Gd/sub x/Au₂₀ (0 less than or equal to x less than or equal to 80) were studied. Zero-field susceptibility, high-field magnetization (up to 75 kOe) and saturated remanence were measured between 1.8 and 290⁰K. Data were analyzed using a cluster mean-field approximation for the spin-glass and mictomagnetic alloys (x less than or equal to 56). Mean-field theories can account for the experimental freezing-temperatures of dilute spin-glasses in which the Ruderman-Kittel-Kasuya-Yosida interaction is dominant. For the dilute alloys, the role of amorphousness on the magnetic interactions is discussed. By extending the mean-field approximation, the concentrated spin-glasses are represented by rigid ferromagnetic clusters as individual spin-entities interacting via random forces. Scaling laws for the magnetization M and saturation remanent magnetization M/sub rs/ are obtained and presented graphically for the x less than or equal to 32 alloys in which M/x = g(H/x*, T/x), M/sub rs/(T)/x = M/sub rs/(0)/x/ exp (-α*T/x/sup p/) where x* is the concentration of clusters, α* is a constant, and p is the freezing-temperature exponent given by T/sub M/ infinity x/sup p/. It is found that p = 1 and 1.3 for the regions 4 less than or equal to x less than or equal to 40 respectively. An attempt is also made to account for the freezing temperatures of concentrated spin glasses. The strength of the interaction among clusters is determined from high-field magnetization measurements using the Larkin-Smith method modified for clusters. It is shown that for the x < 24 alloys, the size of the clusters can be correlated to the structural short-range order in the amorphous state. More concentrated alloys are marked by the emergence of cluster percolation

[en] Results of bulk magnetic measurements are reported for splat-cooled amorphous alloys (La/sub 100-x/Gd/sub x/)₈₀Au₂₀ over the concentrated region between the dilute (x less than or equal to 1 at.%) and the ferromagnetic range (x > 70 at.%). For x less than or equal to 40 at.% alloys, low field susceptibility maxima occur at temperatures proportional to x, and the reduced remanent magnetization M/sub rs/(T)/x depends only on the reduced temperature T/x. Our results are analyzed in the spirit of a phenomenological model of uncompensated magnetic clouds. The interplay of the RKKY interaction and of the anisotropic dipolar forces is sufficient to account for the order of magnitude of the remanent magnetization for x less than or equal to 40 alloys

[en] The ternary diagram of the amorphous phase in the splat-cooled Fe--P--B system was investigated by x-ray diffraction measurements. Alloys were found to be amorphous within the Fe concentration range 75 less than or equal to c/sub Fe/ less than or equal to 83 at. percent. For a fixed Fe concentration, B substitutes for P over a large scale, the maximum substitution occurring for c/sub Fe/ = 80 at. percent. Concentration dependence of the electronic and magnetic properties was systematically studied in alloys within the whole amorphous diagram by means of bulk magnetization, anti μ (4.2⁰K), Curie temperature, T/sub c/, and electrical resistivity, rho, measurements from 4.2⁰K up to the amorphous-crystal transformation. The composition dependence of anti μ and T/sub c/ when B substitutes for P at c/sub Fe/ constant suggests the existence of two different short-range orders in the amorphous alloys, corresponding to the epsilon and epsilon₁ crystal structures in the Fe₃P/sub 1-x/B/sub x/ compounds. The concentration dependence of anti μ and T/sub c/ when P (or B) substitutes for Fe at c/sub B/ (or c/sub P/) constant is explained by a comparison between the amorphous Fe and the fcc Fe. A minimum in the variation of rho as a function of the temperature T occurs at T/sub m/. The temperature dependence of rho is expressed by a phenomenological law: rho(T) = rho₀ + A log T + B T² + CT. The concentration dependence of T/sub m/ and of the coefficients A and B is discussed in relation with the magnetic and structural properties of the amorphous Fe--P--B alloys. 10 figures, 1 table

[en] From magnetization measurements on some amorphous dilute La/sub 79-x/Gd/sub x/Au₂₀ alloys with x less than or equal to 1, it is shown that the magnetic behavior follows the scaling laws of a spin-glass system, characteristic of the 1/r³ dependence of the pairwise interaction. The strength of the Ruderman-Kittel-Kasuya-Yosida interaction, V(r) = (V/sub o/cos 2k/sub F/r)/r³, was determined to be V/sub o/ = 0.20 x 10^-37 erg cm³. The corresponding value of the s-f exchange integral is vertical bar J/sub sf/ vertical bar = 0.14 eV, which is compared with values determined from other experiments. 4 figures, 1 table

[en] Magnetization and resistivity measurements are performed on amorphous La₈₀Au₂₀ alloys doped with Gd impurities. The anomalous behavior observed in the dependence of the superconducting transition temperature on Gd concentration is related to the onset of spin-spin correlations among the Gd impurities. We have identified the coexistence of superconductivity and magnetic ordering as a spin glass

[en] Magnetization was measured between 4 and 290⁰K in fields up to 70 kOe on liquid-quenched Gd₈₀Au₂₀ amorphous alloys. The Curie temperature and critical exponents β, γ and delta are found to be 149.45⁰K, 0.439, 1.294 and 3.948 respectively. These values are compared with the Heisenberg and molecular field values. The data are fitted to an equation of state previously derived for second order phase transition in fluid systems. The results illustrate clearly a second order phase transition in the amorphous state. A discussion in terms of the Heisenberg model is presented. The effective magnetic moment in the paramagnetic state has a value of 9.37 μ/sub B/ per gadolinium atom. The saturation moment extrapolated to 0⁰K is 7.0 μ/sub B/ per gadolinium atom. The low temperature saturation magnetization observes the T/sup ³/₂/ law from 0.13 T/sub c/ to 0.80 T/sub c/. The effective exchange integrals J/sub n/ determined from the Rushbrooke--Wood formula and spin-wave theory are found to be 2.28 and 1.34⁰K respectively. The differences in J/sub n/ and that between the effective moment and saturation moment are attributed to the nearest-neighbor antiferromagnetic couplings below T/sub c/. Possible effects of structural disorder on the magnetic properties of Gd in the amorphous state are discussed