Solid State Phenomena Vols. 172-174

Abstract: In recent years, spintronics whose principle is based on controlling the spin of electrons in semiconductor layers is presented as a complementary or even an alternative solution for production of logic devices in microelectronics. It relies on the fact that electric current in a magnetic layer can be spin polarized. Manufacture of such components is based on the use of materials or heterostructures whose electronic properties depend on their magnetic state. The magnetic Mn-Ge system is interesting because of its possible development at high Curie temperature and its integration on Si substrate. Among all of the Mn-Ge phase compounds of the diagram, Mn₅Ge₃ seems the most interesting one for spintronics applications: it is a stable and ferromagnetic phase at room temperature. In this paper, we present first results of the study, by Reflection High Energy Electron Diffraction (RHEED), X-ray diffraction (XRD) and Atomic Force Microscopy (AFM), of the sequence of formation of the Mn_xGe_y phases in the case of reaction of a nanometric-thick Mn film (200nm) deposited by MBE on Ge (111).

Abstract: The study was devoted to the thermal stability of the phases present in chromium-less and practically carbon-less Ni27Ti2AlMoNb steel. Steel with an austenitic structure was subjected to martensitic transformations using a thermal treatment, namely cooling to below the temperature M_s (beginning of the martensitic transformation), or by plastic deformation using the TRIP effect. The cooling in liquid nitrogen gave a martensitic-austenitic structure with athermal martensite α’_a. The martensite had a lenticular morphology, and its volumetric share was 51%, which is typical of duplex type steels. The 50% squeeze of the austenitic steel resulted in the formation of deformation martensite α’_d with a lamellar structure (this transformation is only possible below a certain temperature M_d). During the experiments, the thermal stability of the two phases, α’_a and α’_d, at temperatures of 450°C and 550°C was examined. The changes taking place in the steel structure when the athermal martensite and deformation martensite were annealed below and above the temperature A_s (beginning of the austenitic transformation) were observed using a vibrating sample magnetometer and an optical microscope.

Abstract: Single crystals of Ni-25.6 at.% Pt and Ni-87.8 at.% Pt were investigated by diffuse x-ray scattering for states of thermal equilibrium (923 K and 603 K, respectively). The separated short-range order scattering showed local maxima at 100 positions. Effective pair interaction parameters, as determined by the inverse Monte Carlo method, show a strong composition dependence of the nearest-neighbor interaction parameter. First-principles calculations are consistent with this finding and reveal a large contribution due to lattice strain. Based on values of the ordering energy, NiPt₃ with L1₂ structure was considered as a plausible new intermetallic phase, with Monte Carlo simulations giving an order-disorder transition temperature of 650 K. A single crystal of Ni-75.2 at.% Pt, quenched from 1073 K and aged at 613 K, showed L1₂-type ordering, reaching a long-range order parameter of 0.50(4) after 800 h.

Abstract: The present authors recently presented the time-dependent Ginzburg-Landau (TDGL) formulation for L1₂ type ordering process in binary alloys, taking into account the symmetrical relationships of these ordered phases. Extending the formulation, the authors have developed the TDGL model for microstructural evolution of D0₁₉ type ordering. The D0₁₉ structure based on hcp is divided into four equivalent sublattices. The site occupation probabilities are given as a function of three order parameters and a composition parameter. Multiple types of variants of the structures are represented by the order parameters. Mean-field free energies are defined in a form of Landau type expansion with the order parameters and the composition parameter. Interfacial energies due to local variations of degrees of order and composition are given in a gradient square approximation. Kinetic equations for time-evolution of the order parameters and the composition one are derived from the Ginzburg-Landau type potential consisting of the mean-field free energies and the interfacial energy terms. Three-dimensional numerical simulations based on the kinetic equations have been performed, and the domain structures obtained are compared with a TEM image of Cu₃Sn alloy.

Abstract: The tetragonal lattice relaxation has been included in the thermodynamics of the fcc→L1₀ ordering to produce a first-order character of the transition within the mean field description of the binary solution energetics. In view of growing interest in such systems e.g. Fe-Pd and Co-Pt alloys, which display a wide range of applications relevant to current and futuristic technologies, the fcc→L1₀ two-phase field is re-examined utilizing a generalized Bragg-Williams approach including first and second nearest neighbor interactions. The thermodynamic behavior is examined in the limit of T→0K and discussed in terms of the implications of the Third Law of Thermodynamics.

Abstract: Short-range order formation in dilute Fe-Si and Fe-Al alloys has been investigated by statistical Monte Carlo simulations with effective interactions deduced from first principles calculations for different magnetic structures of bcc Fe. We find that the variation of the magnetic order from ferromagnetic to paramagnetic leads to significant changes in effective cluster interactions and, as follow, in short-range order parameters of alloys. It is shown in agreement with experiment the B2 type short-range order is formed above the Curie temperature, T_C, while the D0₃ type short-range order is preferred below T_C.

Abstract: Several samples of (U,Nd)O₂ with different Nd content (ranging from 2 to 50 in Nd₂O₃ mass. %) were prepared. For composition ranging from 6 to 20 in Nd₂O₃ mass %, two crystalline phases are unambiguously evidenced by X-ray and neutron diffraction, by optical microscopy, and by SEM-EDX and EPMA. The influence of the preparation route on the distribution of the domains made by these two phases is evidenced and discussed.

Abstract: Silicide sequential phase formation during tens-of-nanometer-thick metallic film reaction on Si substrate has been extensively studied. Nevertheless, the reasons of sequential phase formation are still under debate, and have been poorly studied at the atomic scale. Using atomistic kinetic Monte Carlo simulations, we show that considering a binary fcc non-regular solid solution, without diffusion asymmetries, the diffusive reaction of a sub-nanometer-thick film (~5 atomic monolayers) on a semi-infinite substrate leads to the sequential formation of all the phases present in the binary phase diagram, starting with the film atom richest phase. These predictions are supported by experimental observations: the dissolution of a 4 monolayer-thick Si film on a Ni(111) substrate, during in-situ ultra high vacuum Auger electron spectroscopy, shows delays and kinetic changes in the dissolution process that may correspond to the sequential formation of the Ni-Si compounds, i.e. NiSi₂, NiSi, Ni₃Si₂, Ni₂Si, Ni₃₁Si₁₂ and Ni₃Si.

Abstract: The formation of metal (Ni and Pd) silicide thin films on a Si wafer is analyzed using differential scanning calorimetry (DSC) and isothermal X ray diffraction measurements. The sensitivity of DSC is remarkable even in this experimental Ni/Si and Pd/Si(001) and allows to show two steps of growth for a phase formation (lateral and normal growth). This technique is shown being of main interest for characterization of silicide formation during microelectronic industrial processes. Combining X-ray diffraction measurements and DSC measurements, the interface mobilities and the effective diffusion coefficient characterizing Ni₂Si and Pd₂Si growth are measured. These quantities as well as the interface mobilility for lateral growth have been determined by using a model taken into account the nucleation and lateral growth as well as a normal growth controlled by diffusion and interface reaction.

Abstract: The phase formation sequence of Ni silicide for different thicknesses is studied by in situ X ray diffraction and differential scanning calorimetry measurements. The formation of a transient phase is observed during the formation of δ-Ni₂Si; transient phases grow and disappear during the growth of another phase. A possible mechanism is proposed for the transient phase formation and consumption. It is applied to the growth and consumption of θ-Ni₂Si. A good accordance is found between the proposed model and in situ measurement of the kinetics of phase formation obtained by x-ray diffraction and differential scanning calorimetry for higher thickness.