Engineering of the magnetic properties of Finemet based nanocrystalline glass-coated microwires

The magnetic properties of Finemet nanocrystallized ribbons have been extensively studied and well understood. However, results obtained on microwires of such compositions are ambiguous considering the form of the hysteresis loop after the crystallization thermal treatment. In this paper, we show results on the static characterization of annealed Finemet based microwires (Fe_73.2Si_15.8B_6.9Cu_1.2Nb_2.9) of different geometries and illustrate how the magnetic anisotropy of Finemet based nanocrystalline microwires depends on the annealing temperature and the geometric characteristics of the microwires.     

Torsion and tension stress induced transformation of surface magnetic structure in Co-rich amorphous microwires

The effect of torsion and tensile stress on the surface magnetic properties in Co-rich glass covered amorphous microwires has been studied. The magnetization reversal in microwires has been studied by magneto-optical Kerr effect. Calculations of hysteresis loop was performed taking into account the existence of helical magnetic anisotropy in the outer shell of the microwire. The comparative analysis of the results of the experiment and the calculation demonstrates that the observed transformation of the surface hysteresis is related to the variation of the angle of the helical surface structure.     

Magnetostatic bias in soft/hard bi-phase layered materials based on amorphous ribbons and microwires

Bi-phase soft/hard magnetic systems have been prepared by electroplating CoNi alloy with relative hard magnetic character (10² Oe coercivity) onto ultrasoft (coercivity less than 1 Oe) FeCo-based amorphous alloys prepared by rapid solidification techniques. Experiments have been performed in planar (ribbons) and cylindrical (microwires) configuration. Low-field hysteresis loops after premagnetizing to saturation are ascribed to the magnetization reversal process of the soft inner phase, while harder component remains close to its remanence. Such low-field loops exhibit a shift typical of magnetic bias phenomena which in this case is ascribed to the magnetostatic coupling originated through the stray fields created by uncompensated charges at the ends of the harder phase.     

Complex primary crystallization kinetics of amorphous Finemet alloy

The complex primary crystallization kinetics of the amorphous Finemet soft magnetic alloys has been analyzed by non-isothermal DSC measurements. The local activation energies E_c(α) were determined by an isoconversional method without assuming the kinetic model function and its average value was about 383 kJ/mol. The nucleation activation energy E_n and growth activation energy E_g were 425 and 333 kJ/mol, respectively. And the apparent local activation energies E_c can be expressed by E_n and E_g as follows: E_c = aE_n + bE_g. The local Avrami exponents lies between 1 and 2 in a wide range of 0.2 < α < 0.9, and it indicates that dominating crystallization mechanism in the non-isothermal primary crystallization of amorphous Finemet alloy is one dimensional growth at a near-zero nucleation rate for surface crystallization. The significant variation of local Avrami exponent and local activation energy for primary crystallization with crystallized volume fraction demonstrates that the primary crystallization kinetics of amorphous Finemet alloy varies at different stages. In addition, the variable local activation energies E_c(α) and local Avrami exponents n(α) are applicable and correct in describing the primary crystallization process of the amorphous Finemet alloy according to the theoretical DSC curve simulation.     

Lateral photovoltage measurements in hydrogenated amorphous silicon and silicon-oxygen thin films

Hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon-oxide alloy films (a-SiO_x:H) were investigated by temperature dependence of lateral photovoltage (LPV) measurements. The suboxide sample with [O] = 27 at.%, was found to exhibit larger LPV compared to the unalloyed sample. It is difficult to simply correlate LPV measurements to related diffusion length measurements, only. On the other hand, the observed magnitude of LPV in a-Si:H and its decrease with temperature, could be explained based on an internal electric field induced by diffusion electron and hole currents, and multiple trapping of the photocarriers.     

Complex permeability and electromagnetic wave absorption properties of amorphous alloy-epoxy composites

Electromagnetic wave absorption properties of amorphous alloy-epoxy composites have been investigated with various amorphous alloy particle sizes and fractions in the 45 MHz to 10 GHz frequency range. The fraction of amorphous alloy in amorphous alloy-epoxy composites varied from 30 to 60 vol.% at a fixed amorphous alloy particle size and the size of amorphous alloy particles was varied from several μm to 125 μm at a fixed amorphous alloy particle fraction. Complex permeability (μ), permittivity (ε) of composites and reflection loss were measured by the reflection/transmission technique. The composites with small sized amorphous alloy particles (<26 μm) and a small amount of particles (<50%) had good reflection loss values less than −20 dB with thin thickness (<1 cm). The decreasing amorphous alloy particle size and fraction in amorphous alloy-epoxy composites resulted in a high minimum reflection loss frequency and small minimum reflection loss thickness. Variations of minimum reflection loss frequency and thickness of composites resulted from variations of materials constants such complex permeability, permittivity and resonance frequency. Changes in materials variables were due to the demagnetization effect and the eddy current effect, which operate differently in composites according to amorphous alloy particle size and fraction.     

Distribution of electronic states in amorphous Cd-As thin films on the basis of optical measurements

Transmission and fundamental reflectivity studies, completed on amorphous Cd-As thin films, allowed us to obtain parameters describing the fundamental absorption edge, i.e. the optical pseudogap E_G^opt, Urbach energy E_U and exponential edge parameter E_T. All these data, together with the results of earlier transport measurements, have been utilized in developing simple models of electronic structure (distribution of electronic states) for amorphous Cd-As thin films of various compositions.     

Influence of the amorphous/crystalline silicon heterostructure properties on planar conductance measurements

We report a quasi-analytical calculation describing the heterojunction between hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) at equilibrium. It has been developed and used to determine the carrier sheet density in the strongly inverted layer at the a-Si:H/ c-Si interface. The model assumes an exponential band tail for the defect distribution in a-Si:H. The effects of the different parameters involved in the calculation are investigated in detail, such as the Fermi level position in a-Si:H, the density of states and the band offsets. The calculation was used to interpret temperature dependent planar conductance measurements carried out on (n) a-Si:H/ (p) c-Si and (p) a-Si:H/(n) c-Si structures, which allowed us to confirm a previous evaluation of the conduction band offset, ?E_C = 0.18 ± 0.05 eV, and to evaluate the valence band offset: ?E_V = 0.36 ± 0.05 eV at the a-Si:H/ c-Si heterojunction. The results are placed in the frame of recent publications.     

ESR,Seebeck and Hall measurements on an amorphous system with hopping conduction

To get further information about the conduction mechanism, ESR, Seebeck and Hall measurements were performed on thermally degraded phenol-formaldehyde resins whose dc conductivity had proved to be proportional to exp $\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$. The Hall experiments yield no direct result. Although a sensitive double-frequency test set-up was used, no Hall voltage could be detected. As to the ESR and thermoelectric power measurements it was not difficult to get reliable results but rather difficult to interpret them. Most significant in this respect is the change in the sign of the Seebeck coefficient from minus to plus with rising thermal degradation. An estimation of the number of carriers from the thermo-emf leads to results compatible with those evaluated from the ESR signal.     

Enthalpy recovery, creep and creep-recovery measurements during physical aging of amorphous selenium

The physical aging behavior of amorphous selenium has been investigated using differential scanning calorimetry and conventional and interrupted creep experiments. As a result of physical aging, enthalpy decreases and the creep and recovery curves shift to longer times. The times required to reach equilibrium for enthalpy recovery and creep appear to have different temperature dependences resulting in enthalpy reaching equilibrium before creep at aging temperatures a few degrees below the nominal glass temperature. In the nominal glass transformation range, however, the times required to reach equilibrium are approximately the same. A general picture of aging behavior has emerged from our data on selenium coupled with past work on polyetherimide and polystyrene.     

Electrical resistivity,magnetic susceptibility and thermoelectric power of amorphous niobium-nickel alloys synthesized by rf-sputtering

Thin film samples (10–20 μ thick) of niobium-nickel alloys in the composition range Nb-5 to 95% Ni were vapour quenched by rf sputtering onto fused quartz substrates held at a temperature of 450 K. At room temperature, the electrical resistivity of these alloys lies between 176–210 μΩ cm, and the absolute thermoelectric power S between 2.20–2.52 μV/K. Magnetic susceptibility for Ni_0.5Nb_0.5 and Ni_0.4NB_.6 amorphous alloys show a Pauli magnetic behaviour with values of x of about 1.5 × 10^?4 and 1.8 × 10^?4 emu g^?1, respectively.     

Disorder and optical absorption in amorphous silicon and amorphous germanium

The role that disorder plays in shaping the functional form of the optical absorption spectra of both amorphous silicon and amorphous germanium is investigated. Disorder leads to a redistribution of states, which both reduces the empirical optical energy gap and broadens the optical absorption tail. The relationship between the optical gap and the breadth of the absorption tail observed in amorphous semiconductors is thus explained.     

Model for the amorphous roughening transition in amorphous semiconductor deposition

The evolution of surface roughness on hydrogenated amorphous silicon (a-Si:H) films prepared by plasma-enhanced chemical vapor deposition exhibits initial smoothening due to nuclei coalescence, a region of surface stability, and finally a prominent roughening transition (designated a → a) at a critical thickness. The thickness at which the a → a transition occurs, as measured by real time spectroscopic ellipsometry (SE), is found to correlate closely with the electronic properties of the film. Thus, the transition has been incorporated into deposition phase diagrams that have been applied successfully to optimize solar cell performance and stability. A simple continuum model for the evolution of the 1D surface profile using an initial condition designed to be consistent with the nucleation characteristics measured by real time SE is sufficient for insights into the correlation between the roughness evolution and film properties. Good agreement between the experimental results and model calculations support the concept that optimum electronic properties of the films are associated with weakly reactive surfaces and long lifetime radicals that lead to adsorbed precursors with large surface diffusion lengths.     

Effect of γ-irradiation on non-linear I-V behaviour and thermoelectric measurements in amorphous semiconducting AsSeTe system

An amorphous semiconducting compound of the composition As₂SeTe₂ is studied for its non-linear current-voltage characteristics, conductivity and thermoelectric measurements. It has been observed that γ-irradiation of the sample gives a lower threshold voltage and greater thermoelectric power. The threshold voltage shifts to a lower voltage value for the irradiated sample. The non-linear I-V behaviour and p-type nature of unirradiated and γ-irradiated As₂SeTe₂ are discussed on the basis of charged defect states existing and also created after γ-irradiation in the material. The thermoelectric power for unirradiated and γ-irradiated As₂SeTe₂ is discussed on the basis of a quantity, related to conductivity and thermoelectric power which explains to a great extent the effect observed experimentally.     

Magnetoresistance in amorphous germanium

The magnetoresistance of evaporated amorphous germanium films ranging in thickness from 200 Å to several microns has been measured in the temperature range 77 K < T < 300 K for magnetic fields ranging from 5 G to 100 kG. The results are in general agreement with those of Mell and Stuke. Various models which attempt to account for the magnetoresistance are discussed, but it is concluded that the origins of this effect are not understood at this time. The magnetoresistance measured at 77 K in the presence of a high electric field (45 000 V/cm) shows the same behavior as obtained for low electric fields.