Comparative study of porous amorphous a-Si1−xCx films and a-Si1−xCx membranes on structural and luminescence properties

Electrochemical etching of amorphous SiC in fluoride solution was studied. Anodic dissolution and passivation are observed for p-type electrodes under dark illumination. The dissolution of p-type a-Si_1−xC_x is found to be under mixed transport/kinetic control; the diffusion current is of first order in fluoride concentration. Porous etching was not observed in this case. The surface finish of 6H-SiC depends on the experimental conditions; both uniform and porous etching is observed. In this paper, we report the formation of porous p-type amorphous SiC (a-Si_1−xC_x) films, elaborated previously by DC magnetron sputtering and analyze the porous layers (PSC) using scanning electron microscopy, spectrophotometer and photoluminescence. The crystal structures and the preparation conditions of porous SiC are shown to have an effect on the structural and electrical properties of the material obtained. SEM observation indicates that the porous a-Si_1−xC_x layers have shown some specific feature; a semi-cylindrical structure of the porous network has been observed.     

Surface roughening in Si1−xGex alloy films by 100 MeV Au: Composition dependency

Using a 100 MeV Au beam, the surface roughening kinetics of relaxed Si_1−xGe_x alloy films for x=0.5 and 0.7 are studied by means of ex-situ atomic force microscopy (AFM). Swift heavy ion (SHI) irradiation induced surface roughening behavior is demonstrated using the trend in variation of β as a function of fluence when the data are analyzed in terms of the Edwards-Wilkinson (EW) model. By employing the EW model, the observed surface roughening is explained on the basis of the competition between SHI induced sputtering and smoothening through redeposition of the sputtered atoms. The composition dependent variation of surface morphology with increasing fluence is discussed in the light of the strain distribution along the sample surface.     

Study of the local micro-structure and magnetic and transport properties of CrxGe1−x thin films

The local micro-structure as well as the magnetic and transport properties of Cr_xGe_1−x films prepared by means of magnetron sputtering have been investigated. Structural analysis shows that Cr atoms are situated in substitutional sites in the Ge lattice. Electrical transport properties indicate that Cr introduces a shallow acceptor level at 0.016 eV from the valence band implying Cr substituting for Ge. The low temperature ferromagnetism observed in the films is mediated mainly by ferromagnetic superexchange interactions between diluted Cr ions.     

Properties of MgxZn1−xO thin films sputtered in different gases

Mg_xZn_1−xO alloy films were prepared on sapphire substrates using Ar and N₂ as the sputtering gases. The effect of the sputtering gas on the structural, optical and electrical properties of the Mg_xZn_1−xO films was studied. By using N₂ as the sputtering gas, the Mg_xZn_1−xO film shows p-type conductivity and the band gap is larger than that employing Ar as the sputtering gas. The reason for this phenomenon is thought to be related to the reaction between N-O or N-Zn, and the N-doping.     

Exchange bias in structure-phase separated K0.8Fe2−xNixSe2 (x=0.015) single crystal: Interface evidence

Exchange bias behavior is observed in Ni slightly doped K_0.8Fe_2−xNi_xSe₂ (x=0.015) single crystal. Two distinguished phases with epitaxial growth were observed in X-ray diffraction experiments, indicating the structure phase separation in our samples. The magnetic hysteresis loop shifts in both horizontal and vertical directions when the sample was cooled down to 3 K in a magnetic field. The nature of this magnetic anisotropy could be understood as a result of the freezing properties of the local spin disorders in cluster like spin-glass system. Our results suggest that the sample contains short range weak ferromagnetic clusters (phase 2) embedding in the antiferromagnetic backgrounds (phase 1), in which the random distribution of Ni on Fe or Fe-vacancy sites quenched the superconductivity and induced spin disorders.     

Ferromagnetism and antiferromagnetism in Ni2+x+yMn1−xAl1−y alloys

The magnetic behavior of Ni_2+xMn_1−xAl alloys around the stoichiometric 2:1:1 composition was investigated with several experimental techniques. The results of low-temperature magnetization measurements indicate that a competition mechanism between ferromagnetism and antiferromagnetism is expected in off-stoichiometric alloys. Although the Curie temperature is strongly dependent on the composition, the saturation magnetization has an unsystematic variation for deviations from the stoichiometric Ni₂MnAl alloy. A reentrant-spin-glass behavior is observed below 50 K.     

Temperature-programmed surface reaction (TPSR) of CH4 synthesis by PdxNi100−x nanoparticles

A series of Pd_xNi_100−x nanoparticles were prepared by the co-precipitation method and analyzed using a temperature-programmed surface reaction (TPSR) of their methanation reactions. ESCA measurement suggested that the as-prepared Pd-Ni alloys had Pd-core/Ni-shell structure. Surface Pd segregation occurred during H₂ reduction and resulted in a surface composition close to the nominal value. The TPSR experiments were performed by pre-adsorption of CO with H₂ to form methane. The peak temperature of methanation increased as Pd content increased, indicating that a methanation reaction is favored on Ni and Ni-rich alloy nanoparticles. For physical mixtures of Pd and Ni nanoparticles, methanation behaviors is similar to those of alloy nanoparticles; but the methanation temperatures of physical mixtures are always higher than those of alloy nanoparticles. This may be due to the formation of a Pd-enriched alloy surface layer during reduction in H₂ at 400 °C, or because the CO molecules adsorbed on the Pd sites spill over onto the Ni sites for methanation. Using TPSR technique and measuring methanation temperature, the top-most surface of such bimetallic nanoparticles can be probed.     

Structure, magneto-structural transitions and magnetocaloric properties in Ni50−xMn37+xIn13 melt spun ribbons

Melt spun Ni_50−xMn_37+xIn₁₃ (2≤x≤5) ribbons were investigated for the structure, microstructure, magneto-structural transitions and inverse magnetocaloric effect (IMCE) associated with the first-order martensitic phase transition. The influence of excess Mn in Ni site (or Ni/Mn content) on the martensite transition and the associated magnetic and magnetocaloric properties are discussed. It was found that with the increase in Mn content, the martensitic transition shifted from 325 to 240 K as x is varied from 2 to 4, and the austenite phase was stabilized at room temperature. The x=5 ribbon did not show the martensitic transition. For the x=3 ribbon, the structural and magnetic transitions are close together unlike in the x=4 ribbon in which they are far (∼60 K) apart. The zero field cooled and field cooled curves support the presence of exchange bias blocking temperature due to antiferromagnetic interactions in the ribbons. A large change in the magnetization between the martensite and austenite phases was observed for a small variation in the Ni/Mn content, which resulted in large IMCE. A large positive magnetic entropy change (ΔS_M) of 32 J/kg K at room temperature (∼ 300 K) for a field change of 5 T with a net refrigeration capacity of 64 J/kg was obtained in the Ni₄₇Mn₄₀In₁₃ ribbon.     

Thermomagnetic history effects in LaCo5xNi5−5x and ThCo5xNi5−5x compounds

The magnetization versus temperature curves of several compounds of the series LaCo_5xNi_5?5x were found to give rise to an abrupt increase at temperatures being higher in accordance as the applied field strengths are smaller. The magnetic properties strongly depend on the magnetic as well as on the thermal history. The observed magnetic behaviour is discussed in terms of mictomagnetism and narrow Block wall propagation.     

Room-temperature ferromagnetism in Zn1−xMnxO

In view of recent controversies on above room-temperature ferromagnetism (RTFM) in transition-metal-doped ZnO, the present paper aims to shed some light on the origin of ferromagnetism by investigating annealing effects on structure and magnetism for polycrystalline Zn_1−xMn_xO powder samples prepared by solid-state reaction method and annealed in air at different temperatures. Magnetic measurements indicate that the samples are ferromagnetic at room temperature (RTFM). Room temperature ferromagnetism has been observed in the sample annealed at a low temperature of 500 °C with a saturated magnetization (M_s) of 0.159 emu/g and a coercive force of 89 Oe. A reduction in RTFM is clearly observed in the sample annealed at 600 °C. Furthermore, the saturation magnetic moment decreases with an increase in grain size, suggesting that ferromagnetism is due to defects and/or oxygen vacancy confined to the surface of the grains. The experimental results indicate that the ferromagnetism observed in Zn_1−xMn_xO samples is intrinsic rather than associated with secondary phases.     

Structure, transport and magnetic properties in La2xSr2−2xCo2xRu2−2xO6

The perovskite solid solutions of the type La_2xSr_2−2xCo_2xRu_2−2xO₆ with 0.25≤x≤0.75 have been investigated for their structural, magnetic and transport properties. All the compounds crystallize in double perovskite structure. The magnetization measurements indicate a complex magnetic ground state with strong competition between ferromagnetic and antiferromagnetic interactions. Resistivity of the compounds is in confirmation with hopping conduction behaviour though differences are noted especially for x=0.4 and 0.6. Most importantly, low field (50 Oe) magnetization measurements display negative magnetization during the zero field cooled cycle. X-ray photoelectron spectroscopy measurements indicate the presence of Co²⁺/Co³⁺ and Ru⁴⁺/Ru⁵⁺ redox couples in all compositions except x=0.5. Presence of magnetic ions like Ru⁴⁺ and Co³⁺ gives rise to additional ferromagnetic (Ru-rich) and antiferromagnetic sublattices and also explains the observed negative magnetization.     

Weakferromagnetism in FexNi1−xS2 (x = 0−0.075)

The effects of nonmagnetic impurities (Fe²⁺ in a low spin state) on the weakferromagnetism in NiS₂ were investigated. The weak-ferromagnetic moment decreases rapidly with Fe content (x) and almost vanishes at about x = 0.05. On the other hand, T_c remains quite constant. Above T_c, two anomalies in the temperature dependence of the susceptibility were observed except for samples with x = 0 and 0.006. The effective magnetic determined from the susceptibility above T_c is about 3 μ_B/Ni, and is independent of x. Discussions are given on the origin of weakferromagnetism and on the hysteresis of the virgin magnetization curve.     

Electroreflectance study of the dilure magnetic semiconductor alloy Hg1−xMnxTe

We report the first electroreflectance study of the dilute magnetic semiconductor Hg_1?xMn_xTe in the photon energy region of 0.7–6 eV. The composition dependence of the E₀ + Δ₀, E₁, E₁ + Δ₁, E₀, E₂, E₂ + δ optical features has been determined for 0.015 ≤ × ≤ 0.23. We find that at room temperature the semimetal-semiconductor transition (Γ₆ – Γ₈ crossing) occurs at x = 0.05 ± 0.01. Discontinuities in the composition dependence of all the observed spectral features are seen at this value of x.     

Evidence for the spin-dependent scattering of conduction electrons on Mn2+ ions in Hg1−xMnxTe and Cd1−xMnxSe mixed crystals

Changes in the resistivity of Hg_1?xMn_xTe and Cd_1?xMn_xSe mixed crystals associated with paramagnetic resonance of the Mn²⁺ ions have been observed at liquid helium temperature in a strong magnetic field. The effect was recorded by monitoring the submillimeter radiation induced photoconductivity in a swept magnetic field. An increase in the resistivity associated with EPR of the Mn²⁺ ions is interpreted in terms of the spin- dependent scattering of electrons on magnetic impurities, the spins of which are selectively depolarised by means of paramagnetic resonance. Some additional effects influencing the experiments are also discussed.     

Absence of magnetic ordering in low dimensional mixed magnetic Mn1−xNixCl2·H2O

A new mixed magnet, Mn_1−xNi_xCl₂·H₂O, is examined by dc magnetization and susceptibility measurements across the entire composition range. The pure components are quasi-one-dimensional Heisenberg antiferromagnets ordering at 2.17 K (Mn) and 5.6₅ K (Ni) due to weaker interchain exchange supplementing the dominant exchange along MCl₂MCl₂M… chemical and structural chains. High temperature magnetic susceptibilities yield Curie and Weiss constants in χ_M=C/(T−θ). C(x) is linear but θ(x) displays curvature, which is analyzed to show that unlike-ion exchange is ferromagnetic and similar in size to like-ion. Most notable is the absence of antiferromagnetic susceptibility maxima down to 1.6 K from x=0.10 to 0.95. For x=0.05 a susceptibility maximum appears, with T_max almost 20% lower than in the pure Mn component but T_c reduced by 2%. The size of the susceptibility is enhanced by admixture, the effect of disrupted antiferromagnetic tendencies. Magnetization isotherms evolve with composition. Larger values of magnetization, under the same measuring conditions, occur for mixtures than for pure components, consistent with frustration, which weakens antiferromagnetic alignment tendencies. The competing ferromagnetic (Ni) and antiferromagnetic (Mn) intrachain interactions, along with disorder and low dimensional characteristics, presumably lead to the absence of magnetic order over a remarkably broad composition range.     

Influence of growth and anneal conditions on the surface roughness of L10 Fe50PdxPt50−x thin films

We deposit Fe₅₀Pd_xPt_50−x alloy thin films by magnetron sputtering onto a TiN seed layer. Chemically ordered L1₀ films are obtained which display large perpendicular magnetic anisotropy. We find that the surface roughness of the film depends strongly on the growth and anneal conditions as well as the Pd composition of the film. Smooth films are obtained by deposition above the chemical ordering temperature and by removing Pd from the alloy.     

Ionic conductivity of Zr1−xIn2xO2−x

As x in Zr(In)O_2?x is increased from 0.08 to 0.16 (9–19 mole per cent In₂O₃) the activation energy E(x) for ionic conduction increases from 1.05 to 1.51 eV; the concuctivity decreases from 2 × 10^?5 to 3 × 10^?6Ω^?1cm^?1at 400°C, is composition-independent at about 580°C, and increases from 1 × 10^?2 to 4 × 10^?2Ω^?1cm^?1 at 800°C. The pre-exponential term of the Boltzmann-type conductivity equation depends exponentially on E(x), a much stronger dependence on x than theoretically expected with a model for ionic conductivity that includes nearest-neighbor defect interactions. Analysis of reported conductivity data for Zr(M)O_2?x (M = Sc, Y, Ca and rare earth metals) and other doped oxide electrolytes with fluorite-type structure reveals that the same relationship is observed with these materials when x γ0.08. It is shown that ionic conduction in these oxides is consistent with nearest neighbor vacancy-cation defect interaction forx < 0.08 but that an additional complex interaction with composition-dependent free energy ΔG(x) occurs when xγ 0.08.The lattice constant of Zr(In)O_2?x with the cubic fluorite-type structure is independent of composition, 5.114 ± 0.002 Å, in agreement with ionic size considerations.     

Raman scattering from GaSe1−xTex

The long-wavelength optical phonons of the layer GaSe_1?xTe_x have been investigated at room temperature by means of Raman scattering spectroscopy. The spectra of the Bridgman grown crystals were excited with the 1,06 μm line of the continuously operated YAG:Nd³⁺ laser. Detailed study of the Raman spectra of GaSe_1?xTe_x solid solutions showed that there is an abrupt change in the frequency-composition dependences for all observed modes. It is shown, that a phase transition from hexagonal ?-GaSe to monoclinic GaTe in GaSe_1?xTe_x solid solutions takes place in the composition range 0.27 ? × ? 0.72. Only one mode behaviour of the optical phonons was observed in GaSe_1?xTe_x system.     

Magnetism of surface alloys of the type MxN1−x/Rh(1 1 1)

We present a density functional theory study on the magnetic properties of two-dimensional surface alloys of the type M_xN_1−x (M=Fe, Co and Ni; N=Pt, Au, Ag, Cd and Pb) on Rh(1 1 1) for x=0.0, 0.25, 0.33, 0.5, 0.67, 0.75 and 1.0, in two types of geometric arrangements—striped phases or linear-chain type, and non-striped phases or mixed checkerboard type. Many pairs among these are bulk-immiscible but show mixing on the surface. We find that the trend in the magnetic moment of surface alloys of N with a given M follows the number of valence electrons in N: the higher the number of valence electrons, the lower the magnetic moment. Overlayer atoms when put on hcp sites show higher moment compared to fcc sites. In general, for a given composition x, linear-chain type structures show a reduced magnetic moment compared to checkerboard type structures. We find that Pb, when alloyed with magnetic elements (Fe, Co and Ni), has a lowering effect on their magnetic moments.     

Surface anisotropy energy of thin amorphous magnetic films of (Gd1−xCox)1−yMoy alloys; experimental results

Surface excitations in thin amorphous (Gd_1?xCo_x)_1?yMo_y films obtained by the rf sputtering technique were studied. A microwave spectrometer at X-band was used for magnetic resonance investigation with external magnetic field rotating from perpendicular to parallel resonance orientations. The critical angle and angular dependence of the position of the surface mode and the uniform mode were determined. The Surface Inhomogeneity (SI) model was applied with symmetrical boundary conditions. The surface anisotropy energy term was assumed as a superposition of the uniaxial anisotropy term and a biaxial anisotropy term. The origin of the latter term is not known yet. We also performed the resonance experiment for different temperatures ranging from 180 to 300 K. From the experiment, the uniaxial surface anisotropy constant K′_s1 and the biaxial surface anisotropy constant K′_s2 were found as functions of the temperature; the uniaxial anisotropy energy against temperature changes the sign for y=0.02 from easy axis to easy plane while the biaxial surface anisotropy does not change its character.