Mössbauer studies of ɛ-Fe3−x Ni x N and γ′-Fe4−y Ni y N nanoparticles

Nanocrystalline ?-Fe_3?x Ni _x N (0.0?≤?×?≤?0.8) particles are synthesized by precursor technique and nitridation of decomposed products in NH₃ (g) in the temperature range 673 K-823 K. For x?=?0.1–0.4 compositions, single phase ?-Fe_3?x Ni _x N hexagonal structure with space group P6₃/mmc is formed, while for x?=?0.5–0.8, fcc γ′-Fe_4?y Ni _y N phase is also precipitated. The room temperature Mössbauer spectrum for all the compositions shows the presence of superparamagnetic doublet, which is attributed to ?-Fe_3?x Ni _x N phase. For x?=?0.5–0.8 compositions, two additional sextets are observed corresponding to two different iron sites, the corner position (Fe^c) and the fcc position (Fe^f), in γ′-Fe_4?y Ni _y N. The added Ni atoms preferentially substitute the corner Fe^c positions. The isomer shift, quadrupole splitting and hyperfine field values are found to change with the Ni content.     

Pressure experiment determination of the direct-indirect transition in the quarternary In1−xGaxP1−zAsz

Spontaneous and laser emission from In_1-xGa_xP_1-zAs_z double heterojunction diodes near the direct-indirect crossover (E_Γ = E_X, x ≡ x_c, z ≡ z_c) are studied at 77°K as a function of hydrostatic pressure up to 6 kbar. The pressure coefficients of the spontaneous emission peaks and of the laser modes are ～- 10.5 × 10^-6 eV/bar which is characteristics of the Γ band edge in III–V semiconductors. Laser threshold current is found to rise rapidly as pressure is applied owing to the decreasing Γ-X separation and the resultant carrier transfer to the X minima. Experimental lower limits for the direct-indirect crossover at three points in the In_1-xGa_xP_1-zAs_z quaternary system are determined. These three points and the established crossover in GaAs_1-yP_y (y_c ≈ 0.46, 77°K) give for the quaternary crossover (77°K) x_c ? 0.52z_c = 0.72. and the value x_c ≈ 0.72 for the limiting case of In_1-xGa_xP. Band edge bowing effects along the direct-indirect crossover in the In_1-xGa_xP_1-zAs_z system are discussed. The highest energy laser (77°K) for this quaternary system is estimated from pressure measurements to be ～ 2.155 eV (5752 Å).     

Optical and vibrational properties of CdxHg1-x-y ZnyTe solid solutions

The lattice dynamics of Cd_xHg_1-x-y Zn_yTe solid solutions is studied theoretically and experimentally. The frequencies of the basic optical phonons of Cd_xHg_1-x-y Zn_yTe are calculated in terms of a modified random-element isodisplacement model. As a result, all basic vibrations of the crystal lattice that substantially affect the optical properties of this material in the spectral region corresponding to one-phonon resonance are identified. The optical properties of epitaxial Cd_xHg_1-x-y Zn_yTe layers grown by liquid-phase epitaxy on Cd_1-x Zn_xTe substrates are studied. The calculated and experimental spectral dependences of the dielectric function of Cd_xHg_1-x-y Zn_yTe solid solutions of various compositions are compared at 295 and 78 K, and good agreement between them is reached. The additional lattice vibrations whose frequencies in the phonon density of states are lower than that of the HgTe mode are shown to be caused by the lattice defects of the Cd_xHg_1-x-y Zn_yTe solid solutions.     

Low temperature resistivities of FexNi80-xP14B6 metallic glasses

The resistivities of six Fe_xNi_80-xP₁₄B₆ alloys have been measured between 1.5 and 50 K. It is found that the resistivity variations both below and above the resistivity minima depend on the transition metal composition. The room temperature coefficients of the resistivity indicate the existence of the magnetic contribution to the resistivity.     

Dielectric, ferroelectric and pyroelectric properties of Pb[(Ni1/3Sb2/3)xTiyZrz]O3 ceramics

In the present study, various Pb[(Ni_1/3Sb_2/3)_xTi_yZr_z]O₃ where x+y+z=1, x=0.08 and y=0.44-0.49, ceramics in the morphotropic phase boundary (MPB) range were studied by dielectric and pyroelectric methods. The results of the investigations revealed an MPB composition range of y≅0.46. The study of the dielectric properties of these compounds as a function of temperature suggests that with increase in y the permittivity maximum increases and transition temperature shifts towards higher temperature. Well-saturated polarization versus electric field (P-E) hysteresis loops were obtained and values of P_r were calculated. The samples revealed good pyroelectric properties for y=0.44 and y=0.45 at room temperature with large figures of merit F_v=0.019 m²/C and F_D=1.34×10^-5Pa^1/2.     

Band edge offsets in strained (InGa)As-(AlGa)As heterostructures

The excitonic transitions between the ground electron and hole quantum well sublevels in strained In_xGa_1-xAs-Al_yGa_1-yAs multiple quantum well structures (x = 0.12−0.35 and y = 0.2−0.35) have been investigated by means of photoluminescence and photoconductivity measurements. The molecular beam epitaxy grown structures contained an Al_yGa_1-yAs matrix with one unstrained GaAs and three strained In_xGa_1-xAs quantum wells one of which was in the GaAs cladding layers. The ratio of the conduction band edhe line up to the band gap offset for the strained In_xGa_1-xAs-unstrained Al_yGa_1-yAs interface has been found to be 0.67 ± 0.08 for the studied regions of x and y.     

Correlations between hyperfine parameters of Fe−Ni-B amorphous metals

The correlations between fluctuations in the⁵⁷Fe Mössbauer hyperfine parameters of the amorphous alloys (Fe_0.5Ni_0.5)_100?x B _x (x=16, 18, 20, 22 and 25 at%) and Fe_yNi_80?y B₂₀ (y=20, 25, 40 and 60 at%) have been determined. Values of the correlation between the fluctuations of the isomer shift and the fluctuations of magnetic hyperfine field, μ_N 〈ΔHΔδ〉 together with published values on similar amorphous systems are compared with correlation values for related crystalline phases. The lack of characteristic values suggests that the correlation values do not allow a link to be made between local structural units in amorphous alloy and crystalline phases.     

Magnetic field dependence of the energy levels in CdTe-Cd(Mn)Te double quantum well structures

We show that if the valence band offset between CdTe and Cd_1-xMn_xTe alloys is not too large, magneto-photoluminescence experiments performed on CdTe-Cd_1-xMn_xTe-Cd_1-yMn_yTe double quantum wells should yield direct informations on this offset: the structure changes from a type I configuration (where electron and hole are mostly localized within CdTe layer) prevailing at zero magnetic field to a type II configuration (electron in CdTe layer, hole in Cd_1-yMn_yTe layer) at large magnetic field.     

Optical properties of GaN x As y P1?x?y semiconductor quaternary solid solutions

The photoluminescence method was used in the temperature range of 15?C300 K to study the optical properties of quantum-dimensional heterostructures based on GaN _x As _y P_(1?x?y) solid solutions, synthesized on the surface of a GaP (100) substrate. The calculations for the band-gap width of the GaN _x As _y P_(1?x?y) solid solution with the use of the band anticrossing model are shown to agree well with experimental data.     

Thermodynamics of type A1−xBxC1−yDy III–V quaternary solid solutions

An improved theory based on the pair approximation is given for the thermodynamic properties of type A_1?xB_xC_1?yD_yIII–V quaternary solid solutions. The theory takes into account the quasi-chemical nature of the nearest neighbor pair distribution, which has been neglected or inadequately treated in previous work. With a suitable thermodynamic treatment, a quasi-chemical equilibrium relation for the nearest neighbor pair distribution is derived. Numerical calculations using available thermodynamic data show that the distribution will deviate appreciably from random mixing values in the In_1?xGa_xP_1?yAs_y and Al_1?xGa_xAs_1?ySb_y systems due to a short-range clustering effect of nearest neighbor pairs with strong bond energy, whereas the deviation will be small in the Al_1?xGa_xP_1?yAS_y system. The expressions for the chemical potentials and the activity coefficients of the binary compound components are given. These are readily applicable to phase diagram calculations.     

X-ray photoelectron diffraction (XPED) patterns from: III-V compound mixed crystals (Ga1-xAlxAs AND GaAs1-yPy). Comparison of experiment and single scattering calculations

X-ray photoelectron diffraction (XPED) measurements, together with single scattering calculations, were made for two III—V group compound semiconductor mixed crystals (Ga_1-xAl_xAs(110) and GaAs_1-yP_y(001)). Each pair of photoelectrons excited at equivalent atomic sites in the crystal (Ga 3d and Al 2p in Ga₁-xAl_xAs, and As 3d and P 2p in GaAs₁-yP_y) showed essentially the same XPED patterns. Single scattering calculations reproduced the observed XPED patterns fairly well. An obvious site dependence of XPED patterns was observed in both the experimental and calculated results. These results clearly show that, for a given kinetic energy, XPED patterns mainly depend on the site of emitter atoms in the crystal, not on the species of emitter atoms. Thus, XPED measurements can be used for crystal site determinations in a fingerprint manner.     

NMR studies of hydrogen diffusion in β-LaNi5−yAly hydrides

Proton relaxation times have been measured in β-phase LaNi_5?yAl_yH_x, where 0<y<1.2, to determine the role of Al on hydrogen diffusion. Al substitution significantly increases the observed activation energy with a corresponding 100-fold decrease in the room temperature (300K) diffusion constant between y=0 and y=1.2. Comparisons are also made with previous studies of β-LaNi₅H_x and possible diffusion mechanisms are suggested.     

Diffusion of Fe,Co, Nd,and Dy in R2(Fe1−xCox)14B where R=Nd or Dy

Transition metal and rare earth diffusion coefficients at 1323 K in Dy_2−yNd_y(Fe_1−xCo_x)₁₄B were determined by field emission energy dispersive spectroscopy compositional analysis of diffusion couple specimens. Various arrangements of component materials and temperatures were examined in order to understand the mechanisms affecting diffusion of the components and to predict the stability of functionally graded microstructures consisting of a dysprosium-rich (Dy_2−yNd_y(Fe_1−xCo_x)₁₄B) outer layer and a neodymium-rich (Nd₂(Fe_1−xCo_x)₁₄B) interior. Estimates of the mutual interdiffusion coefficients of Dy, Nd, Fe, and Co in this system were obtained from the preparation of arc melted and annealed polycrystalline specimens, assuming that the diffusion coefficients were independent of concentration (Grube solution). Fifteen diffusion couples were prepared and heat treated at 1323 K for various times in order to provide data for calculation of the diffusion coefficients. The results indicate that the diffusion coefficients of Fe and Co (D_Fe=3.28×10⁻¹⁰ cm²/s and D_Co=7.63×10⁻¹⁰ cm²/s) were significantly higher at 1323 K in this system than those for Dy and Nd (D_Nd=2.3×10⁻¹² cm²/s and D_Dy=2.9×10⁻¹² cm²/s).     

A Raman study of lattice vibrations in II–VI semiconductors doped with 3<Emphasis Type="Italic">d</Emphasis> elements

Room-temperature Raman spectra were obtained for powder samples of Zn_1?xNixSe and Zn_1?yCr_ySe compounds and for a single-crystal Zn_1?xNi_xSe (x = 0.0025) sample in the temperature range 5–140 K. The results obtained are interpreted in terms of large-scale lattice shear strains induced by 3d elements in these solid solutions.     

Giant magnetoresistance of electrodeposited Cu-Co-Ni alloy films

Electrodeposition of CuCoNi alloys was performed in an acid-citrate medium. Nickel density parameter was varied in order to analyse its influence on the magnetoresistance. The structure and giant magneto-resistance (GMR) effect of CuCoNi alloys have been investigated. The maximum value for GMR ratio, at room temperature is 1% at a field of 12 kOe, and at 20 K is 2.1% at a field of 8.5 kOe for 3.1 Ni. The MR ratio of Cu_100-y-x Co_yNi_x alloys first increases and then decreases monotonically with increasing Ni content. The GMR and its dependence on magnetic field and temperature were discussed.      

Hyperfine magnetic fields on Cd impurity in the Pd2MnIn1-xSnx and Pd2MnSn1-ySby heusler alloys

The TDPAC technique was used to measure the magnetic hyperfine field (mhf) acting on Cd impurity in the Heusler alloys Pd₂MnIn_1-xSn_x and Pd₂MnSn_1-ySb_y for various values of x and y in the range 0 ? x, y ? 1. The alloys of Pd₂MnIn_1-xSn_x are antiferromagnetic at the In-rich end and ferromagnetic at the Sn-rich end, with a transition region x ≈ 0.5?0.7 where both phases coexist; the alloys containing Sn/Sb are ferromagnetic for all values of y. The mhf on the Cd impurity in the antiferromagnetic, transition and ferromagnetic regions of Pd₂MnIn_1-xSn_x are respectively zero, -150 and -200 kOe. For the Sn/Sb alloys the field changes from -200 at the Sn-rich end to -235 kOe at the Sb-rich end. The values of the field very closely follow the trend of the ferromagnetic Curie temperatures for the same alloys as a function of the s-p electron concentration. The observed large distribution of field intensities (～20%) and the lower values of the field in the region x = 0.5?0.6 are attributed mainly to the effect of antiferromagnetic domains. The results are compared with previous Mössbauer mhf measurements at the sites of Sn and Sb in the same alloys as well as with measurements in other Heusler alloys.     

Microstructure characterization and magnetic behavior of NiAlxFe2−xO4 and Ni1−yMnyAl0.2Fe1.8O4 ferrites

NiAl_xFe_2−xO₄ and Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements. The single spinel phase was confirmed for all prepared samples. A proper explanation of data is possible if the Al³⁺ ions are assumed to replace Fe³⁺ ions in the A and B sites simultaneously for NiAl_xFe_2−xO₄ ferrites, and if the Mn²⁺ ions are assumed to replace Ni²⁺ ions in the B sites for Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites. Microstructural factors play an important role in the magnetic behavior of Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites with large Mn²⁺ content.     

Magnetization and NMR study of amorphous Ni-P alloys in the paramagnetic concetration range

The temperature- and field-dependence of magnetization, and the ³¹P NMR linewidth and Knight shift of some amorphous Ni_100?xP_x alloys with 18? x ? 22 prepared by melt-quenching and electrodeposition were studied below room temperature. Although all the alloys investigated contain magnetic inhomogeneities, the amount and nature of which depend on impurities and on the details of sample preparation, it could be established that the amorphous matrix of Ni_100?xP_x alloys with x ? 18 exhibit Pauli paramagnetism and the Pauli susceptibility was found to decrease rapidly with P content. From the results, a similarity of the electronic structure and the short-range order in amorphous Ni-P alloys and in the crystalline Ni₃P compound is deduced.     

Magnetization reorientation in HoCo5-xNix and HoCo5-xCux compounds

The studies of magnetic anisotropy of HoCo_5-xNi_x and HoCo_5-xCu_x compounds in the temperature range between 60 and 293 K are presented. The magnetization reorientation from the c-axis to the base plane has been observed. The experimental results are analyzed on the base of the Irchin and Rozenfeld model.     

Homogeneity examination of a disordered magnetic alloy by means of CEMS

The depth sensitivity of the Mössbauer effect conversion electron spectroscopy (CEMS) offers a possibility to check the homogeneity of an alloy as a function of the sample thickness. We present experimental results on (Fe_0.65Ni_0.35)_1-xMn_x (x=0.026) prepared by diffusion of Mn into a Fe_0.65Ni_0.35 foil. On an inhomogeneous sample we determined the diffusion coefficient of Mn in Fe_0.65Ni_0.35 at 1300 K to be about 10^?11 cm²/s.