A mean-field study on the thermo-magnetic properties of GdxCo1−x amorphous alloys (0.16⩽x⩽0.25)

We present a mean-field study on the thermo-magnetic properties of Gd_xCo_1−x amorphous alloys in the 0.16⩽x⩽0.25 composition range. A single set of exchange integrals and fixed values of the angular momenta of Gd and Co fairly describe the temperature dependence of magnetization. The magnetic specific heat and magnetic entropy show field and composition dependence. Both the specific heat anomaly and the saturated entropy, at the temperature of the magnetic phase transition, increase with increasing Co concentration. The two magnetic subnetworks and their cross-interactions contribute differently to the specific heat.     

57Fe Mössbauer spectroscopy and magnetization study of YFexAl12−x (4.4⩽x⩽5)

YFe_xAl_12−x in the composition range 4.4⩽x⩽5 was prepared by induction melting followed by annealing in vacuum at 1270 K. Magnetization data below 150 K show complex magnetic behaviour dependent on applied field, composition and temperature. The transition temperature T_c, corresponding to the main maximum of the magnetization vs. temperature curves and below which magnetic interactions are observed for a significant fraction of the Fe atoms in the Mössbauer spectra, decreases from 180 K for x=4 down to 100 K for 4.2⩽x⩽4.7 and rises again up to 160 K for x=5. The analysis of the spectra obtained at 5 K is consistent with full occupation of the 8f sites by Fe atoms and sharing of the 8j sites by Fe and Al as deduced from the Rietveld analysis of X-ray powder diffraction data. The Mössbauer spectra further show a dependence of magnetic hyperfine fields and isomer shifts on the crystallographic site and on the number of the Fe nearest neighbours similar to that observed in UFe_xAl_12−x (4⩽x⩽6) and RFe_xAl_12−x (R=Y, Lu, x=4, 4.2). The magnetic properties of the UFe_xAl_12−x and YFe_xAl_12−x series are compared and the magnetic interactions between the different Fe sublattices are discussed.     

Pressure effect on magnetic properties of NdMn2−xFexGe2 (0.1⩽x⩽1.0) series of solid solutions

Temperature and pressure dependence of magnetic properties in the NdMn_2−xFe_xGe₂ series of solid solutions (0.1⩽x⩽1.0) are reported. The (P, T) magnetic phase diagrams are determined on the basis of the AC magnetic susceptibility measured in a weak magnetic field. The measurements were carried out under hydrostatic pressure up to 1.5 GPa in the temperature range 80−430 K. The reported data show that in the studied series of solid solutions, a drastic change in magnetic properties takes place in a narrow dilution parameter range (0.4⩽x⩽0.5). While taking into account the magnetic properties, the studied range of Fe content could be divided into four regions. Only in the case of x=0.3 and 0.4, the external pressure significantly influences the magnetic properties of the samples.     

A magnetic study of HoMn6Sn6−xGax single crystals (0.14⩽x⩽1.89)

Single crystals of hexagonal HfFe₆Ge₆-type HoMn₆Sn_6−xGa_x compounds (0.14⩽x⩽1.89) have been obtained by a flux method and studied by magnetisation measurements. All the compounds order ferrimagnetically (308⩽T_c⩽386 K) with moments lying in the (0 0 1) plane and undergo a moment reorientation transition at lower temperatures (156⩽T_SR⩽195 K). At 5 K, the moments are aligned along an intermediate direction (44⩽φ_c⩽50°). These results are discussed and compared with the neutron diffraction results related to the isotypic TmMn₆Sn_6−xGa_x and TbMn₆Sn_6−xGa_x series where a change of the easy direction is observed with increasing gallium contents.     

Properties of Hg1−xZnxSe (0.09 ⩽ x ⩽ 1.0) thin films prepared by flash evaporation technique

Flash evaporated mercury zinc selenide films are observed to grow as single phase ternary alloys of the type Hg_1−xZn_xSe in the composition range 0.09 ⩽ x ⩽ 1.0, with a f.c.c./sphalerite structure on substrates maintained at T_s between 30°C and 175°C. The grain size is observed to increase with increase in T_s for all compositions. The films are observed to have a direct optical band gap which increases from 0.01 to 2.60 eV as the Zn concentration x is varied between 0.09 and 1.0. The band gap V_scomposition shows a bowing, typical of pseudobinary solid solutions. Zn rich films were observed to be p-type whereas Hg-rich films were n-type. Room temperature resistivity was observed to increase with Zn concentration x, which can be attributed to the increase in the band gap of the semiconductors. Higher resistivity in films deposited at high substrate temperatures is due to the decrease in contribution to conduction from the highly conducting grain boundaries.     

Electrical and thermal properties of a-(Se70Te30)100−x(Se98Bi2)x (0⩽x⩽20) alloys

In electrical properties, the dc conductivity and photoconductivity measurements have been made in vacuum evaporated thin films of a-(Se₇₀Te₃₀)_100−x(Se₉₈Bi₂)_x system, in the temperature range (308–355 K). It has been observed that dc conductivity and activation energy depend on the Bi concentration. Photocurrent dependence on incident radiation has also been observed which follow the power law (I_ph∝F^γ). Transient photocurrent exhibits the non-exponential decay time. All these parameters show that the recombination within the localized states is predominant. In crystallization kinetics, the heating rate dependence of glass transition and crystallization temperatures is studied to calculate the activation energy for thermal relaxation and activation energy for crystallization. The composition dependence of the activation energy for thermal relaxation and activation energy for crystallization is discussed in terms of the structure of Se–Te–Bi glassy system.     

The magnetism of Zn-doped colossal magnetoresistance materials Fe1−xZnxCr2S4 (0⩽x⩽1)

The polycrystalline samples of Zn-doped Fe_1−xZn_xCr₂S₄ (0⩽x⩽1) were prepared by the conventional solid-state synthesis method. The magnetic homogeneity has been proved by the measurements of the alternating current (AC) magnetic susceptibility. We presented a detailed study of Zn-doping effect on the magnetism. It is found that the magnetism does not change monotonically with substituting nonmagnetic Zn²⁺ ions for Fe²⁺ ions. The difference between the magnetic moments for the samples with respective doping level can be ascribed to the variation of compensation between the Fe²⁺ and Cr³⁺ sublattices as the Fe²⁺ concentration is varied. The Zn-doping leads to spin reversal of Cr³⁺ ions. Based on the spin orientation of Fe²⁺ and Cr³⁺ ions in FeCr₂S₄ and ZnCr₂S₄ at 0 K, we suggest a phenomenological model describing the magnetism and the doping level dependence of the up-spin proportion of Cr³⁺ ions. The up-spin proportion of Cr³⁺ ions, denoted by y, as a function of doping level in zero fields is linear. However, y in magnetic fields cannot be fitted linearly, which shows a maximum. The above results can be described qualitatively by the effect of the applied magnetic field on the AB, AA, and BB interactions of the spinels with the formula ABS₄.     

Effects of Er substitution for Nd on structural and magnetic properties of Nd1−xErxMn2Ge2 (0⩽x⩽1)

The structure and magnetic properties of Nd_1−xEr_xMn₂Ge₂ (0.0⩽x⩽1) were studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in the ThCr₂Si₂-type structure with space group I4/mmm. Substitution of Er for Nd led to a linear decrease in the lattice constants and the unit cell volume. Increasing substitution of Er for Nd in NdMn₂Ge₂ shows a depression of ferromagnetic ordering and the gradual development of antiferromagnetic ordering. A typical SmMn₂Ge₂-like behaviour is observed for the x=0.4 sample. The results are summarized in α preliminary magnetic phase diagram.     

X-ray diffraction and Mössbauer Studies of γ′-(Fe1−xSnx)4N compounds (0.0⩽x⩽0.3)

The structure and magnetic properties of γ′-(Fe_1−xSn_x)₄N samples have been studied using X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy. It has been shown that the single-phase (Fe_1−xSn_x)₄N compounds can be prepared in the composition range of 0.0⩽x⩽0.3, which have the similar structure as γ′-Fe₄N The lattice parameter with the increase of Sn concentration can be well fitted with two linear relationships a₀(x)=3.795+0.019x (with x⩽0.10 ) and a₀(x)=3.797+0.228(x−0.10) (with 0.1⩽x⩽ 0.3). The fitting results of Mössbauer spectra indicate that the hyperfine parameters have the same changing tendency with lattice parameter, and the Sn atoms have a preference to be located at the corner site.     

Colossal magnetoresistance in La0.7Ca0.3−xHgxMnO3 (0⩽x⩽0.2) system over wide temperature range

The divalent substitutions in rare-earth manganites create quenched disorders; those are responsible for the modification of magnetoelectronic properties through competition of two phases. In this regards, the substitution of divalent Hg is rarely studied. Here, we present the results on effect of Hg²⁺ substitution in LCMO manganites. We have synthesized polycrystalline samples with nominal compositions La_0.7Ca_0.3?xHg_xMnO₃ (0?x?0.2) by the standard solid-state reaction method. The X-ray diffraction data revealed monophasic nature without a signature of Hg related or any other impurity phase. The ac susceptibility measurement showed variation in the magnetic transition temperature as well as the transition width. The electrical resistivity measurement showed variation in metal-insulator transition temperature. The magnetoresistance (MR) measurements were carried out at 5 T magnetic field. The samples show varying magnitude of magnetoresistance as high as 74.48%. The distinct feature seen with Hg substitution is the increase in the magnitude of MR and temperature range over which MR value is sustained. It also improves the microstructure of the samples.     

Exchange integrals and magnetic short range order in the system CdCr2−xGaxSe4(0⩽x⩽0.06)

High-temperature series expansions are derived for the magnetic susceptibility and two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice. The calculations are developed in the framework of the random phase approximation and are given for both nearest and next-nearest neighbour exchange integrals $J_1$ and $J_2$, respectively. Our results are given up to order 6 in $\beta =(k_{\mathrm{B}}T{)}^{-1}$and are used to study the paramagnetic region of the ferromagnetic spinel CdCr_2−xGa_xSe₄. The critical temperature $T_{\mathrm{c}}$ and the critical exponents $\gamma$ and $\nu$ associated with the magnetic susceptibility $\chi (T)$ and the correlation length $\xi (T)$, respectively are deduced by applying the Padé approximate methods. The results as a function of the dilution x obtained by the present approach are found to be in agreement with the experimental ones.     

Critical points and points of the bifurcation of the rotating magnetized newtonian polytropes with a 1 ⩽ n ⩽ 1.6 index

The presence of critical points and the bifurcation points of rotating Newtonian polytropes with an index of 1 ? n ? 1.6 has been shown for the first time in this paper. The symbolic-numerical calculation error in metric L ₂ has reached the size of the 10^?4 order. The approximate analytical solution to the problem to the abovementioned accuracy has been set forth. A critical value of the polytropes index n = n _k = 1.51025 has been calculated which is the highest among the critical points and bifurcation points. The value n _k corresponds to the infinitely slow polytropes rotation. Furthermore, in this paper the presence of the period jump at the bifurcation point T _b has been predicted and the relative value of this jump ΔT _b /T _b ～ B _0in ^4/3 has been estimated.     

Elastic scattering of 11B from 209Bi in the energy range 49.8 ⩽ E ⩽ 84.1 MeV

The elastic scattering of ¹¹B from ²⁰⁹Bi has been measured at laboratory energies of 49.8, 51.3, 52.2, 52.8, 54.3, 55.8, 59.8, 64.8, 69.8, 74.8 and 84.1 MeV. These data have been analyzed using a microscopic optical model and the energy dependence of the real and the imaginary parts of the optical potential at near barrier energies has been determined. The “threshold anomaly” observed in the real part of the potential is found to be consistent with the dispersion relation which connects the real and the imaginary parts of the potential. Inelastic scattering and transfer reactions have also been measured at energies of 51.3, 55.8, 59.8 and 74.8 MeV. DWBA calculations for the 3⁻ state in ²⁰⁹Bi are made. From the measured transfer probabilities, using a semiclassical approach the strength of the form factors have been obtained. The fusion cross sections have been derived at these energies from the corresponding quasi-elastic scattering angular distribution data. The fusion cross sections calculated using the energy dependent barriers extracted from the energy dependent real parts of the potential compare well with, that determined from quasi-elastic scattering data and are also in good agreement with simplified coupled channels calculation for fusion incorporating important inelastic and transfer channels.     

Synthesis,magnetic properties,surface modification and cytotoxicity evaluation of Y3Fe5−xAlxO12 (0⩽x⩽2) garnet submicron particles for biomedical applications

Yttrium aluminum iron garnet Y₃Fe_5−xAl_xO₁₂ compounds were produced by the citrate gel process by varying the aluminum content x from 0 to 2. Particles with an average size of about 100 nm and a specific surface area ranging from 6 to 28 m²/g were obtained. Curie temperatures (T_C) were determined from the temperature dependence of the field-cooled magnetization (FCM) and the remnant magnetization (REM) and range from –40°C to 280°C. According to the magnetic moment variation, the aluminum cation seems to replace the iron cation in the tetrahedral site for compounds with x<0.7. 3-Aminopropyltrimethoxysilane (γ-APS) was grafted onto the particles surface and its presence confirmed by infra-red spectroscopy. Cytotoxicity assays were carried out during a period of 13 days using human microglia and rat glioblastoma cell lines. The presence of the amine-functionalized particles did not affect the cell viability or the culture growth rate. Finally, amine-functionalized particles were successfully labeled with covalently linked fluorescing molecules.     

AC and DC susceptibility of powder solid solutions of TmFe1−xAlxO3 and TmFe1−xGaxO3 systems (x⩽0.15)

The paper presents the results of static and AC susceptibility measurements for powder solid solutions of two systems: TmFe_1−xAl_xO₃ and TmFe_1−xGa_xO₃. The magneto-thermoremanence effect occurring in these systems is explained, and it is shown as to how this effect can be used to determine phase transition temperatures in powder compounds. It has been found that spin reorientation temperatures are dependent on the concentration of non-magnetic atoms in the iron sublattice, and that the compensation temperature is dependent on the type of non-magnetic atoms. Also, it has been found that the non-magnetic atoms have a strong dampening effect on the intensity of higher harmonic components of AC susceptibility.     

A study of the three- and four-photon final states produced in e+e− annihilation at 35⩽s⩽46.8 GeV

The reactions e⁺e⁻→γγγ and e⁺e⁻→γγγγ have been studied at center-of-mass energies between 35 and 46.8 GeV with an integrated luninosity of about 130 pb⁻¹accumulated with the CELLO detector at PETRA. The measurements are compared to QED calculations up to third and fourth orders of perturbation theory. Excellent agreement is observed.     

Specific heat of Eu x Sr1−x Te

In this paper, we report on measurements of the specific heatC of single-crystalline Eu _x Sr_1–x Te at temperatures between 60 mK and 15 K and in magnetic fields up to 6 T. Pure antiferromagnetic EuTe shows unusual critical behavior in the vicinity of the Néel temperatureT _N=9.8 K with a positive critical exponent instead of the 3d-Heisenberg exponent =–0.12. Possible reasons for this discrepancy between theory and experiment include magnetic anisotropy effects due to magnetic dipole-dipole interactions, which may give rise to a cross-over of the critical behavior very close toT _N. This anisotropy is also seen in the specific heat below 1 K where an exponential decay ofC is observed, and in the dependence of the magnetic susceptibility on the direction of the applied field. With increasing dilution of EuTe with nonmagnetic Sr, the critical behavior changes: becomes negative and decreases continuously towards –1 atxx _c. This concentration dependence of was previously observed in the diluted ferromagnetic system Eu _x Sr_1–x S. Our data thus support that the apparent change in the critical behavior depends on the degree of disorder. Samples with concentrationx lower than the critical concentrationx _c reveal spin-glass behavior in the specific heat. In addition, the dependence ofT _N on magnetic fields is discussed. The data yield a normalized magnetic phase boundaryB _c(T)/B_c(T=0) vs.T _N(B)/T_N(B=0) which is independent of concentration.