Growth and magnetism in amorphous Si1−xMnx thin films grown by thermal deposition

Si_1−xMn_x   (x?0.22$x?0.22$) thin films were grown by using a thermal evaporator, and their magnetic and electrical properties were investigated. The Si_1−xMn_x semiconductors are amorphous when Mn concentration is 9.0 at% and less. The electrical resistivities of amorphous Si_1−xMn_x   (x?0.09$x?0.09$) semiconductor thin films are in the range of 9.86–6.59×10⁻⁴ Ω cm at room temperature and decrease with increasing Mn concentration. The amorphous Si_1−xMn_x   (x?0.09$x?0.09$) semiconductor thin films are p-type and hole densities are 3.73×10¹⁸–1.33×10²² cm⁻³ at room temperature. Low temperature magnetization characteristics reveal that amorphous Si_1−xMn_x   (x?0.09$x?0.09$) semiconductor thin films are paramagnetic.     

Magnetic entropy-change in La0.67−xBixCa0.33MnO3 compound

Bi doped lanthanum manganites with the chemical composition of La_0.67−xBi_xCa_0.33MnO₃ (x=0$x=0$, 0.05, 0.1, 0.2) were prepared by the standard solid-state process. The Curie temperatures were measured to be 267 K for x=0$x=0$, 248 K for x=0.05$x=0.05$, 244 K for x=0.1$x=0.1$ and 229 K for x=0.2$x=0.2$ samples. It was found that the maximum value of the magnetic entropy change ∣ΔS_m∣ has reached the highest value of 6.08 J/kg K at 3 T for the composition with x=0.05$x=0.05$. Nearly the same maximum entropy change was observed for the x=0$x=0$ sample. A large decrease in the magnitude of the entropy change was observed for the x=0.2$x=0.2$ sample.     

Magnetocaloric effect of Gd5SixSn4−x Alloys

The crystal structure and magnetocaloric effect of Gd₅Si_xSn_4−x   (with x=2.4$x=2.4$, 2.6 and 2.8) alloys were studied by means of X-ray power diffraction (XRD) and magnetic measurements. From the XRD results, these alloys adopt a Gd₅Si₄-type structure for x=2.8$x=2.8$, Gd₅Si₄-type and Gd₅Si₂Ge₂-type mixed structures for x=2.4$x=2.4$ and 2.6, while some minor phases can also be found. The Curie temperatures of the Gd₅Si_xSn_4−x increases gradually when x increases from 276 K for x=2.4$x=2.4$, to 301.5 K for x=2.8$x=2.8$. Magnetic entropy changes of these alloys at a magnetic field change of 0–1.8 T are 1.88, 2.26 and 1.69 J/kg K for x=2.4$x=2.4$, 2.6 and 2.8, respectively. The temperature-dependent XRD analysis shows that there is no crystallographic transition for these alloys, which can explain their low magnetic entropy changes.     

Structural,electronic and optical properties of CdxZn1 − xS alloys from first-principles calculations

Structural, electronic and optical properties as well as structural phase transitions of ternary alloy Cd_xZn_1 − xS have been investigated using the first-principles calculations based on the density functional theory. We found that the crystal structure of Cd_xZn_1 − xS alloys transforms from wurtzite to zinc blende as Cd content of x=0.83$x=0.83$. Effect of Cd content on electronic structures of Cd_xZn_1 − xS alloys has been studied. The bandgaps of Cd_xZn_1 − xS alloys with wurtzite and zinc blende structures decrease with the increase of Cd content. Furthermore, dielectric constant and absorption coefficient also have been discussed in detail.     

Magnetic properties of novel FeSe(Te) superconductors

Magnetization studies were carried out for the novel FeSe_1−xTe_x superconductors (0≤x≤1$0\le x\le 1$) to investigate a behavior of the intrinsic magnetic susceptibility χ$\chi$ in the normal state. The magnetic susceptibility was found to increase gradually with Te content. The temperature dependencies of the magnetic susceptibility χ$\chi$ and its anisotropy Δχ=_χ∥−_χ⊥$\mathrm{\Delta }\chi ={\chi }_{\parallel }-{\chi }_{\perp }$ were measured for FeSe in the temperature range 4.2–300 K, and a growth of susceptibility with temperature was revealed. For FeTe a substantial increase of χ$\chi$ under pressure was found. Ab initio calculations of the band structure and magnetic susceptibility have shown, that FeSe_1−xTe_x systems are close to magnetic instability with dominating enhanced spin paramagnetism. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume V   and especially the height of chalcogen species from the Fe plane. With appropriate values of these parameters the calculations have reproduced the experimental data on χ(T)$\chi (T)$ and χ(P)$\chi (P)$ for FeSe and FeTe, respectively.     

Effect of Mg substitution on electromagnetic properties of (Ni0.25Cu0.20Zn0.55)Fe2O4 ferrite prepared by auto combustion method

The ferrite compositions of (Ni_0.25−xMg_xCu_0.2Zn_0.55)Fe₂O₄ with x=0.0$x=0.0$, 0.07, 0.13, 0.18, and 0.25 were synthesized through nitrate-citrate auto-combustion method. The as-burnt powders showed the presence of crystalline cubic spinel ferrite with about 19–22 nm crystallite sizes. The resultant powders were calcined at 700 °C/2 h and pressed ferrites were sintered at 950 °C/4 h. The initial permeability, magnetic loss and AC resistivity were measured in the frequency range 10 Hz–10 MHz. The permeability and AC resistivity were found to increase and the magnetic loss decreased with Mg substitution for Ni, up to x=0.18$x=0.18$. The very high permeability in the composition x=0.18$x=0.18$, was due to better densification, lower magnetostriction constant and inner stresses, etc. The AC resistivity of the composition was also highest. The composition would be better than NiCuZn-based material for more miniaturization of multi layer chip inductor.     

Topological recursion for chord diagrams,RNA complexes,and cells in moduli spaces

We introduce and study the Hermitian matrix model with potential _Vs,t(x)=x²/2−stx/(1−tx)$V_{s,t}(x)=x^2/2-stx/(1-tx)$, which enumerates the number of linear chord diagrams with no isolated vertices of fixed genus with specified numbers of backbones generated by s and chords generated by t. For the one-cut solution, the partition function, correlators and free energies are convergent for small t and all s   as a perturbation of the Gaussian potential, which arises for st=0$st=0$. This perturbation is computed using the formalism of the topological recursion. The corresponding enumeration of chord diagrams gives at once the number of RNA complexes of a given topology as well as the number of cells in Riemann?s moduli spaces for bordered surfaces. The free energies are computed here in principle for all genera and explicitly in genus less than four.     

Effects of Ag addition on electrical transport and magnetic properties of La0.67Ba0.33Mn0.88Cr0.12O3

We have prepared a series of polycrystalline manganites with the nominal compositions, La_0.67Ba_0.33Mn_0.88Cr_0.12O₃/Ag_x (LBMCO/Ag_x) (x   is the mole fraction) with x=0$x=0$, 0.05, 0.1, 0.15, 0.2, 0.23, 0.27, 0.3, 0.35. The X-ray diffraction patterns show that the samples with x>0.05$x>0.05$ are two-phase composites. The Ag addition in LBMCO improves the properties of grain surfaces/boundaries and reduces the resistivity of the composites. For x=0.30$x=0.30$ sample, a minimum resistivity is obtained and a maximum room temperature magnetoresistance up to −54.5% was observed at 288 K, 1 T field. The room temperature T_C and the reduced resistivity are responsible for the enhancement of room temperature MR.     

Structural and magnetic properties of CoPt clusters

The geometrical structures, electronic and magnetic properties of Co_n − xPt_x (n=2–13,38,55$n=2\text{–}13,38,55$) alloy clusters have been systematically investigated by using the density functional theory within the generalized gradient approximation (DFT–GGA). It is found that CoPt alloy clusters adopt the structures of corresponding monatomic Co clusters, where Pt atoms localize at the surface sites and tend to bond together forming a Pt exterior shell. The ferromagnetic coupling between atoms is determined in CoPt clusters, and the Co local magnetic moments can be enhanced by the increase of Pt concentration.     

Structural,magnetic, and transport properties in La(2+4x)/3Sr(1−4x)/3Mn1−xCuxO3 (0⩽x⩽0.20) system

A series of the double-doping samples La_(2+4x)/3Sr_(1−4x)/3Mn_1−xCu_xO₃(0?x?0.2)$(0?x?0.2)$ with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 have been prepared. The structural, magnetic, transport properties and magnetoresistance of the series samples have been investigated. It is found that no apparent crystal structure change is introduced by Cu doping up to x=0.20$x=0.20$. But the Curie temperature _TC$T_{\mathrm{C}}$ and magnetization M   are strongly affected by Cu substitution. A remarkable magnetotransport behavior, characterized by double bumps, is observed, and an obvious low-temperature upturn is found in the range of 0.07?x?0.12$0.07?x?0.12$. As a result, the temperature range of colossal magnetoresistance (CMR) is greatly broadened. Moreover, it is found that the room temperature magnetoresistance (MR) of double-doping samples is obviously larger that the undoped La_2/3Sr_1/3Mn_1−xCu_xO₃ at 300 K, which can give a guide for the adequate selection of the room temperature CMR materials.     

Effect of Co ions on the microstructure and magnetic properties of nanocrystalline CoxFe3−xO4 films

Co_xFe_3−xO₄ nanocrystalline films (x=0.2-0.8$x=0.2-0.8$) on SiO₂ substrates were prepared by a sol–gel method. The microstructural and magnetic properties of samples were measured by an X-ray diffractometer (XRD) and a vibrating sample magnetometer (VSM), respectively. Atomic force microscopy (AFM) was used to investigate the surface image of the sample. The measurement results of XRD at room temperature show that the pure spinel structure of the film could be obtained at x=0.8$x=0.8$. The magnetic measurements reveal the magnetic properties of the samples depend strongly on Co²⁺ ions content, and the optimal parameters of the saturation magnetization and coercivity in Co_xFe_3−xO₄films are obtained at x=0.8$x=0.8$. Here the coercivity reaches 1.954 kOe. The average grain sizes of the film are less than 30 nm obtained from the microscopy images. The situ measurement at high temperatures of range from 293 to 773 K shows that the microstructures of Co_0.8Fe_2.2O₄ film have good thermal stabilization.     

Effect of carbon content on structure and magnetic properties for (Fe,Ni)1−xCx by mechanical alloying

FCC (Fe₅₅Ni₄₅)_1−xC_x   supersaturated solid solution was prepared in a wide concentration range (0?x?0.9)$(0?x?0.9)$ by mechanical alloying of nanocrystalline Fe₅₅Ni₄₅ with graphite. The lattice constant of Fe₅₅Ni₄₅ increases linearly with increasing carbon content up to x=0.25$x=0.25$. At the same time, it is found that the magnetic moment per metal atom (Fe, Ni) decreases linearly with increasing carbon content for 0?x?0.25$0?x?0.25$ with a slope of 1.2 μ_B/at. For high carbon content, x?0.5$x?0.5$, it is observed that the decrease of lattice constant and increase of moment per metal atom (Fe, Ni) with increasing C content, indicates that the dissolution of carbon is hindered by the high-volume fraction of graphite in the initial powder mixture. The complete amorphization of x=0.5$x=0.5$ does not occur after the extended ball milling. The alloying effect of carbon on the magnetization is compared with other metalloid B, P, and Si in Fe- and Ni-based binary system.     

First-order density matrices in one dimension for independent fermions and impenetrable bosons in harmonic traps

To complement existing knowledge of the density matrix γF(x,y)${\gamma }_{\mathrm{F}}(x,y)$ of independent fermions for N   particles in one dimension under harmonic confinement, the corresponding matrix γIB(x,y)${\gamma }_{\mathrm{IB}}(x,y)$ for impenetrable bosons is given for N=2$N=2$ and 3 (with the N=4$N=4$ form available also). For fermions the momentum density is then obtained and illustrated numerically for N=10$N=10$. The boson momentum density is studied analytically at high momentum p  , the coefficients of the p⁻⁴$p^{\mathrm{-4}}$ and p⁻⁶$p^{\mathrm{-6}}$ terms being tabulated for N=2–5$N=2\text{–}5$ inclusive. Their dependence on powers of N   is exhibited numerically. Finally, the functional relationship between γIB(x,y)${\gamma }_{\mathrm{IB}}(x,y)$ and γF(x,y)${\gamma }_{\mathrm{F}}(x,y)$ is formally set out and illustrated.     

The position–momentum uncertainty relations for a Pöschl–Teller type potential and its squeezed phenomena

We present the position–momentum uncertainties for the Pöschl–Teller potential. We observe that the Δx decreases with the potential depth λ but increases with quantum number n. Interestingly, we find that the Δp first increases and then decreases with the n  . The ΔxΔp$\mathrm{\Delta }x\mathrm{\Delta }p$ first decreases and then increases with the λ  , but almost becomes a constant (n+1/2)?$(n+1/2)?$ for a larger λ. Particularly, there exists a squeezed phenomenon in position x for the lower states. The squeezing in x compensated for by an increase in momentum p  , such that ΔxΔp??/2$\mathrm{\Delta }x\mathrm{\Delta }p??/2$ is still satisfied.     

Analysis of confinement potential fluctuation and band-gap renormalization effects on excitonic transition in GaAs/AlGaAs multiquantum wells grown on (1 0 0) and (3 1 1)A GaAs surfaces

The competition between confinement potential fluctuations and band-gap renormalization (BGR) in GaAs/Al_xGa_1−xAs$\mathrm{GaAs}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells grown on [1 0 0] and [3 1 1]A GaAs substrates is evaluated. The results clearly demonstrate the coexistence of the band-tail states filling related to potential fluctuations and the band-gap renormalization caused by an increase in the density of photogenerated carriers during the photoluminescence (PL) experiments. Both phenomena have strong influence on temperature dependence of the PL-peak energy (E_PL(T))$(E_{\mathit{PL}}(T))$. As the photon density increases, the E_PL can shift to either higher or lower energies, depending on the sample temperature. The temperature at which the displacement changes from a blueshift to a redshift is governed by the magnitude of the potential fluctuations and by the variation of BGR with excitation density. A simple band-tail model with a Gaussian-like distribution of the density of state was used to describe the competition between the band-tail filling and the BGR effects on E_PL(T).     

Testing the interaction model with cosmological data and gamma-ray bursts

We use the new gamma-ray bursts (GRBs) data, combined with the baryon acoustic oscillation (BAO) observation from the spectroscopic Sloan digital sky survey (SDSS) data release, the newly obtained A   parameter at z=0.6$z=0.6$ from the WiggleZ dark energy survey, the cosmic microwave background (CMB) observations from the 7-year Wilkinson microwave anisotropy probe (WMAP7) results, and the type Ia supernovae (SNeIa) from Union2 set, to constrain a phenomenological model describing possible interactions between dark energy and dark matter, which was proposed to alleviate the coincidence problem of the standard ΛCDM model. By using the Markov chain Monte Carlo (MCMC) method, we obtain the marginalized 1σ   constraints _Ωm=0.2886±0.0135${\Omega }_m=0.2886\pm 0.0135$, _rm=−0.0047±0.0046$r_m=-0.0047\pm 0.0046$, and _wX=−1.0658±0.0564$w_X=-1.0658\pm 0.0564$. We also consider other combinations of these data for comparison. These results show that: (1) the energy of dark matter is slightly transferring to that of dark energy; (2) even though the GRBs + BAO + CMB data present less stringent constraints than SNe + BAO + CMB data do, the GRBs can help eliminate the degeneracies among parameters.     

Observation of spin reorientation in layered manganites La1.2Sr1.8(Mn1−yRuy)2O7 (0.0⩽y⩽0.2) by Lorentz transmission electron microscopy

The effect of Ru substitution for Mn in bilayered oxides La_1.2Sr_1.8(Mn_1−yRu_y)₂O₇ (0?y?0.2$0?y?0.2$) was investigated by magnetization measurements and low-temperature Lorentz transmission electron microscopy. It was found that the magnetic anisotropy is controlled by the Ru content y and temperature T. The easy axis changes from 〈1 1 0〉 for the y=0$y=0$ crystal to the c  -axis for y=0.2$y=0.2$, and it rotates away from the c-  axis for the y=0.05$y=0.05$ and y=0.07$y=0.07$ crystals with decreasing temperature. Furthermore, maze-shaped magnetic domain structures were observed in the (0 0 1) thin crystals with 0.05?y?0.2$0.05?y?0.2$. Changes in domain size and structure indicate that the uniaxial magnetic anisotropy becomes stronger as Ru content y increases.     

Characterization of magnetic phases in the FexMn0.65−xAl0.35 disordered alloys

Melted alloys of the Fe_xMn_0.65−xAl_0.35 disordered system, 0.25?x?0.65, were experimentally studied by Mössbauer spectrometry, vibrating sample magnetometry and AC magnetic susceptibility. All the alloys exhibit the BCC structure with a nearly constant lattice parameter (2.92 Å). Mössbauer studies at room temperature (RT) show that in the 0.25 ?x?0.45 range the alloys are paramagnetic (P) while in the 0.50?x?0.65 range, they are ferromagnetic. At 77 K, Mössbauer studies show that the alloy with x=0.25$x=0.25$ presents weak magnetic character that is consistent with an antiferromagnetic (AF) behavior due to the high Mn content, while those with 0.30?x?0.40 are paramagnetic, and those in the 0.45?x  ?0.65 range are ferromagnetic (F) with a mean field increasing with the Fe content. Hysteresis cycles at RT prove the paramagnetic character of the alloys between x=0.25$x=0.25$ and 0.40 and the ferromagnetic character for x?0.45$x?0.45$. Complementary measurements using AC magnetic susceptibility permit a magnetic phase diagram to be proposed, with the P phase for high temperature and all the compositions, the AF phase for low Fe content and at low temperature, the F phase for high Fe content above RT and the spin glass phase for all the compositions and at temperatures lower than 46 K. In addition, the mean field renormalization group (MFRG) method, applied to a random competitive and site dilute Ising model with nearest-neighbor, gives rise to magnetic phase diagram, which fairly agrees with previous experimental one.     

Structural and magnetic properties of nanostructured Mn–Al–C magnetic materials

Pre-alloyed Mn_50+x−yAl_50−xC_y   (x=0$x=0$, 2, 4, 6, 8; y=0$y=0$, 1.7, 3) powders were mechanically milled (MM), and the as-milled powders subsequently annealed at temperatures from 350 to 600 °C to produce the ferromagnetic metastable L1₀-structured τ-phase. Bulk Mn₅₄Al₄₆ specimens were also annealed under the same conditions for comparison. The effects of the Mn concentration and C additions on phase formation, microstructure, magnetic properties, as well as on the magnetization mechanism of the Mn–Al–C alloys were systematically investigated. It was found that the magnetic properties are strongly dependent both on the fraction of the τ-phase and its microstructure. There exists a strong influence of the microstructural refinement, due to the ball milling, on the rate of ε-phase to τ-phase transformation and on the stability of the τ-phase. The kinetics of formation and subsequent decomposition of the magnetic τ-phase were markedly different in the MM and bulk alloys. Both remanence curves and δM plots showed no exchange coupling among the τ-phase nanograins. The mechanism for the magnetization process was determined to be domain wall pinning.