The structure and magnetotransport properties of La2/3Sr1/3MnO3/Ba(La)TiO3 composites

The effect of Ba(La)TiO₃ doping on the structure and magnetotransport properties of La_2/3Sr_1/3MnO₃(LSMO)/xBa(La)TiO₃ (x=0.0, 1.0, 5.0 mol%) have been investigated. The X-ray diffraction patterns and microstructural analysis show that BaTiO₃ and LSMO phases exist independently in BaTiO₃-doped composites. The metal-insulator transition temperature (T_MI) decreases whereas the maximum resistivity increases very quickly by the increase of BaTiO₃ doping level. The partial substitution of Ba by La(0.35 mol%) results in a decrease in resistivity of LSMO/xBa(La)TiO₃ composites. Magnetoresistance of BaTiO₃-doped composites decreases monotonously in the temperature range 200-400 K in a magnetic field of 5 T, which is completely different from that of LSMO compound. The value of MR decreases at low field (H<1 T) and increases at high fields (H>1 T) with increasing the BaTiO₃ doping level at low temperatures below 280 K. These investigations reveal that the magnetotransport properties of LSMO/xBa(La)TiO₃ composites are dominated by spin-dependent scattering and tunneling effect at the LSMO/BaTiO₃/LSMO magnetic tunnel junction.     

Room-temperature magnetoresistance effects of Ag-added Fe3O4 films with single-domain grains

Polycrystalline Ag_x(Fe₃O₄)_1−x films (x=0, 0.1, 0.2 and 0.3) have been prepared by the sol-gel method in combination of the spin-coating technique with a precursor solution containing polyvinyl alcohol (PVA) on fused quartz substrates. XRD analysis and SEM images indicate that the Fe₃O₄ grains are nearly spherical single-domain particles. The coercivities of the films are about 290 Oe for x=0.1 and 360 Oe for x=0.3, respectively, which are nearly the same as the magnetocrystalline anisotropic effective field H_K of Fe₃O₄. At 300 K, the x=0.1 film has a maximal magnetoresistance of −8.7% at a magnetic field of 50 kOe and −3.5% at 8.8 kOe, while the pure Fe₃O₄ film is only −2.2% at 8.8 kOe. This enhancement of the MR can be attributed to the contribution from the spin-dependent scattering at the Ag-Fe₃O₄ interfaces as well as the spin-polarized tunneling at boundaries of Fe₃O₄ grains of the spin-polarized electrons. In addition, different MR behaviors for Ag-added Fe₃O₄ bulk polycrystalline samples and polycrystalline films are discussed.     

The magnetic susceptibility of hydrogen-doped partially crystalline (Zr76Ni24)1−xHx metallic glasses

The magnetizations of Zr₇₆Ni₂₄ metallic glass and hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses have been measured in the temperature range 10-300 K and magnetic fields up to 2 T for various dopant concentrations (x=0, 0.024, 0.043, 0.054). It is found that the samples are paramagnetic and magnetic susceptibility at room temperature, χ(300 K), shows a nonmonotonic behaviour upon hydrogenation. The values of χ(300 K) of the hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses are reduced with increase in hydrogen content up to x=0.043, whereas for x=0.054, an enhancement of χ(300 K) has been revealed. The magnetic susceptibility is weakly temperature dependent down to 110 K, below which an increase is observed. A shallow minimum exists between 90 and 120 K. The form and magnitude of the observed temperature dependence of the magnetic susceptibility are well accounted for by the sum of the quantum corrections to the magnetic susceptibility. Hydrogen reduces the electronic diffusion constant and influences strongly the quantum interference at defects, slowing down the spin diffusion and enhancing the magnetic susceptibility in the temperature range from 110 down to 10 K.     

Excess current in point contacts on two-band superconductor MgB2 in magnetic field

A series of I(V) characteristics and bias-dependent differential resistance dV/dI(V) curves of point contacts made between a single crystal of two-band superconductor MgB₂ and Cu were measured in magnetic fields up to 9 T. The magnetic field dependences of the excess current in the I(V) curves were obtained and analyzed using Koshelev and Golubov's [Phys. Rev. Lett. 90, 177002 (2003)] theoretical results for the mixed state of a dirty two-band superconductor. Introducing a simple model for the excess current in the point contact in the mixed state, our data can be qualitatively described using the theoretical magnetic field dependence of the superconducting order parameter of the σ and π-bands and the averaged electronic density of states in MgB₂.     

The magnetic state of diamagnetically diluted antiferromagnetic cobalt and nickel monoxide

The nearest neighbour J₁(x) and the next-neighbour super-exchange J₂(x) interactions are evaluated by using the mean field theory for Mg1−xBxO (B=Co and Ni) systems. The magnetic energy E(x) is obtained. A magnetic phase diagram of the Mg1−xBxO (B=Co and Ni) solid solutions with 0?x?1 is drawn by high-temperature series expansions (HTSE) combined with the Padé approximants method (PA). The critical exponents associated with the magnetic susceptibility (γ) and with the correlation length (ν) are deduced in order phase.     

Manifestation of vortex depinning transition in nonlinear current-voltage characteristics of polycrystalline superconductor Y1−xPrxBa2Cu3O7−δ

We present our recent results on the temperature dependence of current-voltage characteristics for polycrystalline Y_1−xPr_xBa₂Cu₃O_7−δ superconductors with x=0.0, 0.1 and 0.3. The experimental results are found to be reasonably well fitted for all samples by a power like law of the form V=R(I−Ic)^a(T). Here, we assume that a(T)=1+Φ₀IC(T)/2πkBT and IC(T)=IC(0)(1−T/TC)^3/2 for the temperature dependences of the power exponent and critical current, respectively. According to the theoretical interpretation of the obtained results, nonlinear deviation of our current-voltage characteristics curves from Ohmic behavior (with a(TC)=1) below TC is attributed to the manifestation of dissipation processes. They have a characteristic temperature Tp defined via the power exponent as a(Tp)=2 and are related to the current induced depinning of Abrikosov vortices. Both TC(x) and Tp(x) are found to decrease with an increase of Pr concentration x reflecting deterioration of the superconducting properties of the doped samples.     

Spin correlations in YBa2(Cu1−x Fx)3O7+y ceramic

To detect scattering by magnetic correlations and to estimate their characteristic space scale, YBa₂(Cu_1?x Fe_x)₃O_7+y ceramic with x=0.13 and y=0.4 is investigated by the small-angle scattering of polarized neutrons. The measurements are carried out in the range of temperatures 15 K?T?315 K and magnetic fields 0<H?4500 Oe. Anomalies in the temperature curves of the intensity I(T,q) (where q is the momentum transfer) and the polarization P(T,q) are observed in the temperature range T<40 K. Interference between nuclear and magnetic scattering is also observed in this temperature range. The observed phenomena are interpreted as scattering by magnetic correlations having a scale 70 Å<R<370 Å. Irreversible effects and the type of magnetic ordering are discussed.     

The generalized exponential-integral V(x,y)=∫1∞ exp(−xt)ln(t+y)dtt and computer algorithms for y = 0, 1

The generalized exponential-integral function V(x, y) defined here includes as special cases the function E⁽²⁾₁(x) = V(x, 0) introduced by van de Hulst and functions M₀(x) = V(x, 1) and N₀(x) = V(x, -1) introduced by Kourganoff in connection with integrals of the form ∫ E_n)t)E_m(t±x), which play an important role in the theory of monochromatic radiative transfer. Series and asymptotic expressions are derived and, for the most important special cases, y = 0 and y = 1, Chebyshev expansions and rational approximations are obtained that permit the function to be evaluated to at least 10 sf on 0<x<∞ using 16 sf arithmetic.     

Effect of surface preparation on the properties of Au/p-Cd1−xZnxTe

The effect of bromine methanol (BM) etching and NH₄F/H₂O₂ passivation on the Schottky barrier height between Au contact and semi-insulated (SI) p-Cd_1−xZn_xTe (x ≈ 0.09-0.18) was studied through current-voltage (I-V) and capacitance-voltage (C-V) measurements. Near-infrared (NIR) spectroscopy technique was utilized to determine the Zn concentration. X-ray photoelectron spectroscopy (XPS) for surface composition analysis showed that BM etched sample surface left a Te⁰-rich layer, however, which was oxidized to TeO₂ and the surface [Te]/([Cd] + [Zn]) ratio restored near-stoichiometry after NH₄F/H₂O₂ passivation. According to I-V measurement, barrier height was 0.80 ± 0.02-0.85 ± 0.02 eV for Au/p-Cd_1−xZn_xTe with BM etching, however, it increased to 0.89 ± 0.02-0.93 ± 0.02 eV with NH₄F/H₂O₂ passivation. Correspondingly, it was about 1.34 ± 0.02-1.43 ± 0.02 eV and 1.41 ± 0.02-1.51 ± 0.02 eV by C-V method.     

Anisotropy and FOMP in Tb3 (Fe28−xCox) V1.0 (x=0, 3 and 6) compounds

In this work, the structural and magnetic properties of Tb₃ (Fe_28−xCo_x) V_1.0 (x=0, 3, 6) compounds have been investigated. The structural characterization of compounds by X-ray powder diffraction is an evidence for a monoclinic Nd₃(Fe, Ti)₂₉-type structure (A2/m space group). The refined lattice parameters a and b (but not c) and the unit cell volume V, obtained from the XRD data by the Rietveld method, are found to decrease with increasing Co concentration. The unit cell parameters behavior has been attributed to the smaller Co atoms and a preferential substitution of Fe by Co. The anisotropy field (H_a) as well as critical field (H_cr) was measured using the singular point detection (SPD) technique from 5 to 300 K in a pulsed magnetic field of up to 30 T. At T=5 K, a FOMP of type 2 was observed for all samples and persists at all temperatures up to 300 K. For sample x=0, H_cr=10.6 and 2.0 T at 5 and 300 K, respectively, is equal to that reported earlier. The occurrence of canting angles between the magnetic sublattices during the magnetization process instead of high-order anisotropy contributions (at room temperature are usually negligible) has been considered to explain the survival of the FOMP at room temperature. The anisotropy and critical fields behave differently for samples with x=0, 3 compared with x=6. The observed behavior has been related to the fact that the Co substitution for Fe takes place with a preferential entrance in the inequivalent crystallographic sites of the 3:29 structure. The contribution of the Tb-sublattice in the Tb₃(Fe, V)₂₉ compound with uniaxial anisotropy has been scaled from the anisotropy field measured on a Y₃(Fe, V)₂₉ single crystal with easy plane anisotropy.     

Physical and chemical properties of nanostructured CoO-AlOx granular films

A series of nano CoO-AlO_x granular films were prepared by Ar-O₂ reactive rf sputtering and their resistive and magnetoresistive properties were measured for studying the spin-dependent properties. Transmission electron microscopy and X-ray photoelectron spectroscopy are also utilized to investigate the structures and chemical states of these samples. Based on our results, the particles of CoO with radius around 2 nm could be magnetic at room temperature and provide a large tunneling magnetoreststance of −6% through the AlO_x barrier.     

Enhanced low-field magnetoresistance in LSMO/SFO composite system

The granular composites of (1−x)La_0.7Sr_0.3MnO₃/xSrFe₁₂O₁₉ [(1−x)LSMO/xSFO] were prepared. The magnetic, electrical, and magnetoresistive properties of the composites were investigated systematically. Two magnetic transitions originating from LSMO and SFO are observed for x=0.05, 0.10, and 0.20. The addition of hard-magnetic SFO ferrite leads to the increased substantially resistivity of the composites and the shift of insulating-metallic transition temperature T_IM correlated with LSMO. Enhanced low-field magnetoresistance (LFMR) in the composites can be mainly attributed to the enhanced spin disorder and spin-dependent tunneling at LSMO grain boundaries induced by the interaction between LSMO and SFO ferrite. The transport mechanisms in detail are analyzed in LSMO/SFO composite system.     

Low-field transport properties of (1−x)La0.7Ca0.2 Sr0.1MnO3+x(ZnO) composites

Composite samples (1−x)La_0.7Ca_0.2Sr_0.1MnO₃(LCSMO)+x(ZnO) with different ZnO doping levels x have been investigated systematically. The structure and morphology of the composites have been studied by the X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD and SEM results indicate that no reaction occurs between LCSMO and ZnO grains, and that ZnO segregates mostly at the grain boundaries of LCSMO. The magnetic properties reveal that the ferromagnetic order of LCSMO is weakened by addition of ZnO. The results also show that ZnO has a direct effect on the resistance of LCSMO/ZnO composites, especially on the low-temperature resistance. With increase of the ZnO doping level, T_P shifts to a lower temperature and the resistance increases. It is interesting to note that an enhanced magnetoresisitance (MR) effect for the composites is found over a wide temperature range from low temperature to room temperature in an applied magnetic field of 3 kOe. The maximum MR appears at x=0.1. The low field magnetoresistance (LFMR) results from spin-polarized tunneling. However, around room temperature, the enhanced MR of the composites is caused by magnetic disorder.     

Optical,physical and structural studies of vanadium doped P2O5–BaO–Li2O glasses

Glasses in the compositions (Li₂O)₂₅–(BaO)₂₅–(P₂O₅)_50−x–(V₂O₅)_x (with x=0.5,1.0,1.5,2.0,2.5, and 3.0 mol%) have been prepared by the conventional melt quenching technique. X-ray powder diffractrogram show broad peaks which conforms glassy nature of the sample. Differential scanning calorimetry (DSC) thermograms show characteristic glass transition temperature (T_g) and it increases with increasing substitution of V₂O₅ for P₂O₅. The measured physical parameters like density, refractive index, ionic concentration and electronic polarizability are found to vary linearly with increasing x. Infrared spectra exhibits few bands, which are attributed to (P=O)_AS, (P=O)_S, (V=O), (P–O–P)_AS,P–O–V, (P–O–P)_AS and O–P–O vibrations. The optical absorption spectra of VO²⁺ ions in these glasses show three bands and are assigned to the ²B₂→ ²E,²B₂→ ²B₁ and ²B₂→ ²A₁ transitions. Electron paramagnetic resonance spectra of all the glass samples exhibit resonance signals characteristic of VO²⁺ ions. The values of Spin-Hamiltonian parameters indicate that the VO²⁺ ions are present in octahedral sites with tetragonal compression and belong to C_4V symmetry.     

Investigation of the solid solution series 2(MnX)-CuInX2 (X=S, Se)

(2MnX)_x(CuInX₂)_1−x with X=S and Se were prepared by solid state reaction from the end members α-MnS, β-MnS and CuInS₂ in the range 0<x≤0.2 (≤0.6 for β-MnS) as well as MnSe and CuInSe₂ in the range 0<x≤0.1. Mixed crystals with 0≤x≤0.1 crystallize in the tetragonal chalcopyrite type structure, (2α-MnS)_x(CuInS₂)_1−x samples with 0.1<x≤0.2 and (2β-MnS)_x(CuInS₂)_1−x samples up to x=0.6 consist of two phases, occuring as tetragonal domains (x∼0.1 for X=S) within a cubic matrix with zinc-blende type structure (x∼0.4 for X=S), indicating a miscibility gap. For tetragonal single phase samples the band gap energy, the lattice constants and the anion parameter have been determined. The first and the latter ones show a different composition dependent behaviour caused by the modification of the MnS (α-MnS with NaCl type structure, β-MnS with zinc-blende type structure) used during the synthesis. Additionally a CuMn_xIn_1−xS₂ powder sample, in which Mn substitutes the M^III site, was investigated. The SQUID measurements revealed a well-distinct magnetic transition between 15 and 16 K as well as ferromagnetic-like hysteresis loops pronounced for temperatures below the transition temperature. Below this temperature a clear splitting between the zero field cooling (ZFC) and the field cooling (FC) curves indicate to the existence of a long-range magnetic ordering phenomenon. This behaviour was not found in the other samples were Mn substitutes both sites M^I as well as M^III.     

Effects of Co substitution on structural and magnetic properties of R3(Fe1−xCox)29−yVy (R=Tb,Dy)

Synthesis of two novel series of intermetallic compounds Tb₃(Fe_1−xCo_x)_27.4V_1.6 (x=0,0.1, 0.2, 0.3, 0.4) and Dy₃(Fe_1−xCo_x)_27.8V_1.2 (x=0, 0.1, 0.2, 0.3) with the monoclinic Nd₃(Fe,Ti)₂₉-type structure (3:29) is presented. In the Dy series for x=0.4 a disordered variant of the hexagonal Th₂Ni₁₇-type structure is formed. The cell parameters decrease and the Curie temperature increases with increasing of the Co content. In the case of the Tb₃(Fe_1−xCo_x)_27.4V_1.6 series in the M(T) curve a magnetic transition is observed which is attributed to spin reorientation phenomena. This critical temperature decreases with increasing Co from 473 K for x=0.1 to 393 K for x=0.3, and was not observed in the case of 0.4. XRD patterns of magnetically aligned powder samples reveal the presence of a tilted magnetic structure.     

Magnetoresistive properties of Zn1−xCoxFe2O4 ferrites

The magnetic and magnetoresistive properties of spinel-type Zn_1−xCo_xFe₂O₄ (x=0, 0.2 and 0.4) ferrites are extensively investigated in this study. A large negative magnetoresistance (MR) effect is observed in Zn_1−xCo_xFe₂O₄ ferrites of spinel structure. These materials are either ferrimagnetic or paramagnetic at room temperature, and show a spin-(cluster) glass transition at low temperatures, depending on the chemical compositions. The MR curves as a function of magnetic fields, MR(H), are parabolic at all temperatures for paramagnetic polycrystalline ZnFe₂O₄. The MR for ZnFe₂O₄ at 110 K in the presence of 9 T applied magnetic field is 30%. On the other hand, MR(H) are linear for x=0.2 and 0.4 ferrimagnetic Zn_1−xCo_xFe₂O₄ samples up to 9 T. The MR effect is independent of the sintering temperatures, and can be explained with the help of the spin-dependent scattering and the Yafet–Kittel angle of Zn_1−xCo_xFe₂O₄ mixed ferrites.     

Dynamical Localization for the Random Dimer Schrödinger Operator

We study the one-dimensional random dimer model, with Hamiltonian H _ω =Δ+V _ω , where for all x∈ $\mathbb{Z}$ , V _ω(2x)=V _ω(2x+1) and where the V _ω(2x) are i.i.d. Bernoulli random variables taking the values ±V, V>0. We show that, for all values of Vand with probability one in ω, the spectrum of His pure point. If V≤1 and V≠1/ $\sqrt 2$ , the Lyapunov exponent vanishes only at the two critical energies given by E=±V. For the particular value V=1/ $\sqrt 2$ , respectively, V= $\sqrt 2$ , we show the existence of new additional critical energies at E=±3/ $\sqrt 2$ , respectively, E=0. On any compact interval Inot containing the critical energies, the eigenfunctions are then shown to be semi-uniformly exponentially localized, and this implies dynamical localization: for all q>0 and for all ψ∈ $\ell$ ²( $\mathbb{Z}$ ) with sufficiently rapid decrease $${\mathop {\sup }\limits_t} r_{\psi ,I}^{\left( q \right)} {\kern 1pt} \left( t \right): = {\mathop {\sup }\limits_t} \left\langle {P_I \left( {H\omega } \right)\psi _t ,\left| X \right|^q P_I \left( {H\omega } \right)\psi _t } \right\rangle < \infty $$ Here $\psi _t = e^{- iH_{\omega ^t}} \psi$ , and P _I(H _ω) is the spectral projector of H _ωonto the interval I. In particular, if V>1 and V≠ $\sqrt 2$ , these results hold on the entire spectrum [so that one can take I=σ(H _ω)].     

Determination of IVC breakpoint for Josephson junction stack. Periodic and nonperiodic (with γ = 0) boundary conditions

We prove that in the case of periodic and nonperiodic (with γ = 0) boundary conditions, the calculation of the current-voltage characteristic for a stack of n intrinsic Josephson junctions reduces to solving a unique equation. The current-voltage characteristic V(I) has the shape of a hysteresis loop. On the back branch of the loop, V(I) rapidly decreases to zero near the breakpoint I _b . We succeeded to derive an equation determining the approximate breakpoint location.     

Transition from the Kondo regime to long-range magnetic order in the FexV1−x S system

A study is reported on the electrical and magnetic characteristics of the Fe_xV_1?x S solid-solution system with x≤0.5. A maximum in the temperature dependence of resistivity ρ(T) characteristic of the Kondo effect has been observed for small x(x<0.01). For x>0.1, long-range magnetic order sets in in the system with T _K ≈ 100 K. Near x=0.05, the Fe²⁺ impurity behavior crosses over to a magnetically ordered phase. The electronic properties of Fe_xV_1?x S are typical of those of strongly correlated electronic systems. Both the electrical and magnetic data imply that carrier delocalization is the strongest at x=0.4.