Electrical transport properties of a quasi-one-dimensional Nb3Te4 single crystal

The electrical resistance of a linear chain metal Nb₃Te₄ were measured from 1.3 to 320 K. The residual resistance ratio $\text{R(300}\text{K}\text{)}\text{R(4.2}\text{K}\text{)}$ is about 3. Nb₃Te₄ shows an anomaly in the resistivity vs temperature at about 80 K, suggesting an occurrence of a charge-density-wave transition. The transverse and longitudinal magnetoresistance at 4.2 K are proportional to the magnetic field in the range of 2–58 kOe. In the superconducting region close to the transition temperature T_c, the critical magnetic field H_c2 is proportional to δT=T_c?T. The angular dependence of H_c2 fits well with the fluxoid model of the Ginzburg-Landau theory. The ratio of the critical fields parallel and perpendicular to the chain direction is 4.8.     

Upper critical fields of Nb3Ge thin film superconductors

The upper critical field H_c2(T) of the highest T_c(～23K) Nb₃Ge superconducting films has been found to be ≈370kG at 4.2K. Measurements on lower T_c films show very broad transitions reflecting nonuniformity. The H_c2(T) characteristics are consistent with other Nb₃X type II superconductors.     

On the mixed valence of the chromium ions in the noncollinear ferrimagnetic spinels Mn1−xCuxCr2S4 (Where x = 0.85, 0.90, 0.95)

For the polycrystalline samples of Mn_1?xCu_xCr₂S₄ (x = 0.85, 0.90, 0.95) the magnetization was measured in the temperature range between 77 K and the Curie temperature, T_C, using a magnetic balance (Faraday's method) and pulsed magnetic fields up to 2.0 T. The magnetic susceptibility was measured between T_C and about 600 K. The Curie temperatures were obtained using the kink point method.In the temperature range between 4.2 and 77 K the magnetization was measured in stationary magnetic fields up to 14 T. The data indicate a noncollinear ferrimagnetic structure. The compounds under investigation can be treated as CuCr₂S₄ slightly doped with Mn, with a valence distribution Mn²⁺_1?xCu¹⁺_xCr³⁺_2?xCr⁴⁺_xS^2?₄.     

Specific heat and magnetic susceptibility of single-crystalline ZnCr2−xAlxSe4 (x=0.15, 0.23)

A correlation between the second critical field H_c2 of the helix to paramagnetic transition and the magnetic specific heat C-peak was found in ZnCr_2−xAl_xSe₄ spinel single crystals with x=0.15, 0.23. The specific heat peak is anomalously sharp for all finite magnetic fields used here and this points to a first order magneto-structural transition (from cubic to tetragonal symmetry). The C(T)-peak is increasingly suppressed as the external field increases. Approaching the Neel temperature T_N, a broad ac-magnetic susceptibility peak is observed for zero dc-magnetic field. That peak does not show an energy loss and thus points towards a return to a second order type of transition. The magnetic contribution to the specific heat displays a sharp peak at T_N and is maximal at the spin fluctuation temperature T_sf=34 K. T_sf is related to the maximum of the magnetic susceptibility at T_m=40 K (at 50 kOe) in the spin fluctuation region, as evidenced by the entropy exceeding 90% of the entropy calculated classically for the complete alignment of the Cr spins, (2−x)R ln(2S+1). The X-ray photoelectron spectroscopy (XPS) data indicate that Al-substitution does not affect Cr³⁺ 3d³ electronic configuration.     

Upper critical fields of cubic and tetragonal single crystal and polycrystalline Nb3Sn in DC fields to 30 tesla

Measurements of the upper critical field, H_c2(T), in single crystal Nb₃Sn were extended to 30 tesla (300 kG) with dc fields produced by a Hybrid magnet. Observations of H_c2(T) were made for materials which remain in the cubic (c) phase and those which show a martensitic transformation at the tetragonal (t) phase. H_c2(T) measurements of Nb₃Sn for a pure crystal for which the de Haasvan Alphen (DHVA) effect was observed and for polycrystalline (t) phase and (c) phase materials are also reported. Measured values of H_c2(4.2 K) and calculated values of H_c2(0) are: 1) along the [100] direction for our earlier Nb₃Sn, H_c2(4.2 K) = 26 T for the (c) phase and 21.5 T for the (t) phase; H_c2(0) = 29T for the (c) phase and 24 T for the (t) phase; 2) along the [100] direction for the DHVA material H_c2(4.2 K) = 18 T and H_c2(0) = 20 T; 3) for polycrystalline Nb₃Sn (t) phase material H_c2(4.2 K) = 23 T and H_c2(0) = 25 T and for (c) phase material H_c2(4.2 K) = 26 T and H_c2(0) = 29 T. The values of (dH_c2/dT)_T=Tc vary from 2.4T/K for the highest H_c2(T) material to 1.6T/K for the DHVA material. The anisotropy for various Nb₃Sn single crystal materials is small and independent of temperature from T_c to 0.1 T_c. δT_c between the (c) and (t) phase is <0.3 K. Within experimental error excellent fits of H_c2(T) with theory are obtained assuming a dirty or clean Type II superconductor with no Pauli paramagnetic limiting. Experimental details and strong-coupling effects are discussed. When strong-coupling is included, the effects of any paramagnetic limiting would be small and not detectable within our present experimental error. Brief comments also are made concerning H_c2 of V₃Si.     

Transport properties of n-type ferromagnetic semiconductor HgCr2−xInxSe4(x = 0.100)

Measurements of the electrical conductivity, magnetoresistance, and Hall effect were performed on a n-type ferromagnetic semiconductor HgCr_2?xIn_xSe₄(x = 0.100) single crystal from 6.3 to 296 K in magnetic fields up to 1.19×l0⁶A/m. The conductivity decreases rapidly near the Curie temperatureT_c (≈120 K) as the temperature is raised. A large peak in the magnetoresistance is observed near T_c. The Hall effect measurements indicate that the temperature dependence of the conductivity and the magnetoresistance are due mostly to a change in electron mobility. The electron mobility is 1.2 × 10^?2 m²/V · s at 6.3 K, and decreases rapidly near T_c with the rise in temperature. Then it increases slowly from 5.5 × 10^?4 m²/V · s at 160 K to 7.5 × 10^?4 m²/V · s at 241 K. This temperature dependence of the electron mobility can be explained in terms of the spin-disorder scattering which takes into account the exchange interaction between charge carriers and localized magnetic moments.     

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.     

Superconducting and magnetic properties of La3−x In Cex

Measurements of the critical temperature, T_c(x), the critical field, H_c2(x,T) and the susceptibility, χ (T), as a function de la concentration, x, in the system La₃₋x Ce_xIn, are compared with the theories of Muller-Hartmann and Zittsrtz, and Abrikosov and Gor'kov. The H_c2(x, T = 0) and χ(T) measurements indicate the appearance of short range antiferromagnetic order around x = 0.04 which is probably responsible for the anomalous behavior of T_c(x).     

Magnetic properties of Sn1-xCrxTe crystals

Magnetization measurements of magnetic semiconductor Sn_1-xCr_xTe (x <5 at%) crystals with the Curie temperature T_c = 150–300 K were made down to 2 K. The magnetic properties are sensitive to isothermal annealing under Zn vapor. The overall magnetizations of the Zn-annealed crystals have paramagnetic and ferro- or antiferromagnetic contributions.     

Normal state resistivity and Tç studies of superconducting Ti1−xSbx system

The superconducting transition temperature (T_c) and the temperature dependence of the normal state resistivity of the Ti_1?xSb_x system between T_c and 300 K have been studied. The T_c values are found to depend on the heat treatment of the samples. Below 40 K, all alloys show a T² dependence of the resistivity. However, the sample with x = 0.53 is not superconducting and shows a different behaviour of the resistivity.     

Chaleur spécifique et transformation martensitique dans le système Nb1−xSnx

Experimental data on the superconducting temperature, and low temperature specific heat, together with X-ray and neutron investigations are reported for two sets of Nb_xSn_1?x, samples with the A15 type structure. The first one, intended for a study of the effect of stoichiometry, consists of cast alloys with lattice constants ranging from 5.273 to 5.289 Å, γ's from 4.9 to $\text{9}\text{mJ}\text{K}{}^2$ g-at and T_c's from 6 to 17.9 K. The second one consists of nearly stoichiometric sintered alloys with lattice constants ranging from 5.288 to 5.291 Å, γ's from 5.6 to 13.5 $\text{mJ}\text{K}{}^2$ g-at and T_c's close to 18 K. The martensitic transformation ratio has been determined by neutron diffraction in the latter set and the values 32, 77 and 95% were obtained. The analysis of the specific heat jumps at T_c shows that the superconducting T_c of the tetragonal phase is 17.9 K and both T_c and γ are reduced in the cubic phase.Phonon information inferred from specific heat data are indicative of a structure with a typical energy of 9 meV in the density of states. The agreement with spectra obtained by inelastic neutron scattering is good.     

High field magnetization of Pr1-xGdxNi single crystals

Magnetization measurements were carried out on Pr_1−xGd_xNi single crystals for x=0.1, 0.2, 0.3 and 0.4 using pulsed magnetic fields up to 55 T and at 4.2 K. For the data observed along the c-axis there are clear indications of transitions, of the combined system of Pr and Gd moments, from a ferrimagnetic to a ferromagnetic state. The observed saturation magnetizations at 55 T and intermediate magnetizations at 1 T were well modeled assuming ferro- and ferri-magnetic structures, respectively.     

Magnetocaloric effect of GdCo2−xAlx compounds

The structure, magnetic property and magnetocaloric effect of GdCo_2−xAl_x (x=0, 0.06, 0.12, 0.18, 0.24, 0.4) compounds have been investigated by X-ray diffraction (XRD) and magnetic measurement techniques. The experimental results show that the GdCo_2−xAl_x (x≤0.4) compounds are single phase with a Laves-phase MgCu₂-type structure. The Curie temperature T_c initially increases, and then decreases with increasing Al content. The maximum value of T_c, 418 K, is reached for the compound with x=0.06. The magnetic entropy change, which is determined from the temperature and field dependence of the magnetization by the Maxwell relation, decreases almost linearly with increasing Al content.     

Magnetic susceptibility anomaly connected with a displacive phase transition in MnAs1−xPx and CrxMn1−xas compounds

With the second order B31 [rlhar2] B8₁ phase transition T_t in MnAs_1-xP_x and Cr_xMn_1-xAs compounds a high-spi low-spin transition occurs simultaneously with a drastic change in the magnetic coupling. An interpolation scheme is proposed which yields C and T_c for each temperature below T_t separately.     

Structural and magnetic studies of the alloy system GdInxAg1-x

The structural and magnetic studies below room temperature of the alloy system GdIn_xAg_1-x(0?x?1) are reported. It has been found that alloys with x?0.5 crystallize in CsCl type cubic structure, but the distortion of the unit cell starts for alloys with x#62;0.5 and for these alloys the unit cell becomes tetragonal. No phase change is evident from the low temperature (295-8 K) structural studies. However, a break appears in the X^-1_m vs. T linear plot of each alloy of this system at a specific temperature (designated as break temperature T_B). The variation of T_B with x is similar to the variation of phase transition temperature with x reported for LaIn_xAg_1-x. Close agreement has been found in the values of effective magneton number (p), magnetic ordering [Néel (T_N) or Curie (T_C)] and paramagnetic Curie (θ_p) temperature for materials studied by us and earlier workers. The variation of magnetization with applied field strength (2.5-65)×10⁵ Am^-1) at 4.2 K has also been reported for ferromagnetic of this system. It has been concluded that alloys with 0.4?x?0.6 are simple ferromagnets with parallel alignment of magnetic moment in the ground state. The angular arrangement of the magnetic moment starts appearing in the ground state for alloys with x?0.4 for x#62;0.6 and continues till x becomes closure to 0.17 or 0.84. The alloys with x=0.17 or x=0.84 have θ_p and T_C equal to zero and appear paramagnetic. Angular arrangement in spins again appear for alloys with x?0.17 or x#62;0.84, however all materials with 0?x?0.17 or 0.84?x?1 remain antiferromagnetic.     

Magnetic hyperfine interaction and curie temperatures of amorphous (FexNi1−x)80P14B6

Amorphous magnetic glasses (Fe_xNi_1?x)₈₀P₁₄B₆ (0.1 ? x ? 1) have been studied by Mössbauer spectroscopy. At 4.2 K, well-defined and very similar spectra have been observed for samples with different Fe concentration. This suggests that the Fe hyperfine field distribution is insensitive to the amount of dilutant. The value of the average hyperfine field at 4.2 K increases with Fe concentration. A value of 232 kOe is obtained by extrapolating to zero Fe concentration. The relationship $\text{H}{}_{\mathrm{eff}}\text{(O) = (232 + 33}\text{μ}\text{)}\text{kOe}$, similar to that of crystalline Fe-Ni alloys describes the results. The values of T_c = 60 K, 234 K and 425 K have been determined for samples with x = 0.125, 0.25 and 0.375 respectively. The T_C vs. x relation suggests a critical concentration of x_c ? 0.1, above which ferromagnetism exists. By fitting the measured values of T_C to the results of a coherent potential approximation, exchange interaction temperatures of J_FeFe = 617 K, J_FeNi = 800 K and J_NiNi ? 0 K have been obtained.     

Behavior of the irreversibility line in the new superconductor La1.5+xBa1.5+x−yCayCu3Oz

The irreversibility properties of high-T_c superconductors are of major importance for technological applications. For example, a high irreversibility magnetic field is a more desirable quality for a superconductor [1]. The irreversibility line in the H–T plane is constituted by experimental points, which divides the irreversible and reversible behavior of the magnetization. The irreversibility lines for series of La_1.5+xBa_1.5+x−yCa_yCu₃O_z polycrystalline samples with different doping were investigated. The samples were synthesized using the usual solid estate reaction method. Rietveld-type refinement of x-ray diffraction patterns permitted to determine the crystallization of material in a tetragonal structure. Curves of magnetization ZFC–FC for the system La_1.5+xBa_1.5+x−yCa_yCu₃O_z, were measured in magnetic fields of the 10–20,000 Oe, and allowed to obtain the values for the irreversibility and critical temperatures. The data of irreversibility temperature allowed demarcating the irreversibility line, T_irr(H). Two main lines are used for the interpretation of the irreversibility line: one of those which suppose that the vortexes are activated thermally and the other proposes that associated to T_irr a phase transition occurs. The irreversibility line is described by a power law. The obtained results allow concluding that in the system La_1.5+xBa_1.5+x−yCa_yCu₃O_z a characteristic bend of the Almeida–Thouless (AT) tendency is dominant for low fields and a bend Gabay–Toulouse (GT) behavior for high magnetic fields. This feature of the irreversibility line has been reported as a characteristic of granular superconductors and it corroborates the topological effects of vortexes mentioned by several authors 1 and 2.     

Dielectric properties of SrBi2−xPrxNb2O9 ceramics (x=0, 0.04 and 0.2)

SrBi_2−xPr_xNb₂O₉ (x=0, 0.04 and 0.2) ceramics were prepared by a solid state reaction method. X-ray diffraction analysis indicated that single-phase layered perovskite structure ferroelectrics were obtained. A relaxor behavior of frequency dispersion was observed among Pr-doped SrBi₂Nb₂O₉. The degree of frequency dispersion ΔT increased from 0 for x=0-7 °C for x=0.2, and the extent of relaxor behavior γ increased from 0.94 for x=0-1.45 for x=0.2. The substitution of Pr ions for Bi³⁺ ions in the Bi₂O₂ layers resulted in a shift of the Curie point to lower temperatures and a decrease in remanent polarization. In addition, the coercive field 2E_c reduced from 110 kV/cm for an undoped specimen to 90 kV/cm for x=0.2.     

Magnetic studies in amorphous GdxY1−x alloys

We report here our magnetic study on amorphous Gd_xY_1?x alloys with x = 0.17 and 0.70. The alloy with x = 0.17 is paramagnetic down to 4.2 K. For x = 0.70, magnetization shows a peak in low magnetic fields (H < 0.3 T). Magnetic saturation is difficult to obtain even for fields up to 15 T. An extrapolated value gives a moment of $\text{6.2 μ}{}_{\mathrm{B}}\text{Gd at}$. T_c is about 70 K. This led us to the conclusion that ferromagnetic and antiferromagnetic clusters are present in this alloy. The results are discussed by comparing them with crystalline Gd-Y alloys and amorphous Gd-Al, Gd-Au, etc. The effect of Y seems to be specific.     

Structural and magnetic phase transitions in CoxNi1−xMnGe system under pressure

The interaction of magnetic (T_c) an first-order distortion-type structural (T_D) transitions in the Co_xNi_1?xMnGe system was investigated using an ac susceptibility method under hydrostatic pressure up to 1.3 GPa. For all compositions T_C increases and T_D decreases with pressure. At the first triple point (0.3–0.6 GPa depending on x) these transitions go through a simultaneous magnetic-structural transition, as observed by Anzai and Ozawa in NiMnGe. However, for the composition x = 5, at the second triple point (at 0.8 GPa) the magnetic transition separates from the structural one, with T_C greater than T_D. This appears to be the first example of physically different, first and second-order phase transitions merging at a particular value of an external parameter, i.e. pressure P_TR1 and decoupling again at P_TR2 > P_T1. The (P, T) phase diagrams and the nature of the triple points are discussed using a simple Landau-type theory