N.Y. Ayoub

Processing and superconducting properties of the electron-doped compounds Ln2−xMxCuO4−y (Ln=Pr,Nd,Sm,Eu,Gd;M=Ce,Th)

Abstract The variations in transport and magnetic properties as a function of annealing conditions are discussed for the Ln 2−x M x CuO 4−y compounds. A composition x∼0.15−0.16 and a small oxygen deficiency (y∼0.02) are required to optimize superconducting properties. For Nd 1.85 Ce 0.15 CuO 3.98 , critical current densities, inferred from magnetization measurements, are estimated to be as high as 10 6 A/cm 2 at 4.2 K in zero field. From resistive measurements of polycrystalline samples in fields up to 8 tesla, upper critical field H c2 slopes of −dH c2 /dT∼1.5T/K are obtained at low temperatures.

Sm1.85Th0.15CuO4−y: A new electron-doped copper oxide superconductor

Abstract We report here results from a study of superconductivity in the Sm 1.85 (Ce 0.15−x Th x )CuO 4−y system. We observe resistive superconducting transitions for all values of x, with transition midpoints ranging from 19.3 K for x =0 to 1.9 K for x =0.15. This is the first reported observation of superconductivity in the compound Sm 1.85 Th 0.15 CuO 4−y . Magnetic susceptibility measurements performed on this compound verify the presence of superconductivity. Additionally, electrical resistivity measurements done under pressure show a decrease in T c in Sm 1.85 Th 0.15 CuO 4−y at a rate dT c /dP=-0.043 K/kbar.

The low field susceptibility of a textured superparamagnetic system

Abstract A theory is presented which describes the effects of orientational texture on the low field susceptibility of a fine particle system in a solid environment. The texture is produced by cooling the system slowly through its freezing point ina field H . The texture is a result of the coupling between the magnetic moments and easy axes of particles. A general expression is derived for the variation of susceptibility with H .

Magnetism, specific heat, and pressure-dependent resistivity of the electron-doped compounds Ln2−xMxCuO4−y (Ln = Pr, Nd, Sm, Eu, Gd; M = Ce, Th)

Abstract Measurements of the magnetic susceptibility z the specific heat C, and the electrical resistivity ρ are presented for the Nd 2 CuO 4 -type structure insulating parent compounds Ln 2 CuO 4 (Ln = Pr, Nd, Sm, Eu, and Gd), the doped superconducting compounds Ln 2−x M x CuO 4−y (Ln = Pr, Nd, Sm, and Eu, M = Ce; Ln = Pr and Nd, M = Th), and the doped, semiconducting compound Gd 2−x Ce x CuO 4−y . The z and C data indicate that antiferromagnetic (AFM) order occurs in the undop compounds for Ln = Nd, Sm, and Gd at temperatures below 1.7 K, 5.95 K, and 6.6 K, respectively. The coexistence of superconductivity and AFM order is observed in Ln 1.85 M 0.15 CuO 4−y , with reduced Neel temperatures T N = 1.2 K for Ln = Nd, M = Ce, Th and T N = 4.6 K for Ln = Sm, M = Ce. Magnetic susceptibility data for Gd 2−x Ce x CuO 4−y display evidence for AFM ordering of Cu 2+ spins at ≈ 280 K for x = 0 and at ≈ 150 K for x = 0.15. For Nd 1.85 M 0.15 CuO 4−y , a very slight increase (decrease) in T c with applied pressure is observed for M = Ce (Th); this relative insensitivity and the depression of T c for M = Th is in contrast to the behavior observed for other high T c compounds.

A comparative compositional study of Ln2-xMxCuO4-y electron-doped superconductors

Abstract We present the results of temperature-dependent magnetization and electrical resistivity measurements for a systematic series of compositions and concentrations of the electron-doped copper-oxide superconducting compounds Ln 2− x M x CuO 4− y (Ln = Pr, Nd, Sm, Eu; M = Ce, Th). The concentration dependences of normal and superconducting properties for both the Ce- and Th- doped systems are nearly identical, indicating that the Ce dopant ions exhibit simple tetravalent behavior. A number of distinct measures of superconducting properties display similar sharply-peaked concentration dependences; such variation is discussed in terms of phase homogeneity.

Angular dependence of the levitation force on a small magnet above a superconducting cylinder in the Meissner state

The dipole–dipole interaction model was used to calculate the levitation force on a small permanent magnet above a superconducting cylinder in the Meissner state lying on the cylinders axis. We obtained analytical expressions for the levitation force and energy in terms of the physical dimensions of the cylindrical superconductor as well as the height and the angle between the magnetic moment and the surface of the sample. Results show that the levitation force and energy are maxima when the magnetic moment is perpendicular to the surface and minima when it is parallel. Special cases of infinite radius and infinite thickness were discussed.

The effect of particle interactions on Curie–Weiss behavior in ferrofluids

The initial susceptibility of a number of Fe3 O4 particle ferrofluids with volume concentrations, e=0.08, 0.07, 0.05, 0.03, 0.02, and 0.01 (prepared by diluting a stock fluid of Ms =40 kA m−1) has been measured in the temperature range 220–450 K. The mean particle sizes Dm (magnetic) and Dp (physical) are 74 and 81 A, respectively. The ferrofluids exhibit Curie–Weiss‐type behavior with negative (Neel) temperature temperature intercepts To . The form of the Curie–Weiss behavior is interpreted as arising from particle interactions which are strong enough to reduce the initial susceptibility below the noninteracting value in the temperature range of measurements. It is suggested that interactions lead to the formation of closed particle structures of zero moment in zero field and, as would be the case for an antiferromagnetic material, leads to Curie–Weiss behavior with a negative ‘‘Neel’’ temperature. Negative values of To are thus characteristic of an interacting particle system.

The Interaction Force Between a Permanent Magnet and a Superconducting Ring

The interaction force between a small magnet and a superconducting ring in assumptions of the Meissner state was analyzed using the dipole-dipole interaction model. Two configurations were evaluated: the vertical configuration when the magnetic moment lies parallel to the symmetry axis of the ring and the horizontal when it is perpendicular. The levitation force on the magnet increases by decreasing the distance from the center of the ring up to a maximum value and then decreases down to zero when the magnet is exactly at the center of the ring. The levitation force when the magnet is in the horizontal configuration is mostly higher than that when it is in the vertical configuration. Simulation results were compared with experimental results reported in literature.

Viscosity, resistivity and surface tension measurements of Fe3O4 ferrofluid

Abstract The viscosity η of Fe 3 O 4 ferrofluid at volume concentrations in the range ϵ=0.0−0.013 are measured. The corresponding range of values of η is 1.6−1.8 cp. A linear relationship exists between the relative viscosity and the concentration of the fluid. Measurements of relative resistivity of Fe 3 O 4 ferrofluid at various concentrations (ϵ=0.00526−0.0526) are also reported. The relative resistivity is taken with respect to the resistivity of the highest value of concentration available (ϵ=0.0526) which is 2.05×10 6 Ω-m. The results show that the relative resistivity is inversely proportional to the concentration of the fluid. The variation of the resistivity with applied magnetic field is also presented. Measurements of the surface tension of Fe 3 O 4 ferrofluid show a continuous increase with increasing concentration and have values ranging from 25.0 to 26.63 dyn/cm in the concentration range ϵ=0.0−0.0526.

Effects of Magnet Size and Geometry on Magnetic Levitation Force

We obtain analytical relations for the levitation force as a function of dimensions of the superconductor-magnet system. The force has been calculated on the basis of the dipole-dipole interaction model. The effect of thickness of the superconductor on the levitation force is investigated. The results show that the influence of geometry and thickness of the magnet becomes significantly large at small levitation distances. Furthermore, approximating the permanent magnet as a point dipole results in an inaccurate estimation of the levitation force.