Anisotropies in the magnetic properties of aligned high-T [subscript c] superconducting Y [subscript 1] Ba [subscript 2](Cu [subscript 1-x] Fe [subscript x])[subscript 3] O [subscript z] powder composites

Knowledge of the anisotropy of the penetration depth, lower critical field, and critical current density of the high-T_c oxide superconductors is important for the fundamental mechanism for superconductivity and as a guide for practiced applications. Uniaxially-aligned powder composites of Y₁Ba₂(Cu_1-xFe_x)O_x for x ≤ 0.1 were formed by magnetic alignment of particles suspended in epoxy. The voltune distributions of single crystal lites were obtained by light scattering particle size analysis. To good approximation, we observed log-normal distribution functions yielding particle diameters at the distribution peak 10.2 μm ≤ X_o ≤ 13.6 μm.

The substitution of Fe for Cu in Y₁Ba₂Cu₃O_x causes a rapid and nearly linear depression of the superconducting transition temperature, T_c. Magnetic penetration depths λ(0) were obtained from analysis of the temperature-dependent flux exclusion in an applied field of 11 Oe. With increasing Fe content x, the penetration depth λ_ab(λ_c), corresponding to shielding supercurrents flowing along the ab(c) directions increased monotonically from λ_ab(0) = 0.11 μm to 1.5 μm, and λ_c(0) = 3.0 μm to 13 μm. In order to complement the absolute penetration depths obtained from London analysis, the lower critical field H_c1 was determined by measurement of the field at which the magnetization deviates from linearity. Values of λ obtained from H_c1 were comparable with those found from the London theory. For both field orientations if H || c and H || ab at 4.2 K, H_c1 decreased linearly with increasing Fe content from H_c1 = 298 Oe to 25 Oe and from 101 Oe to 3 Oe, respectively. Also, H_c1 decreased linearly with increasing temperature for both field orientations in a 3 % Fe-substituted sample.

From measurement of the superconductive hysteresis of magnetization loops, the critical current density J_c values in fields to 78 kOe were found to decrease exponentially with both reduced temperature (T/T_c) and Fe content x, for both field orientations.

Above T_c, the magnetic susceptibility χ of unaligned Fe-substituted polycrystalline samples followed a Curie-Weiss dependence, χ = χ₀+C/(T+θ), with positive Curie-Weiss temperature θ. As the Fe concentration was increased, the effective moment per Fe-ion decreased while the temperature independent susceptibility χ₀ increased substantially.