Anisotropic magnetic and superconducting properties of aligned weak-ferromagnetic superconductor RuSr$_2$RCu$_2$O$_8$ (R = rare earths)

TL;DR

This study investigates the anisotropic magnetic and superconducting properties of aligned microcrystalline powders of RuSr2RCu2O8 (R = Pr, Nd, Sm, Eu, Gd, Gd0.5Dy0.5) using powder alignment techniques, revealing weak anisotropy and intragrain superconducting behavior.

Contribution

It demonstrates the use of powder alignment methods to analyze anisotropic properties of RuSr2RCu2O8, highlighting the effects of rare earth elements and grain size on magnetic and superconducting behavior.

Findings

Weak magnetic anisotropy observed in susceptibility measurements.

Superconducting intragrain shielding signal is weaker than bulk due to small grain size.

Crossover in susceptibility anisotropy occurs below 190 K.

Abstract

The powder alignment method is used to investigate the anisotropic physical properties of the weak-ferromagnetic superconductor system RuSr2RCu2O8 (R = Pr, Nd, Sm, Eu, Gd, Gd0.5Dy0.5). The RuSr2GdCu2O8 cuprate is a weak-ferromagnetic superconductor with a magnetic ordering of Ru moments at TN(Ru) = 131 K, a superconducting transition in the CuO2 layers at Tc = 56 K, and a low temperature Gd antiferromagnetic ordering at TN(Gd) = 2.5 K. Due to weak magnetic anisotropy of this tetragonal system, highly c-axis aligned microcrystalline powder (diameter ~ 1-10 $\mu$m) in epoxy can be obtained only for R = Eu and Gd through the field-rotation powder alignment method where c-axis is perpendicular to the aligned magnetic field Ba = 0.9 T and parallel to the rotation axis. For smaller rare earth compound R = Gd0.5Dy0.5, powder alignment can be achieved using the simple field powder alignment method where c-axis is partially aligned along the aligned magnetic field. The anisotropic temperature dependence of magnetic susceptibility for the c-axis aligned powders exhibit weak anisotropy with $\chi_{c} > \chi_{ab}$ at room temperature due to anisotropic rare earth, Eu and Gd, contribution and crossover to $\chi_{c} < \chi_{ab}$ below 190 K where strong Ru anisotropic short-range exchange interaction overtakes the rare earth contribution. Anisotropic diamagnetic superconducting intragrain shielding signal of aligned microcrystalline RuSr2GdCu2O8 powder-in-epoxy below vortex lattice melting temperature at 39 K in 1-G field is much weaker than the intergrain polycrystalline bulk sample signal due to the small grain size (d ~ 1-10 $\mu$m), long penetration depth ($\lambda_{ab}$ ~ 0.55 $\mu$m, $\lambda_{c}$ ~ 0.66 $\mu$m) and the two-dimensional (2D) character of CuO2 layers.