Inter-grain superconductivity of polycrystalline FeSr2YCu2O6+y

• Inter-grain superconductivity was studied by higher-harmonic susceptibilities χn.
• The observed χn were reproduced by the Kim–Anderson (K–A) critical-state model.
• The K–A parameters suggest that Jcinter is small but hardly depends on a weak field.

The fundamental and higher-harmonic susceptibilities χn=χn′−iχn′′ (n = 1, 2, 3, 5 and 7) were measured to study the inter-grain superconducting properties of polycrystalline FeSr2YCu2O6+y. The real part χ1′ and imaginary part χ1″ appeared below Tc1 = 56 K and Tc2 = 45 K, respectively. Tc1 and Tc2 correspond to the onset temperatures of the superconductivity in individual grains and the superconductivity across grain boundaries, respectively. The higher-harmonic susceptibilities χn (n = 2, 3, 5 and 7) appeared below Tc2. The results suggest that the higher-harmonic susceptibilities were generated by the non-linear magnetization caused by the magnetic flux penetration through grain boundaries. The inter-grain magnetization curves were obtained by substituting measured χn in Fourier series. The temperature dependence of χn (n = 1, 2, 3, 5 and 7) calculated by the Kim–Anderson critical-state model were approximately fitted with the observed those of χn. The inter-grain critical-current density at 0 K under zero magnetic field was estimated at about 2 × 104 A/m2 from the fitting parameters of the Kim–Anderson critical-state model. The magnetization curves and these parameters imply that the inter-grain critical-current density hardly depends on a weak magnetic field below 0.1 mT.