Evidence of anisotropic vortex pinning by intrinsic and irradiation-induced defects in Ba(Fe,Co)2 As2 studied by quantitative magneto-optical imaging

Abstract

Here we present a study of the pinning properties of high-quality single crystals of Ba(Fe1−xCox)2As2 (x = 0.07) by means of the quantitative magneto-optical imaging (MOI). This technique allows us to measure the magnetic field and supercurrent distribution as a function of the applied magnetic field and temperature, thus extracting the local parameters such as the critical current density, without any model assumption. Since values of the local supercurrent are obtained on the micron scale, we are able to observe evidence of anisotropic pinning both of intrinsic defects and extrinsic defects induced by high-energy heavy-ion irradiation. In fact, we find that these crystals naturally show anisotropic pinning due to intrinsic planar defects (whose presence is confirmed by the structural measurements through electron microscopy), and with quantitative MOI analysis we are able to analyze the effects of anisotropy coming from irradiation-induced defects. We quantitatively evaluated the dependence of the critical current density on the magnetic field direction with respect to the lattice principal axes.