Evidence of anisotropic vortex pinning by intrinsic and irradiation-induced defects in Ba(Fe,Co)₂As₂studied by quantitative magneto-optical imaging

Abstract

Here we present a study of the pinning properties of high-quality single crystals of Ba(Fe 1−x Co x ) 2 As 2 (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.

Effects of heavy-ion irradiations in K-doped BaFe2As2

“…The T c of the pristine sample is 38. 8 lower than that of the pristine one. Theoretically, perfectly introduced correlated columnar defects do not suppress T c , and it accounts for the weak suppression on T c of the sample with small B U .…”

“…It is known that J c in superconductors can be further enhanced by introducing defects using swift particle irradiations [2]. In our previous studies, remarkable effects have been demonstrated in IBS using heavy-ions [3][4][5][6][7][8] and protons [9] into Co-doped Ba-122 single crystals. In these reports, we irradiated heavy-ions such as 200 MeV Au, 800 MeV Xe, and 2.6 GeV U to create columnar defects, which have excellent geometrical matching with vortices, and are expected to be ideal pinning centers.…”

Effects of disorder induced by heavy-ion irradiation on (Ba1−x K x )Fe2As2 single crystals, within the three-band Eliashberg s± wave model

“…TEM microscopy revealed that 200-MeV Au-ion irradiation of Ba 1− x K x Fe 2 As 2 crystals results in the formation of defects that are linearly correlated along the ion track but – contrary to the case of high- T c cuprates – due to the metallic nature of the compound, they are discontinuous (30–240 nm in length) 27 . In fact, magneto-optical imaging showed the clear signature of anisotropic flux pinning by discontinuous tracks in crystals irradiated with Au ions at the higher energy of 2 GeV 20 , which are present also with irradiation at lower energies (250 MeV) 29 . Moreover, a significant difference with respect to cuprates emerged from a STM analysis of irradiated Fe(Se,Te) crystals, showing that columnar defects produced by 250-MeV Au-ions have a metallic core 28 , instead of the insulating core of the amorphous tracks that the same ions with the same energy produce in cuprates 30 .…”

Imaging atomic-scale effects of high-energy ion irradiation on superconductivity and vortex pinning in Fe(Se,Te)

“…First, the maximum critical field H C2 is very high at low temperatures ( 4 , 5 ), the compounds also exhibit rather isotropic superconductivity. Second, as in the cuprates ( 6 ), J C can be strongly enhanced by high-energy ion irradiation ( 2 , 7 ). Finally, the irradiation leaves T C virtually unchanged to a degree unknown in cuprate high-temperature superconductors.…”