Anisotropy of critical current densities in Ba_1−x K _x Fe₂As₂ and Ba(Fe_1−x Co _x )₂As₂ with splayed columnar defects

Abstract

Abstract. Effects of bimodal splayed columnar defects in Ba 1−xKxFe2As2 and Ba(Fe1−xCox)2As2 single crystals through 2.6 GeV U irradiation and 220 MeV Au irradiation are investigated. Magnetic field profiles measured by magneto-optical imaging clarified the presence of in-plane anisotropy of the critical current density (Jc) in 2.6 GeV U irradiated crystals, where Jc parallel to the splay plane is larger than that perpendicular to the splay plane. However, the in-plane anisotropy of Jc is absent in 220MeV Au irradiated crystals. Possible origin of this discrepancy is discussed based on the structure of introduced defects. IntroductionColumnar defects in type-II superconductors serve as artificial pinning centers, which lead to an enhancement of critical current density J c [1,2,3]. It has been proposed that a further enhancement of J c is possible by dispersing the direction of the columnar defects [4]. In such a system with splayed columnar defects, enhancement of J c has been confirmed in 1.08 GeV Au irradiated YBa 2 Cu 3 O 7−δ single crystals [5] and 2.6 GeV U irradiated Ba 1−x K x Fe 2 As 2 single crystals [6]. Due to the competing effects of suppression of vortex motion by unequal lengths of vortex segments between splayed columnar defects and detrimental effect of misalignment of vortices, the optimal splay angles were found to be ≈ ±5 • in both of the above systems.However, since these results were obtained through magnetization measurements, the estimated J c is a weighted average of J c 's along two directions in the ab plane. Since directions parallel and perpendicular to the splay plane of columnar defects can be distinguished in a system with bimodal splayed columnar defects, J c parallel to the splay direction (J c∥ ) and J c perpendicular to the splay direction (J c⊥ ) can be different. Such an in-plane anisotropy of J c in systems with splayed columnar defects has been confirmed through resistivity measurements in 1 GeV Au irradiated YBa 2 Cu 3 O 7−δ [7] and 3.9 GeV Au irradiated YBa 2 Cu 3 O 7−δ [8], where Ohmic dissipation was higher when the current is passed in the direction perpendicular to the

Effects of Asymmetric Splayed Columnar Defects on the Anomalous Peak Effect in Ba0.6K0.4Fe2As2

“…The crystals were shaped into rectangular parallelepipeds with lengths and widths ~500 μm and thicknesses less than 20 μm, which is small enough compared with the projected range of 2.6 GeV U ions (65 μm). we have confirmed that the introduced CDs are continuous and their density is nearly the same as the dose of injected 2.6 GeV U ions by the STEM observation [27]. It has also been reported that the diameter of CDs generated by 2.6 GeV U ions is 7~8 nm [27].…”

“…we have confirmed that the introduced CDs are continuous and their density is nearly the same as the dose of injected 2.6 GeV U ions by the STEM observation [27]. It has also been reported that the diameter of CDs generated by 2.6 GeV U ions is 7~8 nm [27]. In addition, the variation of the diameter of CDs through the samples is negligible, since the sample is much thinner than the projection range of 2.6 GeV U ions.…”

“…Such an enhancement of J c by introduction of CDs has also been confirmed in iron-based superconductors (IBSs) [3][4][5][6][7][8][9][10][11][12], and IBSs offer new playgrounds for vortex physics in superconductors [22][23][24][25][26]. Transmission electron microscope (TEM) observations showed that introduced CDs in IBSs tend to be discontinuous compared with those in cuprates [3,7,27]. The optimal splay angle for the maximum enhancement of J c , determined by the competition between the beneficial effect of suppressing vortex motion by the variable inter-defect distance and forcing vortices to entangle, and the adverse effect of vortex-field misalignment, were found to be ~±5° in both YBa 2 Cu 3 O 7-δ and Ba 1x K x Fe 2 As 2 [16,21].…”

“…The optimal splay angle for the maximum enhancement of J c , determined by the competition between the beneficial effect of suppressing vortex motion by the variable inter-defect distance and forcing vortices to entangle, and the adverse effect of vortex-field misalignment, were found to be ~±5° in both YBa 2 Cu 3 O 7-δ and Ba 1x K x Fe 2 As 2 [16,21]. Moreover, in such systems with splayed CDs, an anomalous peak effect (APE) in J c at a certain magnetic field determined by the irradiation dose was observed [21,[27][28][29][30][31]. Since such an APE is absent in IBSs with CDs parallel to the c-axis and only present in those with splayed CDs when the magnetic field is applied along the average direction of splayed CDs, it has been proposed that it is caused by the suppression of kink motion and/or the effect of vortex entanglement in systems with splayed CDs [28].…”

Critical Current Density and Vortex Dynamics in Pristine and Irradiated KCa2Fe4As4F2

“…Second, the vortex dynamics of the pristine and 3 MeV proton irradiated crystals are discussed based on magnetization measurements and analyses of magnetic relaxation. [20,23,39].…”