Flux Pinning Properties of 7-filament Cu/Ag-sheathed Ba1-xKxFe2As2 Tapes at 4.2 K

We investigated the magnetic field and angular dependence of the critical current I c ( B , θ ) of 7-filament Cu/Ag-sheathed Ba 1-x K x Fe 2 As 2 (Ba122) tapes at 4.2 K. Below approximately 8 T, I c ( B , θ ) exhibited hysteresis behaviors depending on increasing or decreasing B , I c up ( B ) or I c down ( B ), as well as changes in θ in a positive or negative rotation direction, I c pos ( θ ) or I c neg ( θ ). I c up ( B ) exhibited a non-monotonic dependence on B , which was explained by the flux pinning of Abrikosov vortices in grains and Abrikosov-Josephson vortices at grain boundaries. I c ( B , θ ) exhibited an anomalous anisotropy I c ( B , 90°)/ I c ( B , 0°) < 1 opposed to the effective-mass model. We calculated J c ( B , θ ) numerically by treating the angular dependence of the elementary pinning force owing to a spherical pin with a radius of approximately 8 nm, and successfully reproduced the measured I c ( B > 8 T, θ ) and I c avg ( B , θ ) = [ I c pos ( B, θ ) + I c neg ( B, θ )]/2 at B < 8 T. This suggests that spherical pins play an important role in vortex pinning, and controlling them may improve the I c ( T , B , θ ) characteristics in 7-filament Cu/Ag-sheathed Ba122 tapes.