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.