  In order to establish the doping dependence of the critical current
properties in the iron-based superconductors, the in-plane critical current
density (Jc) of BaFe2As2-based superconductors, Ba1-xKxFe2As2 (K-Ba122),
Ba(Fe1-xCox)2As2 (Co-Ba122), and BaFe2(As1-xPx)2 (P-Ba122) in a wide range of
doping concentration (x) was investigated by means of magnetization hysteresis
loop (MHL) measurements on single-crystal samples. Depending on the dopant
elements and their concentration, Jc exhibits a variety of magnetic-field (H)
and temperature (T) dependences. (1) In the case of K-Ba122, MHL of the
underdoped samples (x < 0.33) exhibits the second magnetization peak (SMP),
which sustains high Jc at high H and high T, exceeding 10^5 A/cm2 at T = 25 K
and H = 6 T for x = 0.30. On the other hand, SMP is missing in the optimally (x
~ 0.36-0.40) and overdoped (x ~ 0.50) samples, and consequently Jc rapidly
decreases by more than one order of magnitude, although the change in Tc is
within a few K. (2) For Co-Ba122, SMP is always present over the entire
superconducting (SC) dome from the under (x ~ 0.05) to the overdoped (x ~ 0.12)
region. However, the magnitude of Jc significantly changes with x, exhibiting a
sharp maximum at x ~ 0.057, which is a slightly underdoped composition for
Co-Ba122. (3) For P-Ba122, the highest Jc is attained at x = 0.30 corresponding
to the highest Tc composition. For the overdoped samples, MHL is characterized
by SMP located close to the irreversibility field. Common to the three doping
variations, Jc becomes highest at the under-doping side of SC dome near the
phase boundary between SC phase and the antiferromagnetic/orthorhombic phase.
Also, the peak appears in a narrow range of doping, distinct from the Tc dome
with broad maximum. These similarities in the three cases indicate that the
observed doping dependence of Jc is intrinsic to the BaFe2As2-based
superconductors.
