Doping-dependent critical current properties in K, Co, and P-doped BaFe2As2 single crystals

In order to establish the doping dependence of the critical current properties in the iron-based superconductors, the in-plane critical current density J(c) of BaFe2As2-based superconductors Ba1-xKxFe2As2 (K-Ba122), Ba(Fe1-xCox)(2)As-2 (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, J(c) exhibits a variety of magnetic-field H and temperature T dependences. (1) In the case of K-Ba122, the MHL of the underdoped samples (x <= 0.33) exhibits a second magnetization peak (SMP), which sustains high J(c) at high H and high T, exceeding 10(5)A/cm(2) at T = 25K and mu H-0 = 6 T for x = 0.30. On the other hand, the SMP is missing in the optimally (x similar to 0.36-0.40) and overdoped (x similar to 0.50) samples and consequently J(c) rapidly decreases by more than one order of magnitude, although the change in T-c is within a few K. (2) For Co-Ba122, the SMP is always present over the entire superconducting (SC) dome from the underdoped (x similar to 0.05) to the overdoped (x similar to 0.12) region. However, the magnitude of J(c) significantly changes with x, exhibiting a sharp maximum at x similar to 0.057, which is a slightly underdoped composition for Co-Ba122. (3) For P-Ba122, the highest J(c) is attained at x = 0.30, corresponding to the highest T-c composition. For the overdoped samples, the MHL is characterized by a SMP located close to the irreversibility field H-irr. Common to the three doping variations, J(c) becomes highest at the underdoping side of the SC dome near the phase boundary between the SC phase and the antiferromagnetic-orthorhombic (AFO) phase. Also, the peak appears in a narrow range of doping, distinct from the T-c dome with a broad maximum. These similarities in the three cases indicate that the observed doping dependence of J(c) is intrinsic to the BaFe2As2-based superconductors. The scaling analysis of the normalized pinning force density fp as a function of the reduced magnetic field h = H/H-irr shows that the peak position in the pinning force h(max) depends on x, indicating a change in pinning with x. On the other hand, high-J(c) samples always attain similar h(max) values of 0.40-0.45 for all the dopants, which may suggest that a common pinning source causes the highest J(c). A quantitative analysis of the T -dependent J(c) indicates that the two pinning mechanisms, namely, the spatial variations in T-c (referred to as delta T-c pinning) and the fluctuations in the mean free path (delta l pinning), are enhanced for the underdoped samples, which results in the enhancement of J(c). Possible origins for the different pinning mechanism are discussed in connection with the x dependence of T-c, the residual resistivity, AFO domain boundaries, and a possible quantum critical point.