Bulk magnetization and strong intrinsic pinning in Ni-doped BaFe2As2 single crystals

We report on measurements of isothermal irreversible magnetization loops (M(H)) of Ni-doped BaFe2−xNixAs2 single crystals with x = 0.1 and 0.14 in magnetic fields up to 14 T. For both samples in fields H ‖ c-axis and H ‖ ab-plane, the critical current density calculated from the M(H) curves exceeds 106 A cm−2 at low temperatures, suggesting strong intrinsic pinning in our crystals. In a broad temperature range of 2–17 K in both field orientations, a nearly optimally doped single crystal displays a second magnetization peak (fish-tail effect). For H ‖ c-axis, curves of the normalized pinning force, fp, as a function of reduced field, h, measured at different temperatures obey the scaling relation fp ∝ hp(1−h)q with peak position hmax ≈ 0.33 for the BaFe1.86Ni0.14As2 crystal and hmax ≈ 0.4 for the BaFe1.9Ni0.1As2 sample, indicating a single normal point pinning mechanism. In striking contrast, in the H ‖ ab-plane orientation, the scaling of fp(h) curves is absent.