Effects of Ni doping on vortex pinning in RbCa2(Fe1−xNix)4As4F2 single crystals

Abstract We investigate the correlation between chemical composition and vortex dynamics in Ni-doped RbCa2(Fe1−xNix)4As4F2 (x = 0, 0.03, 0.05, and 0.07) single crystals using electrical transport and magnetization measurements. In the mixed state, the vortex slush phase, typically observed in a system with moderate disorder, gradually disappears as the Ni doping concentration increases. The activation energy U0, derived from Arrhenius plots of resistivity, exhibits an H^−α dependence and decreases progressively. Magnetic hysteresis loops measurements reveal the second magnetization peaks and its evolution from non-monotonic to monotonic behavior with temperature. A peak in critical current density Jc with Ni doping is observed, and the maximum Jc occurs at x = 0.03, with an estimated value of 1.7 × 10^6 A cm-2. The monotonic decline in Tc and non-monotonic variation in Jc suggest that the partial substitution of Fe by Ni increases the lattice disorder and improves the pinning performance. Referring to the Dew-Hughes model, the fitting results of vortex pinning force density against the reduced magnetic fields (h = H/Hirr and H/Hmax) manifest that both normal point pinning and surface pinning contribute to the pinning effect on the critical current, with normal point pinning gaining prominence at higher Ni doping concentrations. The findings offer valuable insights into the role of chemical doping in vortex pinning and critical current density enhancement, deepening our understanding of the pinning mechanisms in iron-based superconductors with intergrowth structures.