Quasi-periodic magnetic flux jumps in the superconducting state of Ba0.5K0.5Fe1.9M0.1As2 (M = Fe, Co, Ni, Cu, and Zn)

• Flux jumps were observed inpoly-crystalline Ba0.5K0.5Fe1.9M0.1As2 (M = Fe, Co, Ni, and Zn).
• Much high density of flux jumps were observed for the Zn-doped Ba0.5K0.5Fe1.9Zn0.1As2.
• Single crystals of Ba0.5K0.5Fe2As2 and Ba0.5K0.5Fe1.9Zn0.1As2 are absent of flux jumps.

Polycrystals of Ba0.5K0.5Fe1.9M0.1As2 (M = Fe, Co, Ni, Cu, and Zn) synthesized at high pressures show quasi-periodic spikes in the magnetization loop at 2 K owing to successive flux jumps, while the single crystals are observed as absence of jumps. The Zn-doped crystal indicates higher frequency flux jumps than that of Zn-free samples, because of the reduction of grain size and enhancement of grain boundary effect. However, the Co-, Ni- and Cu-doped crystals show less jumps duo to the increased granularity. Our present results suggest that the flux jump in the textured polycrystals is controlled largely by the grain boundary as the flux pining effect rather than the intrinsic dopants.