Vortex images on Ba{1-x}KxFe2As2 observed directly by the magnetic force microscopy

TL;DR

This study uses magnetic force microscopy to directly image vortex arrangements in doped BaFe2As2 superconductors, revealing differences in vortex order, pinning forces, and the presence of vortex chains related to doping levels and structural features.

Contribution

It provides the first direct imaging of vortex states in BaKFe2As2, highlighting how doping influences vortex order, pinning strength, and the formation of vortex chains.

Findings

Optimally doped samples show local triangular vortex clusters.

Pinning force per unit length is weaker in BaKFe2As2 than in Co-doped counterparts.

Vortex chains are observed near twin boundaries in underdoped samples.

Abstract

The vortex states on optimally doped Ba0.6K0.4Fe2As2 and underdoped Ba0.77K0.23Fe2As2 single crystals are imaged by magnetic force microscopy at various magnetic fields below 100 Oe. Local triangular vortex clusters are observed in optimally doped samples. The vortices are more ordered than those in Ba(Fe{1-x}Co{x})2As2, and the calculated pinning force per unit length is about 1 order of magnitude weaker than that in optimally Co-doped 122 at the same magnetic field, indicating that the Co doping at the Fe sites induces stronger pinning. The proportion of six-neighbored vortices to the total amount increases quickly with increasing magnetic field, and the estimated value reaches 100% at several tesla. Vortex chains are also found in some local regions, which enhance the pinning force as well as the critical current density. Lines of vortex chains are observed in underdoped samples, and they may have originated from the strong pinning near the twin boundaries arising from the structural transition.