Behavior of vortices near twin boundaries in underdoped Ba(Fe${}_{1-x}{\mathrm{Co}}_x){}_2{\mathrm{As}}_2$

We use scanning superconducting quantum interference device (SQUID) microscopy to investigate the behavior of vortices in the presence of twin boundaries in the pnictide superconductor Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$. We show that the vortices avoid pinning on twin boundaries. Individual vortices move in a preferential way when manipulated with the SQUID: They tend to not cross a twin boundary, but rather to move parallel to it. This behavior can be explained by the observation of enhanced superfluid density on twin boundaries in Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$. The observed repulsion from twin boundaries may be a mechanism for enhanced critical currents observed in twinned samples in pnictides and other superconductors.

Figure 1

Pinning positions of vortices in the presence of twin boundaries. (a) Optical micrograph of the sensing area of the scanning SQUID susceptometer, showing a 3-μm-diam pickup loop surrounded by a field coil (b) Susceptometry and (c) magnetometry images of the same region of sample S1. (d) Overlay of (b) and (c). This overlay shows that the vortices do not pin on the twin boundaries. Arrows mark the scan direction.

Figure 2

Distribution of distances between the vortex positions and the nearest twin boundary in the area of Fig. 1. The blue histogram shows the distribution of the distances from the nearest twin for random pinning. This distribution is not flat because the twins are not uniformly spaced. The red shaded bars show the distribution of the distance from the nearest twin for vortices that were imaged with this particular twin configuration. The count is normalized to the total number of vortices measured in this region (60 vortices).

Figure 3

Vortex motion near twin boundaries. Magnetometry images showing vortices (a), (b) and susceptometry images showing twin boundaries (c), (d) taken on the same region of sample S2 at 5 K (a), (c) and 15 K (b), (d). At 5 K the vortices do not move during scanning. At 15 K the vortices are dragged by an interaction with the SQUID sensor. They tend to move parallel to the twin boundaries, and to not cross them. A force up to ∼2 pN was applied on the vortex shown in (e) by driving current through the field coil, but still the vortex did not cross the twin but rather traveled along it. The arrows in (b) denote the scan directions: the SQUID position is incremented along the single long arrow (slow scan direction) and moves back and forth parallel to the double arrows (fast scan direction).

Figure 4

Calculations of the in-plane dragging force applied on a vortex by the SQUID field coil and the SQUID pickup loop. The number in the legend indicates by how much the signal was multiplied in order to plot it on this scale. (a) Plotted for a scan height $h$ = 1 μm, ${\lambda }_{\mathit{ab}}$ = 340 nm: (dashed black line) SQUID field coil, $r_{\mathrm{loop}}$ = 10 μm, $I_{\mathrm{loop}}$ = 250 μA; (solid red line) SQUID pickup loop, $r_{\mathrm{loop}}$ = 1.5 μm, $I_{\mathrm{loop}}$ = 10 μA. (b) Calculations of forces with parameters relevant to particular images in this paper: (dashed black line) [Fig. 3e] SQUID field coil, $r_{\mathrm{loop}}$ = 10 μm, $I_{\mathrm{loop}}$ = 5 mA, ${\lambda }_{\mathit{ab}}$ = 450 nm, $h$ = 1 μm; (solid red line)[Fig. 6c] SQUID pickup loop, $r_{\mathrm{loop}}$ = 1.5 μm, $I_{\mathrm{loop}}$ = 10 μA, ${\lambda }_{\mathit{ab}}$ = 740 nm, $h$ = 1 μm.

Figure 5

SQUID magnetometry at 18 K (close to $T_C$ = 18.25 ± 0.25 K) of an area containing twin boundaries in sample S1. The area shown is a portion of the area in Fig. 1a.

Figure 6

Dragging a vortex across a twin boundary. SQUID magnetometry images (jet) show two vortices imaged at increasing temperatures: (a) 4.8 K, (b) 12 K, (c) 17 K, (d) 17.5 K. These images are superimposed on a susceptometry scan of the same location in S1 taken at 17 K (gray scale).