A new approach to determine the effective penetration depth by using Scanning SQUID Microscope
Hui-Ting Lin1*, Sing-Lin Wu2, Cheng-En Wu2, Ji-Wun Wang1, Ming-Jye Wang3, Jeng-Chung Chen1, Maw-Kuen Wu2, Cheng-Chung Chi1
1Department of Physics, National Tsing Hua University, Hsinhu, Taiwan
2Institute of Physics, Academia Sinica, Taipei, Taiwan
3Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
* presenting author:huiting lin, email:huiting1014@gmail.com
The Scanning SQUID Microscope (SSQ) is a sensitive mapping tool of magnetic fields. The spatial resolution is usually limited by the pickup loop size, typically a few microns in radius, so that people do not use SSQ to measure the effective penetration depths which is often in the deep submicron range. However, we have found out that the magnitude of the flux peak, measured by using an SSQ with a circular loop of 5 microns in radius, can be used to determine much smaller effective magnetic penetration depth. After we tried to generate a quantitative numerical fitting to our experimentally obtained the flux map of an isolated vortex in a superconducting Nb film, we realized that fitting is critically depends on the penetration depth╬╗, and the distance between pickup loop and film surface z, given the fact the other parameters in the magnetic field formula obtained by D. Agassi
and J.R. Cullen can be easily determined. Thus, if we know either of the two parameters, we can use our data to determine the other one. Unfortunately, it is not easy to measure z accurately inside a cryogenic dewar. Thus we used the flux peak value of Nb film deposited on FeSe0.3Te0.7 film for comparison to the flux peak value of FeSe0.3Te0.7 alone. Since they were done by a single scanning approach, we are sure that they have the same z value. Using the known penetration depth of Nb, we obtained an effective penetration depth of 1100nm for FeSe0.3Te0.7, which is larger than the values in the literature. Possible explanation for the discrepancy will be addressed.

Keywords: scanning SQUID microscope, the effective penetration depth