Thickness-induced crossover from strong to weak collective pinning in exfoliated FeTe0.6Se0.4 thin films at 1 T

We studied flux pinning in exfoliated ${\mathrm{FeTe}}_0.6{\mathrm{Se}}_0.4$ thin-film devices with a thickness $d$ from 30 to 150 nm by measuring the critical current density ${J}_{\mathrm{c}}$. In bulk ${\mathrm{FeTe}}_0.6{\mathrm{Se}}_0.4$, the flux pinning has been discussed in the framework of weak collective pinning, while there is little knowledge on the pinning mechanism in the thin-film region. From the thickness $d$ dependence of ${J}_{\mathrm{c}}$ at a fixed magnetic field of 1 T, we found that the strong pinning is dominant below $d\ensuremath{\approx}70$ nm, while the weak collective pinning becomes more important above $d\ensuremath{\approx}100$ nm. This crossover thickness can be explained by the theoretical model proposed by van der Beek et al. [Phys. Rev. B 66, 024523 (2002)].