We studied flux pinning in exfoliated FeTe_0.6Se_0.4 thin-film devices with a thickness d from 30 to 150 nm by measuring the critical current density J_c. In bulk FeTe_0.6Se_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_c at a fixed magnetic field of 1 T, we found that the strong pinning is dominant below d ≈ 70 nm, while the weak

more »

... ve pinning becomes more important above d ≈ 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)].