Anisotropic critical current density and flux pinning mechanism of Fe 1+y Te_0.6Se_0.4 single crystals

Abstract

Fe 1 + y Te0.6Se0.4 has considerable application potential due to its large critical current density (J c) and high upper critical magnetic field ( H c2 ). However, the uncertainty of the anisotropy of J c and the unclear flux-pinning mechanism have limited the application of this material. In this study, the J c in three directions were obtained from magnetic hysteresis loop measurements. A large anisotropy of J c a b / J c c ∼ 10 was observed, and the origin of the anisotropy was discussed in details. Flux pinning force densities (F p) were obtained from J c, and a non-scaling behavior was found in the normalized pinning force f p [ F p / F p-max ] versus the normalized field h[H/ H c2 ]. The peaks of pinning force shift from a high h to a low h with increasing temperature. Based on the vortex dynamics analysis, the peak shift was found to originate from the magnetization relaxation. The J c and F p at critical states free from the magnetic relaxation were regained. According to the Dew-Hughes model, the dominant pinning type in Fe 1 + y Te0.6Se0.4 clean single crystals was confirmed to be normal point pinning.

Magnetization loops and non-scaling behavior of flux pinning force of Fe1+y Te1−x Se x studied by numerical simulation

“…where the parameters J c00 = 3.2 × 10 5 A cm −2 and U 00 = 290 K are referred to the experiment results of our previous work [7]. J c00 and U 00 are independent of temperature and field.…”

“…In addition, the J c00 and U 00 were adjusted slightly according to the requirement of calculation. The piecewise function equation ( 8) we adopted was inspired by [7,17]. However, the specific function and parameters were determined by numerical calculation.…”

“…Therefore, we apply of the same formulas for both directions. Temperature dependence of J is determined by referring to the experimental results of [7]. Then, we performed the calculating and compared with experimental results.…”

“…Leo et al [6] reported that f p -peak in Fe 1+y Te 0.5 Se 0.5 could not be scaled but with an anomalous 'peak effect'. Our previous work [7] reported a significant shift of f p -peak position at different temperatures with different applied field sweeping rates in Fe 1+y Te 0.6 Se 0.4 single crystal. Koblischka and Muralidhar [8] systematically studied the non-scaling behavior of f p in 1111-, 111-, 11-, and 122families, and found non-scaling behavior in all of these materials.…”

“…Considering the relatively large relaxation rate observed in IBSs [10,11], it is better to describe the magnetic behavior of superconductors by introducing time dependence into CSMs. Our previous work [7] estimated the critical state J c from the experimental data after relaxation by the collective pinning model. The Dew-Hughes model was used to obtain the pinning type, and the main pinning type is found to be the normal point pinning.…”