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Discovery of superconductivity in LaFeAsO1-xFx with Tc ~ 26K in 2008 led to an outburst of research activity towards finding new Fe-based superconductors and increasing their Tc, which resulted in the identification of at least six family of Fe-based superconductors with the highest Tc of ~ 56K reported in Gd0.8Th0.2FeAsO. In an uncanny resemblance to the importance of Cu-O planes in the cuprate high Tc superconductors, the FeAs or, FeSe/Te layers play a vital role in Fe-based superconductors. Amongst them the tetragonal FeSe/Te system has proved to be promising to understand the basic mechanism of superconductivity in Fe-based materials. The Fermi surface of this tetragonal system is very similar to that reported for the FeAs based superconductors, comprising cylindrical electron sections at the zone corners, cylindrical hole surface sections, and small hole sections at the zone center. Furthermore, these surfaces are separated by a 2D nesting vector at (,,), another characteristic reminiscent of the FeAs based superconductors. Despite an apparent structural simplicity of FeSe/Te vis a vis other Fe-based systems, its physics is already shown to be both rich as well as interestingly complex. There have been reports hinting towards the possibility of an anisotropy in the symmetry of order parameter, multiple band gaps and a dominant Pauli paramagnetic effect in the upper critical field(s) (Hc2). In view of the promising fabrication of superconducting wires of FeSe/Te by powder-in-tube technique, it becomes important to understand its vortex phase diagram and the pinning mechanism. Superconducting FeSe0.5Te0.5 is the optimal Tc compound from the Fe-11 family of iron superconductors, exhibiting a maximum Tc ~ 14:5K. In this talk, we present detailed magnetization measurements on a single crystal of FeSe0:50Te0:50. The results include: (i) observation of the second magnetization peak (SMP) (i.e., a fishtail feature), (ii) calculation of crystalline anisotropy based on torque magnetometry measurements, (iii) the estimation of flux pinning force density (Fp), (iv) obtaining the vortex phase diagram, and (v) magnetic relaxation across SMP. Based on the above results, we try to provide an understanding of the underlying pinning mechanism and compare and contrast with the results in the well studied CeRu2 system.
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