We present results of magnetic measurements relating to vortex phase diagram in a single crystal of FeSe${}_{0.5}$Te${}_{0.5}$ which displays a second magnetization peak anomaly for $H\parallel c$. The possible role of the crystalline anisotropy on vortex pinning is explored via magnetic torque magnetometry. We present evidence in favor of pinning related to spatial variations of the charge carrier mean free path leading to small bundle vortex pinning by randomly distributed (weak) pinning centers for both $H\parallel c$ and $H\perp c$. This is further corroborated using magnetization data for $H\parallel c$ in a single crystal of FeSe${}_{0.35}$Te${}_{0.65}$. Dynamical response across the second magnetization peak (SMP) anomaly in FeSe${}_{0.5}$Te${}_{0.5}$ has been compared with that across the well-researched phenomenon of peak effect (PE) in a single crystal of CeRu${}_2$.

• Received 19 April 2011

DOI:https://doi.org/10.1103/PhysRevB.84.214526