We present magnetic measurements in a single crystal of the newly discovered superconducting iron-pnictide $\text{Ba}{\left({\text{Fe}}_{0.925}{\text{Co}}_{0.075}\right)}_2{\text{As}}_2$. The magnetization loops exhibit a second magnetization peak (SMP) similar to that observed in most high-temperature superconductors (HTSs). Magnetic relaxation measurements reveal a minimum in the normalized relaxation rate, $S=d\text{ln}M/d\text{ln}t$, located in between the SMP onset and the peak fields. The SMP in HTSs is commonly associated with the vortex order-disorder phase transition. However, in $\text{Ba}{\left({\text{Fe}}_{0.925}{\text{Co}}_{0.075}\right)}_2{\text{As}}_2$ the onset and peak fields, as well as the minimum point in $S$, exhibit strong temperature dependence down to low temperatures, excluding the possibility for such a transition. We suggest that the SMP in $\text{Ba}{\left({\text{Fe}}_{0.925}{\text{Co}}_{0.075}\right)}_2{\text{As}}_2$ is associated with a vortex structural phase transition from rhombic to square lattice taking place at field and temperatures corresponding to the minimum point of $S$. A theoretical fit to the transition line, based on a recent theoretical model for vortex structural phase transition, shows good agreement with the experimental results.

• Received 10 December 2009

DOI:https://doi.org/10.1103/PhysRevB.81.092504