Possibility of vortex lattice structural phase transition in the superconducting pnictideBa(Fe0.925Co0.075)2As2

We present magnetic measurements in a single crystal of the newly discovered superconducting iron-pnictide $\text{Ba}{({\text{Fe}}_{0.925}{\text{Co}}_{0.075})}_{2}{\text{As}}_{2}$. 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, $S=d\text{ }\text{ln}\text{ }M/d\text{ }\text{ln}\text{ }t$, 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 $\text{Ba}{({\text{Fe}}_{0.925}{\text{Co}}_{0.075})}_{2}{\text{As}}_{2}$ 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 $\text{Ba}{({\text{Fe}}_{0.925}{\text{Co}}_{0.075})}_{2}{\text{As}}_{2}$ 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.