Possibility of vortex lattice structural phase transition in the superconducting pnictide Ba(Fe_0.925Co_0.075) ₂As₂

We present magnetic measurements in a single crystal of the newly discovered superconducting iron-pnictide Ba(Fe_0.925Co _0.075)₂As₂. 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=dlnM/dlnt, 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 Ba(Fe _0.925Co_0.075)₂As₂ 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 Ba(Fe_0.925Co _0.075)₂As₂ 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.