We measure magnetization as a function of temperature, magnetic field and time in a BaFe(AsP) single crystal with = 27.6 K. The fish-tail observed on many () curves are used to construct isofield () curves which show an anomalous peak at some temperature suggesting a possible phase transition in the irreversible regime. A vortex dynamics study performed along the peaks evidences a minimum in the relaxation rate occurring at the same position of the minimum value of in these peaks. A vortex dynamics study performed on () curves show two distinct minimum in the relaxation rate: a first minimum () for a lower field correlating with and a second () correlating with the peak in the fish-tail. A phase diagram is constructed and the line corresponding to and points obey an expression developed in the literature for a structural rhombic to square lattice phase transition, further supporting this view.

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