Multiharmonic AC magnetic susceptibility analysis of the rhombic-to-square transition in the Bragg vortex glass phase in a BaFe2(As1−xPx)2 crystal

• Multiharmonic AC susceptibility directly links the onset of the Peak Effect to the rhombic-to-square structural transition of the Bragg vortex glass in BaFe 2 (As 1 – x P x ) 2 . • The in-phase third harmonic χ 3 ′ shows a sharp deviation temperature (T on ) that falls precisely on the RST line for all investigated DC fields and AC frequencies. • The end of the Peak Effect, marked by χ 3 ′ ≈ 0 , coincides with the Second Magnetization Peak, revealing a temporary freezing of vortex dynamics and an elastic-to-plastic pinning crossover. • The constructed AC phase diagram unambiguously separates purely rhombic and square vortex glass phases and links the SMP’s critical‐current maximum to the completion of the RST. • The variation of the AC excitation frequency provides dynamic control over the PE onset, offering a tunable handle on vortex structural transitions for superconducting applications. In this study, we investigate the vortex dynamics in a slightly-overdoped BaFe 2 (As 1−x P x ) 2 single crystal using multiharmonic AC magnetic susceptibility measurements. Previous DC magnetic measurements identified a Second Magnetization Peak (SMP) and a Rhombic to Square Structural Transition (RST) in the Bragg vortex glass. However, the non-monotonous temperature dependence of the critical current density J c ( T ), known as the Peak Effect (PE), was not fully understood. Our AC susceptibility analysis, performed with and without a superimposed DC field, reveals that the real part of the fundamental susceptibility (first harmonic) χ 1 ′ and third harmonic χ 3 ′ exhibit standard behavior in the absence of a DC magnetic field, but show complex non-monotonous trends when a DC field is applied. Using the Bean critical state model, we identify different temperature ranges with distinct superconducting responses. By constructing a vortex phase diagram incorporating the SMP, RST, and PE, we elucidate the origin and the end of the PE. In particular, it is shown that by using the AC susceptibility it is possible to clearly identify the purely rhombic and, respectively, square phases of the Bragg vortex glass and to associate the maximum of the critical current density, corresponding to the SMP, to the completion of the RST. Our findings highlight the potential of AC susceptibility as a tool for probing structural transitions of the vortex glass which, associated with the PE, can be crucial for the understanding of the superconductors behavior in high magnetic fields and temperatures.