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

In this study, we investigate the vortex dynamics in a slightly-overdoped BaFe2(As1−xPx)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 Jc(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) χ1′ and third harmonic χ3′ 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.