Investigation of fundamental and higher harmonic AC magnetic susceptibility of FeSe_0.5Te_0.5 superconductor

We present the complex harmonic magnetic susceptibilities χ_n = χ′_n-iχ″_n (n = 1, 3) of FeSe_0.5Te_0.5 polycrystalline superconducting sample. The ac magnetic susceptibility is measured as a function of various external perturbations such as temperature T, the ac magnetic field amplitude H_ac, frequency υ, and the magnitude of dc bias field H_dc. The in-phase (χ′_n) and out of phase (χ″_n) components of the fundamental and third harmonics ofac susceptibility are found to vary as a function of ac driven field. Particularly, the curves shift toward lower temperatures with increasing H_ac. Contrary to ac magnetic field (H_ac), no noticeable change has been observed within the range of applied dc magnetic field (H_dc) of up to 20 Oe. At a fixed ac magnetic field of H_ac = 0.5 Oe, both parts of the third harmonics show frequency dependence. The imaginary part of the third harmonics, χ″₃ shows a small peak followed by a negative minimum at lower temperatures. The small peak diminishes as the frequency increases. The negative minimum suppresses and shifts towards the higher temperatures as we increase the frequency. To better understand the ac magnetic response under the influence of various perturbations, we have analyzed the polar plots (Cole-Cole) of the complex ac susceptibility for both the harmonics. Our analysis suggests that the studied sample is in a vortex glass state, characterized by a collective flux creep within Bean's model, while the Kim-Anderson model is ruled out.