Investigation of fundamental and higher harmonic AC magnetic susceptibility of FeSe0.5Te0.5 superconductor

Abstract

We present the complex harmonic magnetic susceptibilities χn = χ′n-iχ″n (n = 1, 3) of FeSe0.5Te0.5 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 Hac, frequency υ, and the magnitude of dc bias field Hdc. The in-phase (χ′n) and out of phase (χ″n) components of the fundamental and third harmonics of ac susceptibility are found to vary as a function of ac driven field. Particularly, the curves shift toward lower temperatures with increasing Hac. Contrary to ac magnetic field (Hac), no noticeable change has been observed within the range of applied dc magnetic field (Hdc) of up to 20 Oe. At a fixed ac magnetic field of Hac = 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.