Nano Fe-doped MgB2 samples with varying doping levels (0, 1.0, 2.0, 4.0, and 6.0 wt%) were studied for their first- and third-harmonic ac-susceptibility. The applied dc-field, Hdc = 250 Oe and ac-drive field, Hac = 0.5 Oe were taken into account with varying frequency from f = 0.2 to 3 kHz. The peaks in the \(\chi^{\prime\prime}_{1} (T)\) plots, the imaginary part of the ac-susceptibility’s first-harmonic temperature dependency get enhanced as the frequency increases and shifts to the higher temperature area. The 4-wt% nano-Fe-doped sample had the highest peak intensity in the \(\chi^{\prime\prime}_{1} (T)\) curve when compared to other doped and undoped samples. In contrast, the real third-harmonic ac-susceptibility component, \(\chi^{\prime}_{3}\) shows the maxima and minima of periodic oscillation caused by the ratio, δ, a parameter associated with the depth of the ac-applied field, while the imaginary component, \(\chi^{\prime\prime}_{3}\) holds the higher intensity of peak than that of the real component. In comparison to all samples, the 6-wt% nano-Fe-doped MgB2 sample exhibits distinct oscillating behavior with multiple real and imaginary peaks of third-harmonic ac-susceptibility. The irreversibility line (IL) in third-harmonic ac-susceptibility moves to the region where temperature is high as the frequency is amplified. Among all the samples, the 3-kHz frequency exhibits the highest IL.
