Impedance spectroscopy and ac conductivity mechanism in Sm doped Yttrium Iron Garnet

Polycrystalline samples of Y 3- x Sm x Fe 5 O 12 for x =0.0 to 3.0 were prepared in single phase form with cubic structure. The analysis of XRD patterns using Reitveld refinement reveals that these samples crystallize in cubic structure with Ia 3 d space group and lattice constant increases systematically with Sm concentration. The depressed and asymmetric semicircle of complex impedance spectra suggests that the relaxation of charge carriers is of non-Debye type and it is controlled by thermally activated charge carriers. Both grains and grain boundaries are found to contribute for the measured complex impedance spectrum, however at higher temperature (T ≥ 543 K), the relaxation process is dominated by the grain boundaries. The temperature dependence of relaxation frequency and conductivity exhibit a change of slope in the vicinity of ferrimagnetic transition temperature which is attributed to the magneto-electric coupling. The analysis of ac conductivity spectra in terms of Jonscher power law suggests that the conduction process is governed by small polaron hopping model for x ≤ 0.5 and for higher concentration overlapping of large polaron tunneling model is found to play a dominant role. The dielectric constant is found to increase with increase with increase in Sm concentration.