Electrical properties of yttrium iron garnet at high temperatures

The dc electrical conductivity (σ) and the Seebeck coefficient (α) have been measured on n- and p-type single crystals of yttrium iron garnet (YIG) in the temperature range 600-1500 K. The temperature dependence of σ and α at an oxygen partial pressure (P_O₂) of 1 atm shows extrinsic behavior of σ for both n- and p-type samples with a thermal generation of charge carriers as the main cause of the temperature dependence of σ. The intrinsic conductivity was studied at various temperatures by measurements on p-type samples under a low P_O₂. A compilation of the experimental data in an α-σ plot shows consistency between the measurements on different samples. An analysis of the experimental results leads to the temperature-independent relations μ₊N₊e^A₊=3.0×10²¹ (Vseccm)^-1 and μ_-N_-e^A_-=5.2×10₂₀ (Vseccm)^-1 for holes (+) and electrons (-), where μ, N, and A are the drift mobility, the effective density of states, and the transport coefficient, respectively. Further results are the relations βk+A₊+A_-=-5.4, and A_-+ln(g_-)=2.7, where the constant β determines the temperature dependence of the band-gap energy, and g_- is the spin degeneracy factor of Si impurities in YIG. The results are discussed in terms of the localized hopping model for YIG, the small-polaron model, and the large-polaron model. For both n- and p-type YIG the large-polaron model is in best agreement with the results.