Origin of dielectric loss in Ba(Co_1/3Nb_2/3)O₃ microwave ceramics

The microwave dielectric loss of stoichiometric and non-stoichiometric Ba(Co_1/3Nb_2/3)O₃ ceramics have been measured from 2 to 300 K in magnetic fields ranging from 0 to 5 T using a dielectric resonator (DR) technique. The microwave absorption from spin excitations of unpaired d-electrons in exchange coupled Co²⁺ ions dominate the loss of the Ba(Co_1/3Nb_2/3)O₃ ceramics at cryogenic temperatures. Two peaks in the loss tangent (tan δ) vs temperature relation from a distinctly different origin occur at 25-30 K and 90 K, which increase in magnitude with increasing Co content in the bulk dielectric samples. Evidence that these peaks result from polaron conduction from hopping between Co²⁺ and Co³⁺ ions includes (i) the peak's observed temperature range; (ii) the decrease in peak intensity of approximately a factor of two in a large applied magnetic fields (5 T); and (iii) a strong correlation between the peak's magnitude and both the fraction of the minority Co³⁺ in the dominant Co²⁺ matrix and D.C. conductivity at elevated temperatures. A magnetic-field independent high temperature peak with a maximum at 250 K dominates the room temperature microwave loss whose magnitude correlates with those of the low temperature peaks. This suggests that the defects responsible for carrier conduction play an important role in establishing the loss tangent at room temperature.