We report the detailed analysis of specific heat [C P (T)] and ac susceptibility for magnetically frustrated Sr 2−x La x CoNbO 6 (x = 0-1) double perovskites to understand low-temperature complex magnetic interactions and their evolution with x. Interestingly, the observed Schottky anomaly in the x 0.4 samples shifts gradually toward higher temperatures with magnetic field as well as x, and the analysis reveals the persistence of the discrete energy states in these samples resulting from the

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... n-orbit coupling and octahedral distortion. Moreover, the extracted values of the Landé g factor indicate the existence of high-spin state Co 3+ ions energetically close to a nonmagnetic low-spin state. The specific-heat data show the λ-type anomaly for the x 0.6 samples due to evolution of the long-range antiferromagnetic ordering. Our analysis of low-temperature C P (T) data for the x 0.6 samples demonstrates the 3D isotropic Heisenberg antiferromagnetic (AFM) interactions and the temperature-induced second-order AFM-paramagnetic phase transition. More interestingly, we demonstrate the presence of the free Co 2+ -like Kramers doublet ground state in the x = 1 sample. Further, the ac susceptibility and time evolution of the magnetization data reveal the low-temperature cluster-glass-like behavior in the x = 0-0.4 samples, where spin-spin correlation strength decreases with x.