Understanding the magnetic interactions of Sr_2-xLa_xCoNbO₆in a pure ionic picture

The crystal field induced change in the energy levels/ bands of Sr_2-xLa_xCoNbO₆ (x=0-1) samples have been systematically understood by studying their magnetic properties, where substitution of La³⁺ at Sr²⁺ site is expected to completely transform  the Co³⁺  into Co²⁺ across the series.Temperature dependent magnetization measurements indicate the conversion from the weak ferromagnetic to the antiferromagnetic ordering for x≥0.6 due to evolution of the Co-O-Nb-O-Co exchange path resulting from the enhanced B-site ordering with the La substitution. Further, the effective magnetic moment extracted from the temperature dependent magnetization measurements show its significantly higher value (5.62 µ_B/f.u.) than the spin only moment for x=1 sample (S=3/2; 3.87 µ_B/Co²⁺), which indicate the persistence of the triply degenerate (orbital) free ion like ⁴T₁ ground state and hence significant contribution of the orbital magnetic moment. More interestingly, we observe the low temperature Schottky anomaly in the specific heat curves in case of the x≤0.4 samples due to the transition between the low lying energy levels of Co³⁺, resulting from the spin-orbit coupling and octahedral distortion, and presence of the ground state Karmer's doublet in case of Co²⁺ for x=1 sample.