Effect of Ni-doping on critical behavior of La_0.6Pr_0.1Ba_0.3Mn_1−xNi_xO₃ nanocrystalline manganites

We used Banerjee's criteria, modified Arrott plots and the scaling hypothesis to analyze magnetic-field dependences of magnetization near the ferromagnetic–paramagnetic (FM–PM) phase-transition temperature (T_C) of La_0.6Pr_0.1Ba_0.3Mn_1−xNi_xO₃ (0≤x≤0.3) nanocrystalline manganites. Experimental results reveal that all samples undergo a second-order phase transition. The estimated critical exponents obtained for x=0 sample are close to the mean-field model (β=0.503±0.004, γ= 1.024±0.068 and δ=2.821 at T_C=215 K). Whereas for a high amount of Ni, these exponents belong to a different universality class (β=0.599±0.014, γ=0.897±0.013 and δ=2.570 at T_C=162 K for x=0.1 sample) and (β=0.733±0.012, γ=0.753±0.014 and δ=2.320 at T_C=131 K for x=0.3 sample). This is due to the fact that the substitution of Ni ions into the Mn-site leads to the formation of a larger proportion of Mn⁴⁺ with respect to Mn³⁺ which reduces the ferromagnetic double exchange interaction of Mn³⁺–Mn⁴⁺ couples. These exponents indicate that the exchange interaction J(r) decreases with distance r slower than r^−4.5 for x=0 sample, however it ranges from r⁻⁵ and r^−4.5 for x=0.1 and 0.3 samples.