Influences of Ni-doping on critical behaviors of La0.7Sr0.3Mn1-xNixO3

We have studied the critical behaviors of rhombohedral La0.7Sr0.3Mn1 xNixO3 (x=0.01, 0.02 and 0.03) manganites by analyzing isothermal magnetization data recorded at temperatures around the ferromagnetic-paramagnetic phase transition (the Curie temperature, T-c). The experimental results reveal that all the samples undergo a second-order phase transition. Using the modified Arrott plot method, the critical parameters obtained are T-c=357.3 K, beta=0.394, gamma=1.092 and delta=3.99 for x=0.01; T-c=352.6 K, beta=0.400, gamma=1.018, and delta=3.79 for x=0.02; and T-c=342.7 K, beta=0.468, gamma=1.010, and delta=2.67 for x=0.03. With these critical exponents, the isothermal magnetization data around T-c fall into two branches of a universal function M(H, epsilon)= If vertical bar epsilon vertical bar(beta)f(+/-) (H/vertical bar epsilon vertical bar(beta+gamma) where epsilon=(T-T-c)/T-c is the reduced temperature. This proves that the critical parameters determined are reliable, and in good accordance with the scaling hypothesis. In our case, the beta values for x=0.01 and 0.02 are located in between those expected for the mean-field theory (beta=0.5) and the Heisenberg model (beta=0.365). This proves an existence of ferromagnetic short-range order in La0.7Sr0.3Mn1 xNixO3. On the other hand, the beta value for x=0.03 is close to that expected for the mean-field theory with ferromagnetic long-range interactions. The Ni-doping at high contents has thus a tendency leading to ferromagnetic long-range order. (C) 2012 Elsevier B.V. All rights reserved.