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

Abstract We have studied the critical behaviors of rhombohedral La 0.7 Sr 0.3 Mn 1− x Ni x O 3 ( 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, β =0.394, γ =1.092 and δ =3.99 for x =0.01; T C =352.6 K, β =0.400, γ =1.018, and δ =3.79 for x =0.02; and T C =342.7 K, β =0.468, γ =1.010, and δ =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 , e )= |e| β f ± ( H/|e| β+γ ), where e =( 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 β values for x =0.01 and 0.02 are located in between those expected for the mean-field theory ( β =0.5) and the Heisenberg model ( β =0.365). This proves an existence of ferromagnetic short-range order in La 0.7 Sr 0.3 Mn 1− x Ni x O 3 . On the other hand, the β 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.