Cation disorder and size effects upon magnetic transitions in Ln0.5A0.5MnO3 manganites

A systematic study of the magnetic properties of the manganites ${\mathrm{Ln}}_{\mathrm{0.5}}{\mathrm{(A,A}}^{\mathrm{\prime }}{\mathrm{)}}_{\mathrm{0.5}}{\mathrm{MnO}}_{\mathrm{3}}$ (${\mathrm{A,A}}^{\mathrm{\prime }}\mathrm{=Ba},$ Ca, Sr) has been carried out. The variations of $T_C$ and $T_N$ vs two parameters, the average size of the interpolated cations ${\mathrm{〈r}}_A〉$ and the mismatch effect represented by the variance ${\sigma }^2,$ have been studied for the series of manganites ${\mathrm{La}}_{\mathrm{0.5}}{\mathrm{(A,A}}^{\mathrm{\prime }}{\mathrm{)}}_{\mathrm{0.5}}{\mathrm{MnO}}_{\mathrm{3}},$ ${\mathrm{Pr}}_{\mathrm{0.5}}{\mathrm{(A,A}}^{\mathrm{\prime }}{\mathrm{)}}_{\mathrm{0.5}}{\mathrm{MnO}}_{\mathrm{3}},$ ${\mathrm{Nd}}_{\mathrm{0.5}}{\mathrm{(A,A}}^{\mathrm{\prime }}{\mathrm{)}}_{\mathrm{0.5}}{\mathrm{MnO}}_{\mathrm{3}}$ and ${\mathrm{Sm}}_{\mathrm{0.5}}{\mathrm{(A,A}}^{\mathrm{\prime }}{\mathrm{)}}_{\mathrm{0.5}}{\mathrm{MnO}}_{\mathrm{3}}$ (${\mathrm{A,A}}^{\mathrm{\prime }}\mathrm{=Ba},$ Ca, Sr), involving at most three different cations on the perovskite A site. The results obtained for other ${\mathrm{(Ln,\hspace{0.5em}Ln}}^{\mathrm{\prime }}{\mathrm{)}}_{\mathrm{0.5}}{\mathrm{Sr}}_{\mathrm{0.5}}{\mathrm{MnO}}_{\mathrm{3}}({\mathrm{Ln,\hspace{0.5em}Ln}}^{\mathrm{\prime }}\mathrm{=Gd,\hspace{0.5em}Sm\hspace{0.5em}Y,\hspace{0.5em}Pr)}$ have been taken into consideration. A ${\sigma }^2{\mathrm{-〈r}}_A〉$ diagram has been established, which displays the different types of magnetic behaviors that can be exhibited by ${\mathrm{Ln}}_{\mathrm{0.5}}{\mathrm{A}}_{\mathrm{0.5}}{\mathrm{MnO}}_{\mathrm{3}}$ manganites, i.e., when the temperature is decreasing, either a paramagnetic (PM) to ferromagnetic (FM) transition, or PM to FM followed by FM to antiferromagnetic transitions, or a PM to weak ferromagnetic transition.