Effects of magnetic dilution in the ferrimagnetic columnar ordered Sm2MnMnMn4−xTixO12 perovskites

Powder neutron-diffraction experiments have been employed to establish the effects of site-selective magnetic dilution in the ${\mathrm{Sm}}_2{\mathrm{MnMnMn}}_{4-x}{\mathrm{Ti}}_x{\mathrm{O}}_{12} A$-site columnar ordered quadruple perovskite manganites ($x=1, x=2$, and $x=3$). We show that in all three compositions the Mn ions adopt a collinear ferrimagnetic structure below 27, 62, and 34 K, respectively. An unexpected increase in the ordering temperature was observed between the $x=1$ and $x=2$ samples, which indicates a considerable departure from mean-field behavior. This result is corroborated by large reductions in the theoretical ground-state magnetic moments observed across the series, which indicate the presence of spin fluctuations and/or disorder. We show that long-range magnetic order in the $x=3$ sample, which occurs below the percolation threshold for B-B exchange, can only be understood to arise if it is mediated via both A-B and B-B exchange, hence confirming the importance of A-B exchange interactions in these materials. Finally, we show that site-selective magnetic dilution enables the tuning of a ferrimagnetic compensation point and the introduction of temperature-induced magnetization reversal.